A STUDY OF THE FREE RADICALS 1N
IRRADEATED AMlDES BY ELECTRON
SPIN RESONANCE SPECTROSCOPY

Thesis for the Degree 0%: pk. D.
MICHIGAN STATE UNWERSITY
Sara-Jo Kleinhekse! Bolte

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ABSTRACT
A STUDY OF‘THE FREE RADICALS IN IRRADIATED AMIDES
BY ELECTRON SPIN RESONANCE SPECTROSCOPY

by Sara-Jo Kleinheksel Bolte

Polycrystalline amides representative of hydrocarbon substitution
on both the nitrogen and the carbonyl carbon, and two related carboxylic
acids have been irradiated with 1 Mev. electrons at liquid air tempera-
ture. The free radicals formed by irradiation were studied by electron
spin resonance spectroscopy in the 9 ch. region at liquid air tempera-
ture, using a spectrometer built at Michigan State University and a var-
ian E.S.R. spectrometer. The hyperfine spectra observed were used to
determine the number and type of nuclei interacting with the spin of the
unpaired electron and their isotropic coupling constants. From this in—
formation the free radicals formed by irradiation were identified and a
scheme for the prediction of the site of radiation damage in some com-
pounds is suggested.

The spectra were analyzed by comparison with Gaussian derivative
curves calculated by the MISTIC computer. These theoretical curves
were computed for the interaction of the spin of the unpaired electron
with probable numbers of equivalent protons, using various combinations
of peak spacing and line width. Interaction with the odd electron spin
was assumed to be limited to alpha- and beta-nuclei, and the structure
of the free radical was assumed to be similar to that of the parent
compound.

The radiation damage observed in the amides and acids occurred only

within one of the alkyl groups of the molecule. An alkyl group bonded

Sara-Jo Kleinheksel Bolte
to the nitrogen of an amide was always attacked in preference to a group
attached to the carbonyl carbon. In most cases a hydrogen was removed,
but in some cases a carbon-carbon bond was broken rather than a carbon-
hydrogen bond. We have observed no other type of radiation damage. The
carboxylic acids and the amides analogous to them form the same radical
when irradiated.

The particular site of hydrogen removal within the alkyl group is
found to be consistent with the following generalization: (l) a methyl
carbon-hydrogen bond is broken only if no other carbon-hydrogen or
carbon-carbon bond is present in the preferred alkyl group, (2) a pro-
ton is removed from a CH2 group next to a methyl group in preference to
a similar group adjacent to the nitrogen, (3) for a tertiary carbon the
carbon-carbon bond to the carbonyl group is broken in preference to the
carbon—carbon bond to a methyl group.

Radical formation can be accurately described by these generaliza-
tions for N-substituted amides, but some anomalies appear when the only
alkyl group present is adjacent to carbonyl. The radical formed from
2-butyramide cannot be identified with certainty, but carbon-carbon bond
breakage and loss of the terminal methyl group is suggested. For iso-
butyramide significant amounts of two radicals appear to be present,
with either a methyl group or a hydrogen atom being removed from the
tertiary carbon adjacent to the carbonyl group. The radical formed by
the loss of hydrogen is the more stable.

Substitution of two different types of alkyl groups on the nitrogen
gives the only other example observed of the formation of significant

amount of more than one radical. Branched alkyl groups give some extra

Sara-Jo Kleinheksel Bolte
spectral lines which could not be identified accurately, but which prob-
ably indicate the presence of small amounts of radicals other than the
predominent species identified. The presence of two substituents of the
same kind on nitrogen has no significant effect on the site of radiation
damage or on the spectrum observed.

In all cases the coupling constant observed for an alkyl radical
on the nitrogen is smaller than that for a similar alkyl radical bonded
to the carbonyl. Thus N-methylamides give a value of 16.8 to 19.6 gauss,
while the generally accepted coupling constant for acetamide is about
23 gauss. N-ethylamides give a coupling constant from 20.1 to 20.6, while
that of propionamide is 2b.? gauss. This difference indicates that some
of the unpaired electron spin is localized on the nitrogen atom, even
though no interaction from this nucleus is observed.

We feel that these results will give significant assistance in the
identification of radicals in future studies of irradiated organic

molecules.

A STUDY OF‘THE FREE RADICALS IN IRRADIATED AMIDES
BY ELECTRON SPIN RESONANCE SPECTROSCOPY

By

Sara-Jo Kleinheksel Bolte

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF‘PHILOSOPHY

Department of Chemistry

1963

ACKNOWLEDGMENTS

The author wishes to express her sincere appreciation for the con-
tinuing interest and counsel of Professor M. T. Rogers under whose direc-
tion this research was conducted.

The work of Dr. P. S. Rao and Mr. K. R. Way in obtaining the experi-
mental spectra and the work of Dr. F. H. Buelow, Mr. A. Stewart, and
Mr. W. G. Bickert of the Agricultural Engineering Department of Michigan
State University in the irradiation of the samples, the cooperation of
Dr. J. R. Faber in the use of his Gaussian derivative computer program
and that of the Electrical Engineering Department of Michigan State Uni-
versity in the use of the MISTIC computer, and the financial assistance
of the Atomic Energy Commission during a part of this study are all

gratefully acknowledged.

ii

TABLE OF CONTENTS
PAGE
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . l
HISTORICAL AND THEORETICAL BACKGROUND . . . . . . . . . . . . 2

Introduction . . . . . . . . . . . . . . . . . . . . . . 2
General Theony . . . . . . . . . . . . . . . . . . . . . 3
Line Widths . . . . . . . . . . . . . . . . . . . . . . 8
Quantum Theory of Pi Electron Radicals . . . . . . . . . 9
Configurational Interaction . . . . . . . . . . . . 9
Hyperconjugation . . . . . . . . . . . . . . . . . . 12
Discussion . . . . . . . . . . . . . . . . . . 23
Characteristics of Hydrocarbon E. S. R. Spectra . . . . . . 26

EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . ho

Introduction . . . . . . . . . . . . . . . . . . . . . . NO
The Spectrometer . . . . . . . . . . . . . . . . . . . NO
The Magnetic Field . . . . . . . . . . . . . . . . . to
Measurement of Magnetic Field . . . . . . . . . . . b5
Microwave System . . . . . . . . . . . . . . . . . . A7
Signal Detection and Display . . . . . . . . . . . . A9
The Sample Cavity . . . . . . . . . . . . . . . . . . . . 53‘
The Commercial Spectrometer . . . . . . . . . . . . . . . 68

EXPERIMENTAL . . . . . . . . . . . . . . . . . . . . . . . . . 70

Operation of the Spectrometers . . . . . . . . . . . . . 70
Sample Preparation . . . . . . . . . . . . . . . . . . . 72
The Matching of Spectra . . . . . . . . . . . . . . . . . 7h
EXperimental Results . . . . . . . . . . . . . . . . . . 76
N-methylamides . . . . . . . . . . . . . . . . . . . 76
N-ethylamides . . . . . . . . . . . . . . . . . . . 80
N-propylamides . . . . . . . . . . . . . . . . . . . 8h
Amides, Carboxylic Acids and Others . . . . . . . . 89

DISCUSS ION O O O O O O O O O O 0 O O O O O O O O O O O O O O O 103
SWY O O O O O O O O O O O O O O O O O O O O O O O O O O O 1 l 1

LIST OF REFERENCES 0 O O O O O O O O O 0 O O O O O O O O O O O 113

iii

TABLE

II.

III.

IV.

LIST OF TABLES

PAGE
Wave functions for group orbitals of H3 . . . . . . . . . 15

Results in the treatment of isotropic interaction through“
hyperconjugation for two values of the parameter f312 . . 21

Experimental results of some E.S.R. spectra from irradi-
ated polycrystalline organic compounds . . . . . . . . . . 30

Parameters of the E.S.R. spectra of irradiated N-substi—
tuted amides O O O O O O O O O .0 O O O O O O O O O O O O O 88

Parameters of the E.S.R. spectra of irradiated amides and
carbOWIi-C aCidS O O O O O O O O O O O O O O O O O O O O O 102

iv

LIST OF FIGURES

Approximate contours of H3 group orbitals . . . . . . . .

Block diagram of an E.S.R. spectrometer .'. . . . . . . .

power supply for the electromagnet . . . .
multivibrator and its power supply . . .

audioamplifier . . . . . . . . . . . . .

proton frequency meter . . . . . . . . . .
low voltage klystron power supply . . . .
battery high voltage klystron power supply
preamplifier . . . . . . . . . . . . . . .

power supply for the preamplifier and the

phase-sensitive detector . . . . . . . . . . . . . .

phase-sensitive detector . . . . . . . . .

twin tee filter in the phase sensitive

Sample cavity and fittings, indicating the standing wave
pattern for oscillation in the T5102 mode . . . . .

Experimental spectrum of N-methylacetamide and the
matching theoretical spectrum . . . . . . . . . . .

Experimental spectrum of N,N-dimethylacetamide and the
matching theoretical spectrum .... . . . . . . . . .

Experimental spectra of N-methylpropionamide and the
matching theoretical spectra . . . . . . . . . . . .

Experimental Spectra of N,N-dimethylpropionamide and the
matching theoretical spectra . . . . . . . . . . . .

Figure
l.
2.
3. Circuit of the
b. Circuit of the
5. Circuit of the
6. Circuit of the
7. Circuit of the
8. Circuit of the
9. Circuit of the
10. Circuit of the
11. Circuit of the
12. Circuit of the
detector .
13.
1h.
15.
16.
17.
18.

Experimental spectra of N,N-dimethylbutyramide and the
matching theoretical spectra . . . . . . . . . . .

Page
15
bl
53
SS
56
S7
58
59
60

61
62

6h

611

77

77

77

78

78

Figure
19. Experimental spectrum of N-methylisobutyramide and the
matching theoretical spectrum . . . . . . . . . . .
20. EXperimental spectrum of N,N-dimethylacrylamide and the
matching theoretical spectrum . . . . . . . . . . .
21. Experimental spectrum of N,N-dimethylchloroacetamide and
the matching theoretical spectrum . . . . . . . . .
22. Experimental spectrum of N-ethylformamide and the match-
ing theoretical spectrum . . . . . . . . . . . . .
23. Experimental spectra of N,N-diethylformamide and the
matching theoretical spectra . . . . . . . . . . .
2h. Experimental Spectrum of N-ethylacetamide and the
matching theoretical spectrum . . . . . . . . . . .
25. Experimental spectrum of N,N-diethylacetamide and the
matching theoretical spectrum . . . . . . . . . . .
26. Experimental spectra of N,N-diethyl-n-butyramide and the
matching theoretical spectra . . . . . . . . . . .
27. Experimental spectrum of N-ethylisobutyramide and the
matching theoretical spectrum . . . . . . . . . . .
28. Experimental spectrum of N,N-diethylacrylamide and the
matching theoretical spectrum . . . . . . . . . . .
29. Experimental spectrum of N,N-diethylchloroacetamide and
the matching theoretical spectrum . . . . . . . . .
30. Experimental spectra of N,N-dijn-propylacetamide and the
matching theoretical spectra . . . . . . . . . . .
31. Experimental spectrum of N-isopropylacetamide and the
matching theoretical spectrum . . . . . . . . . . .
32. Experimental spectra of N,N-diisopropylacetamide and the
matching theoretical spectra . . . . . . . . . . .
33. Experimental spectrum of acetamide and the matching

LIST OF FIGURES (Cont.)

theoretical spectrum . . . . . . . . . . . . . . . .

vi

Page

79

79

79

81

81

82

82

82

83

83

83

85

86

87

9b

LIST OF FIGURES (Cont.)

Figure Page
3h. Experimental spectra of propionamide and the matching

theoretical spectra . . . . . . . . . .. . . . . . 9h
35. Experimental spectrum of n—butyramide and the matching

theoretical spectrum . . . . . . . . . . . . . . . . 95
36. Experimental spectrum of isobutyramide recorded by K. R.

Way and the matching theoretical spectra . . . . . . 96
37. Experimental spectrum of isobutyramide recorded by P. S.’

Rao and the matching theoretical spectra . . . . . . 97
38. Experimental spectrum of aged isobutyramide and the

matching theoretical spectrum . . . . . . . . . . . 98
39. Experimental spectrum of trimethylacetamide and the

matching theoretical spectrum . . . . . . . . . . . 98
NO. Experimental spectrum of isobutyric acid and the matching

theoretical spectrum . . . . . . . . . . . . . . . . 99
bl. Experimental spectrum of pivalic acid and the matching

theoretical spectrum . . . . . . . . .. . . . . . . . 100
AZ. Experimental spectrum of N-butyl-N-methylformamide . . . . lOl

vii

INTRODUCTION

In recent years the effect of radiation on biological materials has
become of great interest. The immediate result of irradiation is the
breaking of chemical bonds and formation of free radicals. Electron
spin resonance may be used to identify these radicals and to obtain in-
formation about their electronic structure. The radiation damage in
abiological materials is too complex to be directly analyzed, and the
study of simpler compounds with related bonding may be taken as a begin-
ning to the understanding of their results.

The irradiation of amides produces radicals which are stable for
long periods of time at low temperatures, and which may serve as suit-
able models for radiation damage to amide linkages in proteins. In the
work reported here a group of amides have been irradiated as polycrys-
talline solids at low temperatures, and the radicals formed are identi-
fied and discussed. In order to determine the factors dictating the
stability of radicals, amides having a variety of substituents on both
carbon and nitrogen were used.

Most previous work on irradiated simple organic compounds has been
limited to those which are solid at room temperature. In the present

study amides which are liquid at room temperature have been included.

HISTORICAL AND THEORETICAL BACKGROUND

Introduction

 

The absorption of energy fromeihigh frequenqy magnetic field by a
paramagnetic substance placed in a steady perpendicular magnetic field
was first observed in Russia by Zavoisky1 in l9h5, using solutions of
manganese salts. Using similar samples the American investigators
Cummerow and Hallidayz, and Bagguley and Griffiths3 in England shortly
afterwards also observed electron spin resonance (E.S.R.), as this phe-
nomenon came to be called. Studies of the paramagnetic salts of the
transition elements and later those of the rare earths were undertaken
by numerous investigators.

The study of naturally occurring organic freeradicals by E.S.R. did
not begin until l9h9 when the first observation was made by Holden,
Kittel, Merritt and Yager.4 During the same year E.S.R. was first used
in the study of radiation damage when Hutchison5 investigated the color
centers formed in LiF and KCl due to irradiation with neutrons. In 1951
Schneider, Day, and Stein6 irradiated samples of polymethylmethacrylate,
and made the first.study of the E.S.R. spectra of free radicals formed
by radiation damage. Surveys"'12 of the experimental work in E.S.R.,
discussions9‘22 of the general theoretical aspects of E.S.R., and more
specific articles on the theoretical considerations for organic free

radicalsZ3'27 may be found in the literature.

3

General Theory

 

The electron has an internal angular momentum of l/2(h/2n). This
is more simply stated by saying that the electron has a spin of 1/2. As
a result there is an inherent magnetic moment rigidly bound to the axis
of spin. The presence of an external field then will align the electron
in accordance with the restriction that for small particles the component
of angular momentum along the field can take only certain values. For
the case of an electron, the spin and the magnetic moment must be either
parallel or antiparallel to the field. The electron then has two energy
states, -l/2 gBH, the ground state, and 1/2 gflH, the excited state.
Transitions between such states, known as Zeeman levels, give rise to
paramagnetic resonance. In order to induce a transition, the energy of
a quantum of high frequency energy must equal the energy difference be-
tween the states.

hU = AB = ng (1)
Here h is Planck's constant, and B is the Bohr magneton, equal to
eh/anc, which converts angular momentum to magnetic moment in electro-
magnetic c.g.s. units. H, the magnetic field strength and L/, the
frequenqy, are obviously dependent upon the experimental conditions
chosen. 9 is known as the spectroscopic splitting factor, and is exper-
imentally determined. The value of g is a characteristic of the partic-
ular absorption line, and measures the rate at which Zeeman levels di-
verge with an increase in the magnetic field.

For an atom or ion free from all other effects, this 9 factor would

be expected to be the Landg Q factor;11

h

 

J J+1 +-S 5+1 - L L+1
9 z 1 + ( > 2J§J+Ig ( ) (2)

where:

S is the spin angular momentum quantum number

L is the orbital angular momentum quantum number

J is the total angular momentum quantum number.

Dirac derived the value of 2 for g for a free spin. However, even
in molecular beam experiments, where the spins may be considered as free
the use of exactly 2 leads to serious errors. This is because the mag-
netic moment of the electron, due to relativistic effects, is not exactly
equal to the Bohr magneton. The true free electron value which should
be used is g = 2.0023. In practice, the local magnetic and electric
fields existing within the substances studied cause the value of g to
vary greatly from.the free atom value.

Unlike nuclei, electrons are indistinguishable from one another and
in a molecule they are not constrained to a particular area. As stated
in the Pauli Principle, the total wave functions of electrons must be
asymmetric with respect to the interchange of two electrons. Hence the
net electron spin of the molecule under study should be used rather
than the individual electron spins. For all spin paired molecules, this
net spin is obviously zero, and no transitions can be observed.

Taking S as the total Spin of a molecule the magnetic quantum number,

M can have values S, (5-1), ..., -S. That is, there are 2S+1 values,

S)
each of which has energy MsgBH in a field H. The selection rule for
transitions between these states is AMs = i 1. In the absence of any

other effects, all allowed transitions are degenerate.

S

The degeneracy of the 2S allowed transitions can be lifted through
the Stark effect by the presence of electrostatic crystalline fields of
the proper symmetry in the samples under study. The various transitions
then occur at different values of the magnetic field, giving rise to
fine structure in the spectrum. Free radicals such as are under consider-
ation here have S = 1/2.' Therefore, in spectra of these substances fine
structure does not occur.

In a free ion, the motion of electrons in their orbits sets up a
magnetic field which interacts with the spin moments. This interaction
is known as spin-orbital coupling. An external magnetic field would
orient both the spin and the orbit, and transitions would occur between
energy levels determined by a combination of these moments. It is this
effect which gives values of g greatly different from 2 in some systems.
In the presence of internal fields, however, the orbits may be greatly
distorted and the resultant orbital moment will have directional proper-
ties. In such cases the orbital moment can be greatly reduced, and it
is not readily reoriented in the presence of an external field. In free
radicals there is a high asymmetry of the internal field due to the
strongly directed covalent bonds which are not easily redirected. This
quenches the spin-orbital coupling and gives, for free radicals, a 9
value very close to that of the free spin case.

The spin moment of the electron is quantized along the resultant
of the internal and external fields and anisotropy can result. For free
radicals, in which spin-orbital coupling is quenched, such anisotropy
is very small.

An atomic nucleus which has a Spin and the associated magnetic

moment, has orientations with respect to the field and thevelectrOn spin,

6

so that the extra magnetic field due to the nuclear magnetic moment aver-
aged over the electron orbit is not zero. This field adds to the external
magnetic field and there is an interaction between the moment of the
nucleus and the electronic magnetic moment. With a nuclear spin of I,
21 + l orientations are possible for the nucleus with respect to the
external field. The applicable selection rule during electronic transi-
tions is Am, = 0, that is, the nucleus does not change orientation. Thus
each electronic level is split into 2I + 1 components, giving allowed
transitions at 21 + 1 different values of the external field. This
hyperfine splitting is the main source of information in the spectra of
free radicals. As long as the applied field is much stronger than the
field produced by the nucleus, the spacings between adjacent hyperfine
components are equal.

The energy of interaction is normally expressed in the form of a
Hamiltonian. Abragam and Prycez1 have worked out a spin Hamiltonian
for the case of one nucleus contributing to the hyperfine structure.
Spin-orbital coupling contributions to the energy are omitted in the
equation presented here since the coupling is quenched in free radicals.
Also omitted are any interactions between electronic spins. All radicals
discussed are formed by radiation damage giving a very low concentration

making interaction negligible.

 

'S' -I 3(s' ~F)(I 3) _ _
W = gegNfiBN [If—gm ' I; ‘ |5N ' girl ge'INCS‘re‘TN” (3)
e- N e- N

where: = g-value of the electron

g-value of the nucleus

9e
9N
[3

Bohr magneton

7

BN = nuclear magneton
Se = electron spin operator
IN = nuclear Spin operator
r6 = vector position of the electron
rN = vector position of the nucleus
(5(Fé43h) = Dirac delta function for the distance between the

electron and the nucleus normalized in 3 dimensions.

This Hamiltonian is composed of two parts. The first part;

 

 

§ :IN 36 -;)(IN'?)
gegmfifiu If, 3 - _. f- 5 1 (3a)
Ire rNT Ire NI

varies with the angle between the nuclear moment and the direction of
the external field, giving rise to anisotropy of the hyperfine interac—
tion. This anisotropic part is very important in single crystals, but
in non-crystalline systems the directional component is Spread out over
the whole solid angle. In liquids, due to the rapid, random molecular
motion it goes to zero28 and causes no loss of resolution.

The second part;

gegNfiflN [ - % €251, (3 (Fe-END ' (3b)

is often called the Fermi contact term, and is non—directional, giving
isotropic splitting.29 The Dirac delta function in this term goes to
zero for all orbitals which have a node at the nucleus. The unpaired
electron Spin must therefore have a finite probability of being at the

nucleus in question to give a value for the isotropic term.

8

Line Widths

 

In general there are four parameters of interest in any EAS.R. spec-
trum: (l) the g value, (2) fine structure splitting values, (3) line
width, and (h) hyperfine structure splitting values.

The first two parameters have little significance in free radicals,
since the 9 value is always very close to 2 due to the quenching of spin
orbital coupling, and fine structure does not occur unless more than one
unpaired spin is present in the molecule. Since the free radicals con—
sidered here are neither biradicals nor in triplet states, such fine
structure is not possible.

The two factors of interest in the radicals considered are then,
line widths and hyperfine splitting values.

If the effect of the interaction of the odd electron Spin with the
fields cf neighboring dipoles cannot be resolved into its separate fine
or hyperfine components, a broadening of the spectral lines results,
known as dipolar or spin-spin broadening. In non—crystalline samples,
such an effect results from the anisotropic part of the interaction of
the spin with magnetic nuclei. In liquid samples the rapid, random
motion of the molecules is sufficient to average this effect out to
zero, but for amorphous samples, the effect is merely spread out over
the total solid angle and produces line broadening. This type of broad-
ening gives a Gaussian shape to the line. In general, interaction between
various unpaired electron spins in the sample can also give rise to
line broadening. Dipolar interaction is a function of the reciprocal
of the cube of the distance between the two dipoles,however, and with
the small concentration of free radicals present in an irradiated sample,

such interactions can be ignored.

9

Any processes by which the excited electrons lose their excess
energy to the molecule or sample as a whole are known as spin-lattice
interactions. If such interactions are strong, the lifetime of the
excited spin state becomes very short and the uncertainty in the energy
of the excited state then must increase in accordance with the uncertain-
ty principle, causing an increase in the line width. The lifetime of
the excited state is inversely proportional to the absolute temperature,
so that cooling can be used to reduce this broadening of lines. Spin-
lattice interactions take place through the mechanism of spin-orbital
coupling. In free radicals the spin-orbital coupling is effectively
quenched and Spin-lattice interactions cause very little line broadening
for these compounds. .

The third factor influencing line width is again a result of the
uncertainty principle, combined with the small size and indistinguish—
ability of electrons. Exchange of electrons can occur between orbitals
of different molecules, even.when little chemical bonding is present
between them. Such exchange interaction, as it is known, tends to smooth
out the random internal fields experienced by the electron, resulting
in a narrowing of the lines, and a change in the shape from.Gaussian to
Lorentzian.3° Whenever the concentration of free radicals is high in
a sample, this is an extremely important effect, and accounts for the
very narrow lines observed for the spectra of naturally occurring free

.radical species.
Quantum Theory of Pi-Electron Radicals

Configurational Interaction

 

The Spectra of organic free radicals are generally far richer in

10
hyperfine structure and hence in information on molecular bonding,than
the very basic theory thus far presented would indicate. Many of the
free radicals which have been investigated are aromatic. The unpaired
electron associated with such a planar ring system must be considered
to be located in the pi—orbital system, so that delocalization will
add to the stability of the radical through 'resonance', and hence
the name pi-electron radicals. This configuration leads to the unpaired
electron density being concentrated in regions out of the plane of the
aromatic ring, and going to zero at any nucleus at the plane of the
ring. Such an electron could have no interaction with any nucleus at-
tached to the ring, and no hyperfine splitting could result.

Experimentally, aromatic free radicals do exhibit hyperfine split-
ting. The theory which has been developed to account for this apparent
discrepancy is based on the fact that an excited state of‘ cf-orbital
character can be admixed.with the ground state of pure n-orbital character.
This effect is known as configurational interaction. Such interaction
gives a finite density of the wave function of the unpaired electron
spin at the nuclei attached to the ring, as well as at those forming
the ring.

Conditions imposed upon the combination of the excited and ground
state for configurational interaction to occur are that the states have
vthe same symmetry with respect to reflection in the plane of the aromatic
ring, and that the resulting state *have the same spin multiplicity as
the ground state. The ground state to be considered for a system of the
unpaired electron, a ring carbon, and a nucleus, say a proton, bonded

to it is ((3%)2n, (omitting filled orbitals), where (15 is the bonding

.

>05

0..

Yv

.fl‘
.Lu'

m
.‘d' ' ..a

.4")

I“?

ll

orbital between the carbon and hydrogen. An antibonding orbital, (3A,
can be formed similarly to (IE but with opposite signs of the overlap-
ping carbon and hydrogen atomic orbitals. .An excited state of the bond
is formed if one bonding electron is promoted from (ft to (IA, thus giv-
ing (cab)1n(<3k)1 as the excited state. Other possible excited states
do not produce allowable combinations.

Mc Connell suggested31 that the hyperfine splitting observed due
to such concentration interaction is a direct measure of the unpaired
electron distributions on the carbon atoms. The hyperfine splitting,

an due to a proton attached to carbon atom N is then given by the simple

formula,
an = Qqn (A)
where: Q is a constant for all aromatic hydrocarbons,
qn is electron spin density at carbon at m‘N, and
an = 28 (5)
n
where: S is total spin.

Detailed treatments of configurational interaction applied to n-
electron radicals have been carried out by Weissman32, Jarrett33,
Bersohn34, and MtConnell with various co-workers31,35'4°. The value of
Q has been found semi-empirically to be -22.5 gauss for aromatic hydro-
carbons. Theoretically it has been calculated to be -28 gauss.33 In
the admixing of the excited state with the ground state an effective
partial unpairing of previously balanced orbits can result. Thus an
extra odd electron spin density is produced in a direction opposite to

that of the odd electron responsible for it. On some atoms then, qn,

12
may be negative37i39. The hyperfine splitting is independent of the
Sign of qn, and the over all splitting due to the interacting hydrogens

is written:

Z lanl = 0 Z lqnl (6)
n n

very.good agreement with experimental results has been obtained by
the use of this theory. It may be noted that although the summation over
the spin densities in (6) can exceed one, it is usually close to one
for aromatic radicals, and thus the total spread of the hyperfine spectra
due to the ring protons is close to 23 gauss, independent of the number
of protons interacting with the spin.

The theory of configurational interaction in hydrocarbons was
specifically developed to account for the E.S.R. Spectra of aromatic
radicals. However, the development of the concept was far more general,
being carried out by Mc Connellsin terms of a CH fragment. Thus the
basic precepts may be extended to include any n-electron radical, that
is to include the interaction of any proton bonded to a carbon on which
the odd electron is considered to be localized in an orbit which has a

node in the plane of the proton. Such a proton is known as an.a proton.

Hypercogjuggtigg

Experimentally it has been observed that in a methyl group bonded
to an aromatic radical, the methyl hydrogens also can interact with the
unpaired electron. Also the Spectra of aliphatic radicals indicate
that protons on the B carbon (that adjacent to the carbon on which the
odd electron is localized) can have an interaction equivalent to that

of d protons. Both of these effects are illustrations of the interaction

4"

Nil

1M4

«)1

 

13
of E protons and since for such positions neither a multi—pi system nor
-an unbonded pz orbital is available on the B carbon, configurational
interaction as discussed above is not possible.

In the case of the aromatic radicals treated by configurational
interaction considerations, the total Spread of hyperfine lines from
the ring protons was found to be very nearly constant regardless of the
number of protons interacting. For aliphatic radicals on the other
hand, the spread is very sensitive to the number of protons present, and
indeed often appears almost directly proportional to it. To account
for the observed large hyperfine splittings there must be some other
mechanism for the odd electron wave function to include these various
atoms. One means is through hyperconjugation.

Hyperconjugation depends on a spatial overlap of wave functions with
the same pseudo-symmetry, and thus is a far more direct and potent effect
than configurational interaction which depends on the interaction of
electrons involving excited levels. Both calculations and experimental
results bear out the truth of this.

Hyperconjugation can be explained in terms of valence bond (V.B.)
or molecular orbital (M.0.) theory. Detailed discussions of hyperfine
splitting due to hyperconjugation of methyl groups on aromatic free
radicals have been presented in both V.B.41 and 14.0.34 language.

Coulson and Crawford42 have presented a detailed analysis of hypercon-
jugation energies in toluene in terms of M.0. theory, and Chesnut43 has
followed this with a M.0. treatment of hyperconjugation leading to hyper-
fine splitting for the ethyl, isopropyl, and t-butyl radicals. The

model discussed and the calculations set forth here are those of Chesnut.

1h

M.0. theory requires that the electrons of the groups involved
acquire a delocalization into orbitals of pi character. Considering as
an example a methyl group, this delocalization is accomplished by treat-
ing H3 as a Single group, with ¢,,,¢2, and AS indicating the 1 S atomic
orbitals of the three hydrogens HI, HII’ and HIII reSpectively. 'Group
orbitals'44are then formed from these atomic orbitals. The approximate
contours of the three group orbitals are Shown in Figure l.

The orbitals shown in Ib and IC are closely similar to a normal
pi-type orbital, that is there are regions of positive and negative
oUwith a nodal plane between Similar to the nodal plane in a 2pn atomic
orbital.

The three orthonormal group orbitals as given by Chesnut are listed
in Table I, along with the methyl carbon atomic wave functions having
similar symmetry.

The bonding in a methyl group may then be pictured as follows:

3% forms a cf bond with One of the spl hybrid orbitals of the carbon,
the other Sp, hybrid being free to bond to some other group; X2 and X3
form n bonds with carbon p orbitals p2 and p3 respectively.

The H3 group can be considered as a pseudo-atom, and if the methyl
group is bonded to a pi system, this pseudo-atom contributes one elec-
tron to the pseudo-pi or hyperconjugated system. The methyl carbon
also contributes one electron to this pseudo-pi system.

The molecular wave function for this model is:

W). = Z Cix 1iii (7)
1
for the kth energy level. Each 17% in (7) refers to one of the normal—

ized pi orbitals on one of the three types of atoms considered, that is,

Ia Ib Ic

7‘1 * 9/2 + P’s 210/1 ‘ (A2 + 9/3) £52 " P’s
Figure 1. Approximate contours of H3 group orbitals. Orbitals are

shown as in RCH3 looking down the R-C bond indicated by X.
(from Coulson,44 p. 312).

Table I. Wave functions for group orbitals of H3.

 

Group Orbital . Methyl Carbon
Wave Function

 

X1=¢1+552+¢3

 

 

 

:0 + 6511101/2 spi
a 2551 " ($52 + 553)

Z <6-mmwfl ”

X3 = h I ‘53 ' P3
(2 - 25),!)1/2

where SHH = < #1] ¢j>

1%5

 

16
the d-carbon, the methyl carbon, and the pseudo-atom. The wave equation

for the system is;
WW). = w). w).
or , Z ciXWSL/i = W)\ Z Cik )Ui (8)
i i

Each Side of this equation is multiplied by the complex conjugates (yyi)
of all.orbita1 wave functions and integrated over all space. Collection
of terms in the Oil then gives the secular equations, (9).

2 Ci). WTWTGdT = w). 2 Ci). [WT 1”de
1J

1

1.1
or Z. CixHij = w). Z. Cik Sij
1.] 1.]
1% Oil [Hij "WI 51,-] = O (9)

U)
n

*
where ij “/TI/fi Hg d7” = < vyilij >
Hij Wffl Wj d7 = <¢li§lH W/j >

The energy of each level, Wk, and the constants, Cik’ can be determined

by solving the secular determinant:

n
O

Det

 

- w, 3”,

Three assumptions are further made for the model: (i) overlap is
neglected for all but neighboring atomic orbitals, (ii) the a-carbon

atom is considered to be SpZ hybridized, (iii) the resonance integrals,

17
Hij’ are taken as proportional to the corresponsing overlap integrals,

S...45
1J
In writing the secular determinant, the terminology of Coulson and

Crawford42 is used. (Care must be observed due to the interchange of

various symbols here among authors.45)
Sij <L/jin)
ii < U71|H|y7,_>. d + 6‘i
ij ‘ < V71
1' Yo o

J

 

:1:
u

H

Ill
_<

 

HIV“) ij ; IOin0

50 = Yo ‘ “so (10)

90: Y0: and So refer to the values of these quantities for unsubstituted
benzene.

values must be assumed for S, v, and also therefore for B. These
all are functions of bond length, but can be related as above to compar-
able quantities for benzene.

The calculations to be made are illustrated using the ethyl radical.
The pi centers are labelled as: .

C1-C2-X3 corresponding to ~CH2CH3

Two configurations of this radical are possible; the pz orbital of
C1 can conjugate with the pseudo-pi system made up of pa and )(2, or it
can conjugate with that composed of p3 and )(3.

In considering the overlap of C2 and X3, it is apparent that;'

18

<p2l7<2> = <p3|.?<3> (11)

Thus either of these two configurations may be used equally well for
the purpose of calculations. Chesnut chooses instead to consider each
of the states as contributing equally to the true molecular state, that
is, in effect he allows the methyl group to "rotate".

With the substitution of the quantities from (10), the secular

determinant becomes:

 

 

c + f, - w m - prSo 0
Yn-w/Dlzso c+ ez—w st-W/D23So = o
o st-wpzsso (1+ €3—w
or
c + 61 - w [JUMBO + So(a-W)] o
[312[po+so(a-w)] a + 62 - w stnao + S°(a-W)] = o
o puma + So(a-W)] o. + 63 - w (12)

 

 

It is assumed that the radical can be represented by a single elec-
tronic configuration, and that the lowest energy levels are filled in
order, first with electron pairs, and then.with the unpaired electron.

In order to obtain the eigenvalues and constants for the isopropyl
and t—butyl radicals, the same secular determinant may be used if the
p12 values are replaced by (m) 1/2 p12 where m is the number of methyl
groups attached to the a-carbon atom, C1.

In considering the Coulomb term for the pseudo-atom H3, Coulson and
Crawford42 took cognizance of the o-p directing properties of a methyl

group on benzene in electrophilic reactions. The migration of n electrons

19

from the methyl group into the ring is indicated by this, and therefore
H3 must be considered to be more electropositive than C. This electro—
positivity is also shared somewhat with the neighboring C atom. This
' correction is to be found in the addition of (Ti to a, the Coulomb
integral. The numerical values used for this correction, given in terms
of the resonance integral for benzene, are :

Q = o 62 = -o.1po 63 = 41.550 (13)
These were chosen so as to reproduce the dipole moment of toluene, as-
suming it to be due to hyperconjugation.

Using Slater wave functions and 2pm overlap integrals, Crawford
and Coulson calculated the value of Sij as a function of bond length.
The Sij appropriate to the known interatomic distances in toluene and
benzene were then used to determine the ratio designated as f3... The

1J
values used by Chesnut in his calculations are:

P1, = 0 P2, = 2.5 Hz =- o.7 and p12 = 0.93 (11.)
These are the rounded off values determined by Crawford and Coulson, with
the exception of f312 = 0.93. This number is taken from Bersohn,34
who obtained the best agreement between his theoretical calculations
and the experimental results to which they were compared by the use of
this value. Chesnut calculated his results using both values for 7312.

Since the overlap of adjacent atoms has been included in these
calculations, the normalization condition for the molecular wave func-

tion becomes:

1 = <Wklwk> = Z CiXCjX<Wilyjj>
1.]
= Z Cixc S

ij jX lJ

" 2. Cix Yix (15)
1

20
where

Y (16)

ii = Z le Sij
J
We are concerned with the energy level of the unpaired electron,
which will be designated by a subscript zero. The odd electron density

at an atom k, qk,is given by 47,59

qj = Cko Yko (17)

This definition requires, through the normalization condition, that

the sum of the absolute values of all qk is one. That is, spin dens— T
ities are required to be positive. Since this is not necessarily true,
as previously discussed, the theory is not a completely satisfactory
one. Calculated values of qk are shown in Table II.

The hyperfine splitting constants for the methyl protons only are
calculated, using the Hamiltonian given in equation (3b). For a large
applied magnetic field, only the interaction between the 2 components
of electron and nuclear (those parallel to the field) need to be con-
sidered. Aside from a constant, the calculations reduce to the evalua-

tion of,

(55 3 < HJ° |(5(;s)l Hg°i>
= 125] Cio Cjo < Wi I 66:8”ng > (18)

where?s is the distance from the 5 th proton to the electron, and
(565) is the Dirac delta function.

Chesnut defines a function, rfiij, as the average of the term in angular

parentheses, where

21

Results in the treatment of isotropic hyperfine interaction?
through hyperconjugation for two values of the parameter ,fifiz.

Table II.

 

i

 

 

 

Coupling . r
Radical Constant Odqulectron Den51ty lame. lamel ‘
lamel C qx 9. 9..

= 0. o

E. 7 _
Ethyl 16.9 0.9160 0.0815 18.h8 207.6
Isopropyl 15.5 0.8u33 0.07h3 18.33 208.0
t—Butyl 1b.2 0.7825 0.0681 18.19 208.2
= Q.

2. 93
Ethyl 27.8 0.8677 0.1307 31.99 212.h
Isopropyl 2b.2 0.7669 0.1137 31.57 212.9
t-Butyl 21.5 0.6987 0.1008 31.17 213.h

 

*D. B. Chesnut, J. Chem. Phys., 29, (1958), LS.

22

rij [< W1 I 6 Gs)l Wj >]Av.

(19)-
'2'[< P2 ICS(-1:s)l7(2 > + < p3 I 6 63)")(3 >]

for example, r‘is

The averaging is required due to the earlier assumption of the 'rotat-
ing' methyl group, and has the effect of making all methyl protons
equivalent, that is all F8 are equal. Calculations carried out by
Chesnut show that the only elements of importance in the r‘matrix are

those of the form. r13. For the ethyl radical then, (18) reduces to,

C55 = 2, Cio C30 [is (2 ' (Sis) (20)

where (Sis is the Kronecker delta.
The calculated elements of the r‘matrix, and the values used in the
computation of these elements are listed below. Hydrogen-like wave

functions were used:

for 'C-Cgfi
‘H
c-c distance = 1.51713 (291T)by drogen = 1.00
C-H distance = 1.093 (281.1.)carbon a 3.25
28.—cm = 109°28' F}, = 0.96580 (21)
A-HCH = 1090281 [:3 = 0.22050
8(0) = 1.14707 [13 .. 0.00811

The energies calculated from (20) are related to the hyperfine
coupling constants for the methyl protons, ame' Rather than determin-
ing the value of the multiplicative constant in (3b), the abSOIute

values of the ame in gauss may be found using;

0.
Lu

I"

‘0-

‘b

.
a
-

Hg.
‘Uu

 

A .
.3.

V‘M

 

( .

..‘9
i

23

lame! = $270) 506.2 gauss (21)

where 506.2 gauss is the observed coupling constant for atomic hydrogen.
Table II lists the results obtained from this treatment of the

three radicals.

Discussion

 

The approximations made in this rather simple M.O. treatment are
admittedly crude, so that the exact numerical values listed in Table II
are of small significance. However much of value is obtained in the
way of general considerations about the coupling constants. The proper
order of magnitude is obtained for the hyperfine splittings, which for
such radicals are in the region of 20 gauss. The value ofl amel varies
almost linearly with £312 in this region, and the choice of the value
used for this parameter is quite arbitrary. Consideration of the se-
lection of [312 based on Crawford and Coulson's calculations of 312 and
50, shows that variation beyond the limits of 0.7 and 1.0 is doubtful.
A value of [312 = 1.0 gives lamel of about 31.5 gauss for the ethyl
radical. Use of a comparison with experimental results as the criterion
for selection of this value, as Bersohn did, is perhaps the best method.
In any case, the results of the simple M.0. treatment do retain their
order of magnitude correctness.

It is of great significance that the values ofl amel do not vary
to any large extent among the three radicals considered, that is, with
the addition of more methyl groups on the a-carbon atom. This corres-
ponds to the observed fact that the total spread of the E.P.R. spectra

of aliphatic radicals is roughly proportional to the total number of

A!“

h.‘

II.

n.“

u...

 

...,

 

 

IA.

.. U”

 

2b

protons interacting. The slight reduction in lame lwith an increasing
number of methyl groups agrees qualitatively with the experimental
results of Tsvetkov, Rowlands and Whiffen.51

In Table II the fifth column is seen to remain nearly constant among
the three radicals, for a given value of f%2. That is to say, lamel
is roughly proportional to the odd electron density on the central car-
bon. As previously mentioned the splitting due to alpha protons has
(also been found to be proportional to qca. Thus the couplings for the
two types of protons, whose interactions result from mechanisms which
apparently are totally different, 'accidently' become similar simple
functions of the same quantity, th (Thus the need for 'artifiCial' rad-
icals such as have been proposed48 in the past is not necessary.)

The coupling constant for a protons on an sp2 hybridized carbon
has been estimated49 as about -22(i5) gauss per unit of odd electron
l a

me, from this

Spin density from.experimental data. The values of
M.0. treatment vary from 18 to 32 gauss. Due to the fairly large line
widths occurring in the isotropic spectra of polycrystalline radicals,
even quite large discrepancies in these values for the two types of
protons might remain undetected. Thus the apparently equivalent inter-
action of chemically non-equivalent protons is completely within reason.
This simple theory is restricted to methyl hydrogens, and these
constitute a special case, because of free rotation. In most cases51
this free rotation can be verified down to a temperature of 770K, and
rotation has even been observed-52 to be nearly free in methylmalonic

acid as low as h.2°K. In one case53, however, the rotation has been

found to be quenched at 770K. In most cases the beta protons of a methyl

25
group are equivalent in interaction with the electron due to free rota-
tion.

The more general cases of R-CHZ, R-CH-R', and R-C—(R')R", have not
been considered. For the general case, Heller and Mc Connell53 have
suggested that an expression for beta protons equivalent to equation (b)
for alpha protons, should be of the form

a5 = R(6) q (23)

Cu
where G is the angle between the CHB bond and the axis of the free rad-

ical p orbital as seen in projection in the plane perpendicular to the

CC bond. The angular dependence of R(0) is usually given the form:

R(e) = B cos2 0 (2h)
Thus any protons in the nodal plane of the p orbital occupied by the
unpaired electron (this includes all alpha protons) cannot interact
with the electron spin through hyperconjugation, and restriction of the
rotation about the CC bond tends to change the coupling constants. In.
this way chemically equivalent protons can conceivably have non-equiva-
lent interactions.

Several values for B have been suggested by various authors.51:52’
54755 Heller and Mc Connell53 originally suggested B = hO gauss, con-
sidering the known interaction in (CH3)ZCOH. It is possible that B
varies with the electronegativity of substituents. Attempts to locate
the physical positions of beta protons through equation (2h) have not
been very successful due to the uncertainty in the value to be used for
B. The equation can be used to qualitatively check values of coupling
constants obtained for two non-equivalent beta-protons bonded to the same

carbon, when the angle between them is known with some certainty.

26
Again in polycrystalline studies, the rather large line widths ob-
served will hide small amounts of non-equivalence between protons. Where
highly restricted bonds are present, the polycrystalline spectra may be
difficult to interpret and the use of single crystal study necessary.

Characteristics of Hydrocarbon Electron Spin
Resonance §pectra

 

The hyperfine interaction of organic radicals often exhibits aniso-
tropy due to the strongly directed bonds present. The anisotropy is ;
readily apparent in the electron spin resonance study of single crystals,_
in which spectra are taken as a function of the crystals orientation in
the field, The hyperfine coupling of each nucleus to the unpaired elec-
tron can then be expressed as a tensor A, which is symmetric in the ab-
sence of large spin-orbit intereaction. The trace of the tensor A is
the isotropic part of the interaction, that which is observed for liquids
and polycrystalline solids.

As shown in equation (3a), the anisotropic hyperfine interaction
is pr0portiona1 to r-s. As a result, alpha- protons have a large aniso—
tropy, but beta-protons show very little anisotropy, which allows a
distinction to be made between them in single crystal studies. The
effect of this in polycrystalline studies is an increase in line width
when alpha-protons are present.53:57 The greatest amount of informa-
tion is obtained from.the interaction of beta—protons, since this depends
upon the physical position of the proton in the radical.51

DeSpite the great amount of information available from single

crystals, the study of polycrystalline and liquid forms is most signifi-

cant because they are so important physically, and, under certain

n-ll

' 1
|\.I
u

av.-

.-.

1D,—

ci.‘

p"

27

conditions, the isotropic spectra obtained from such samples can provide
a considerable amount of information. After irradiation, whether by y-
rays, x-rays, ultraviolet light, neutrons, or electrons, a surprisingly
large number of simple organic compounds do give simple electron spin
resonance spectra. Combined with a knowledge of the structure of the
parent compound, these spectra can usually be interpreted in terms of

a single radical.

The problem of radical identification is considered in the light
of some basic assumptions; the observed spectra are assumed to be re—
lated to simple radicals trapped in the system, and the structure of.
’these radicals to be directly related to the structure of the parent
compound. It is further generally assumed that only nuclei attached
to the carbon on which the unpaired electron Spin is concentrated and-
those on adjuacent carbons show interaction with the electron Spin.

That is, only a— and p-interaction is considered. The validity of
these assumptions lies in the ability to interpret all spectra recorded
within their limitations.

From the considerations in the theoretical discussion on beta-pro-
ton interaction, it is easy to understand that gamma-proton interaction
would.be very small, if it could occur at all. Hirota and Weissman58
did succeed in observing a Spectrum indicating the interaction of eighteen
equivalent gamma-protons in the sodium and lithium ketyls of hexamethyl-
acetone. The coupling constant observed for these protons was 0.12 gauss.
Such a small coupling would certainly not be observed with the wide

lines usually observed for polycrystalline solids.

28

Before discussing results of experiments, it is necessary to con-
sider the nuclear spins of the atoms present in various compounds
studied, and to determine the possible hyperfine patterns which might
result. In hyperfine interaction with an unpaired electron, a nuclear
Spin I gives rise to a pattern of 21 + 1 lines of equal intensity.

Neither 160 nor 12C has a nuclear spin, and so these atoms cannot
cause hyperfine Splitting. 14M has I = l, which would result in a pat-
tern of three lines of equal intensity. Since the compounds discussed
contain one nitrogen at most, additive features need not be discussed.
The 1H nucleus has a spin of 1/2, giving a hyperfine doublet. If more
than one hydrogen has an equivalent interaction with an unpaired elec-
tron, the effect can be determined by adding all possible combinations
of the spins. Fbr example, for two equivalent hydrogen nuclei, four
combinations are possible: (1/2, 1/2), (1/2, -1/2),(-1/2., 1/2), and (-1/2,
—1/2), giving total spins of l, 0, 0, and -1 respectively, and result-
ing in a pattern of three lines of relative intensities of 1:2:1. In
general, for n equivalent hydrogen nuclei, there are n + 1 lines in the
hyperfine pattern, with relative intensities corresponding to the binom-
ial coefficients. Both 35C1 and 37Cl have nuclear spins of 3/2, giv-
ing a pattern of four lines of equal intensity. Patterns for equiva-
lent nuclei can be calculated as above. For two equivalent nuclei,
seven lines result with relative intensities of 1:2:32h:3:2:1. For
the interaction of non-equivalent nuclei, or non-equivalent groups of
equivalent nuclei interacting with the odd electron spin of a free

radical, each line of the pattern having the larger splitting is further

split into the pattern appropriate to the other nucleus or group. Such

29

combinations may or may not overlap the various lines, but in general
do contribute greatly to the complexity of the resulting spectrum.

Experimental results of some E.S.R. spectra from irradiated poly-
crystalline organic compounds which are ascribed to hydrocarbon free
radicals are shown in Table III. (More comprehensive listings may be
found in Ingramm, pp. 166, 186 and 206.)

Some of the earliest work on irradiated aliphatic compounds was
- done by Gordy and his co-workers.43 This group originally assumed that
the simple, symmetrical spectra obtained which indicated equivalent
proton interaction could be explained only in terms of a symmetrical
molecular fragment, so that the odd electron would be in a non-localized,
symmetrical M.0. covering the entire radical. They postulated that when
the molecule is dissociated hy the radiation an electron is removed,
and whatever parts remain react among themselves to form the most stable
assemblage of simpler molecules and radicals. Thus, such simple products
as H20, NH3, C0 and C02 were assumed to be formed. The radical Species
they suggested were in general positive ion radicals. Observed quintets
were ascribed to the ethylene radical, (C2H4)+, and triplets to the
methylene radical (CH3)+, or possibly to this group attached to some
other. The single line they observed for irradiated formamide was at-
tributed to (C0)+, while the 5 line butyramide Spectrum was interpreted
as two triplets, but not explained.

A large variety of organic compounds have been irradiated using
various radiation sources and the resulting E.P.R. spectra recorded.
With this advance in the experimental data available, and the concurrent

advance of the theoretical explanation of the observed spectra, it

Table III.

30

polycrystalline organic compounds.

Experimental results of some E.S.R. spectra from irradiated

 

 

 

. Type of Temp. No. Peak Total
Compound Irradiated Irradi- 0K of Sep'n Spread Ref.
ation lines gauss gauss
Methane Y 20 ha -- 80 59
CH4
Ethgjws Y 77 we 26b 79C 59
Prwégfmrws Y 77 8 25 175 5 9
B'B‘afijgfizwzms Y 77 7 29 176 59
'-But
i (ng§ZCHCH3 Y 77 (8)d -- 162 59
n-Pentane e
CH3(CH2)3,CH3 e 77 7 f 28 " 60
Neopentane Y 77 10 23 -- 59
(CH3)4C +3 211 -- S9
n-Hexane g
B‘“E§§?3;Z)SCH3 e 77 7 (29) -- 60
E-Oéfi:?gHz)GCHs e 77 79 (29) -- 60
anonane Y 77 7g -- 165 59
CH_~,(CH2)-,CHS e 77 7g (29) -- 60
n-Decane 9
‘Q-Undecane g __
CH3(CHZ)9CH3 e 77 7 (29) 60
B-Oéfifgaigfsws Y 77 6 33 165 59
n-Octacosane Y 77 h-> 26 182 59
aluminum. 6 31 157
Polyethylene Y 77 6 31 15b 59
(“CH2-)n Y 7 “ 9O 61
Methanol Y 77 3 18 36 59
(II-1301i e1 77 3 19 -- 60
x 77 3 -- 30 L18
u.v.*. 77 3 17 -- 62
u.v. J 77 3 19 -- 63,61;
Ethanol Y 77 5 22 89 59
CHsa'IZOH e 77 S 22 "" 59
x ,, 77 S ,, -- 93 b8
u.v.* 77 5(10) (22) -- 63,6h
u.v. 77 3+1 -- -- 65

Table 111 (Cont.)

31

 

 

Type of Temp.

 

. No. Peak Total
Compound Irradiated Irradi- 0K of Sep'n Spread Ref.
ation lines gauss gauss
‘n-Propanol Y 77 (50r7) 20 80 59
CH3(CH2)0H e * 77 5 (18-20) -- 60
u.v.* 77 5 23 -- 65
u.v. 77 5 22 -- 63,6h
u.v. 77 6 -- -- 62
i-Propanol Y 77 7m 19.5 ll5 59
(CH3)2CH0H e * 77 7 (18-20) —- 60
u.v.* 77 7 -- 120 65 ‘
u.v. 77 7 20 -- 63,6h
n—Butanol e * 77 7 (18-20) -- 6O
CH3(CH2)30H u.v. 77 7 20 -- 63,6h
u.v. 77 6 -- -- 62
s-Butanol e * 77 39 (18-20) -- 60
CH3CH2CH(CH3)0H u.v. 77 6 21 -- 63,61)
u.v. 77 6 -- -- 62
i—Butanol e * 77 3 (18-20) -- 60
(CH3)2CHCH20H u.v. 77 8 23 -- 63,6h
E-Butanol e * 77 39 (18-20) -— 6O
(CH3)3C0H u.v. 77 3 2h -- 63,6h
‘geAmyl alcohol e * 77 7 (18-20) -— 6O
CH3(CHZ);0H u.v. 77 7 (18-20) -- 60
u.v. 77 6 -- -- 62
i—Amyl alcohol e * 77 (9) (18-20) -- 60
(CH3)2CH(CH2)20H u.v. 77 Z 21 -- 611
+ 21 --
trAmyl alcohol u.v.* 77 5 22 -- 6b
CHSCHZC(CHS) 20H
'g—Hexyl alcohol
CH3(CHZ)50H e 77 7 (18—20) _ 60
Ethylene glycol e * 77 3g -- (10) 6O
(DI-{(012) 20H UoVo 77 3 -- "" 65
Dimethyl ether Y 77 3 -- 35 59
(CH3)20 e 77 3 (18-20) -- 60
Diethyl ether Y 77 5 22 86 59
(CH30H2)20 e 77 5 20 -— 60
anropyl ether
(CHsCHzCH2)20 e 77 7 (18-20) -- 60
i-Propyl ether 9 _ __
'2- u yl ether e 77 7 (18-20) __ 60

(CH3(CHZ)3)ZO

1'1..- .

 

 

«I
\Eflu

an...

g...
Jul,

J‘

 

32
Table III (Cont. )

 

 

Type of Temp. No. Peak Total

 

Compound Irradiated Irradi- 0K of Sep'n Spread Ref.
ation lines gauss gauss
Acetone
l —- 60
(CH3%§§3 k t e 77 5 7
Methyle e one _ -_
CH3 0112000113 e 77 5 (18 20) 60
Diethyl ketone
18 20 -- 60
(CHSCHZMCO e 77 S ( )
Diisopropyl ketone 18-20 __ 60
(CI-13)2CHC0CI~1(CHg)Z e 77 S ( )
Sodium methoxide e 77 3 " " 60
CHSONa x 300 3 -- 30 118
Formamide __ __
HCONHZ x 77 1 118
Acetamide
CHSCONHZ x 300 3 hS b8
Propionamide __
03301200an x 300 5 98 [*8
Butyramide x 300 2x3 __ 50b h8
(3113(CH2)2C0NH2 .
N-Ethylpropionamide __ __
CH3CH2C0NHCH2CH3 e 195 5 66
N-Bthylbutyramide __ -_
CH3 (CH2) 20011ch 2CH3 e 195 5 66
N- -Ethylhexamide
e l —- 110 66
CH3(CH2)4CONHCH2CH3 9S 5
N-(n-Propyl)propionamide e 195 b __ __ 66
m3m2c0m(mz)zw3
N-(n-Propyl)butyramide e l h __ __ 66
(Engcnzgzcomnmipzcns 95
N- n-Hexyl propionamide _- . 66
m3CH200NH(Gi2)5CHs e 195 1‘ 121
N-(t-Amyl)propionamide 195 5 __ -_ 66
EHSCH2coNHc(c1i3) ZCHZCH32
N-(t-Amyl)butyramide
l -- l 5 66
a... 95 5 3
eopen y propionami e __ 66
CH 3CH 20011ch 2c( 0113) 3 e 195 2+5 1M
Methylamine __ _-
CHsNHZ x 77 1 66
Acetanalide
_- 0 8
CHSCONHC5H5 X 300 3 5 b
Tetra- -n-butyl ammonium
iodide e 300 7 " “ 67

+ ..
(CH3CH2CHZCHZ)4N I

33

Table III (Cont.)

 

 

Type of Temp. No. Peak Total

 

Compound Irradiated Irradia- 0K of Sep'n Spread Ref.
tion Lines gauss gauss
Tetra-g-nutyl ammonium_bromide __ __
(CH30HZCHZCHZ)4N Br 300 7 67
Nylon
(C0(CH2)4C0NH(CHZ)6NH-)n Y “ 1* " " 61
Propane 2 2-dicarboxylic acid __
(CH3)ZC(COOH)2 Y 300 7 22 51
2-Methylpropane-l,l-dicarboxylic
acid Y 300 89 23.5 -- 51
(013)2(CH)2(COOH)2 X 300 89 2.3-5 "' 51
2,2-Dimethy1propane-l,3-
dicarboxylic acid Y 300 2 22.5 -- 51
(C00H)CH2C(CH3)2CH2C00H
Cyclopentane-l,l-bisacetic acid
(C00H)CH2C(CHZ)5CHZCOOH x 300 2 21 " 51
S°dtt§§éizssmimmlate x zoo s 32 a
Cyclobutane-l,l-dicarbosylic
acid x 300 S 32 -- 51
(CHZ)3C(C00H)Z
Sodiggzgiéfigggn§zge carboxylate x 300 5 32 127 51
Ammonium-l,l-dimethylethane-l-
carboxylate (ammonium x 300 10 23 -- 51

trimethylacetate)

 

aIntensities of lines are in the ratio of binomial coefficients unless
otherwise indicated.

bSpacing between center lines of triplets.

CSpacing between the center lines of the outer triplets.

dValue given is uncertain. 'llrradiated with x-rays.

eIrradiated with 2 Mev. electrons. JIrradiated with ultraviolet

f light, primary radicals formed
Two radicals formed. from dissolved H202.

gOther lines present but not identified. kObserved occasionally.

hSpectrum changes from the first listed mIntensities of lines are not
to the second with increased time of irra- in the ratio of binomial
diation. coefficients.

311
became apparent that a more probable explanation lay in rupture of a
single bond by irradiation, with the products thus formed being trap-
ped.64 Although a C—H bond is stronger than a C—C bond the former is
more often broken, perhaps due to the greater vibration occurring in a
C—H bond. When a C-C bond is thought to be broken in a hydrocarbon the
removal of a methyl group or a carboxyl group is almost always the re-
sult. Perhaps then the explanation lies in the size of the group removed.
A small radical could readily escape through the lattice of the solid,
while larger fragments, being trapped would tend to recombine. The
simple removal of hydrogen or a methyl group during irradiation has
considerable verification in chemical evidence.69'73

Smaller and Matheson59 and Alger, Anderson, and Webb60 surveyed
the E.P.R. spectra of a large number of irradiated organic compounds of
various classes, in the effort to characterize the spectra from partic-
ular types of radicals, and in an attempt to establish preferred points
for bond rupture.

The spectra of all alkanes studied were shown to be consistent
with the breaking of a C-H bond and removal of the proton. Smaller and
Natheson were able to record the absorption due to the free hydrogen as
well as that for the main radical. In general, the spectra indicated
equivalent interaction of the spin of the odd electron with all alpha
and beta protons. For ethane,however, Smaller and Matheson obtained a
quartet of triplets, showing the slight non-equivalence of the 2 alpha
and 3 beta protons. The small separation occurring here plainly demon-
strates how this pattern might easily be overlapped to give a sextet if

the lines were broadened through other effects. Their study of ethyl

35
chloride and ethylene showed twelve-line spectra identical to that of
ethane.

Normal propane, butane, hexane, and heptane give Spectra consistent
with the removal of a proton from a methylene group. The recurring ap-
pearance of 7 line spectra in the alkanes beyond propane indicates that
the proton is removed specifically from the carbon next to the terminal
methyl group. For higher alkanes other structure is observed in addi-
tion to the 7 line spectrum. The additional set of lines is not fully
resolved, but Smaller and Natheson did identify it as an odd number of
lines, which indicates it is not due to the formation of methyl or ethyl
radicals, nor to the removal of a proton from the center of the chain,
which radicals would give h, 6, and 6 lines, respectively. Presumably
radicals of the form .CHZCHZ- are formed. The 6-line spectrum expected
from radicals formed hy the removal of a proton from the center of a
polyethylene chain was observed for Marlex 50 (linear polyethylene)
which has exceptionally low percentage of side chains. The 7-line
spectrum obtained by Abraham and Whiffen51is undoubtedly due to proton
removal from side chains present in their sample. The complex spectra
obtained for branched alkanes indicate the presence of more than one)
radical. Apparently some breaking of C-C bonds does occur in these com-
pounds.

The production of stable radicals from frozen alcohols irradiated
with ultraviolet light is not readily accomplished. Some experimenters
have used the alcohol as a solvent, dissolving in it H202 which easily
forms radicals, probably HOZ’, which in turn attack the alcohol molecules,
removing a proton and forming a secondary radical which is observed.

Usually the same radicals are produced by this process as by the more

36
common primary process.

In all cases irradiated alcohols give spectra indicating the removal
of a proton from the hydrocarbon part of the molecule and the equivalent
interaction of the remaining alpha and beta protons with the unpaired
spin. No interaction with the hydroxyl proton is observed. As was ob-
served for the alkanes mentioned above, the longer chain alcohols give
seven-line spectra, indicating radicals of the type -CH2—CH—CH3, and
the complexity of the spectrum increases with branched compounds.

Two groups of investigators have carried out extensive studies on
irradiated alcohols, studying both the effect of the length of irradia-
tion and the temperature. Gibson, Symons, and Townsend64 used ultra-
violet light and dissolved H202, and were able to observe the formation
of the primary free radical and then the secondary radical. The results
of Fujimoto and Ingram62 differ from those of others on the alcohols.
The variation in experimental techniques may account for some of this.
However, they were able to match their results theoretically by assum-
ing radicals similar to those indicated by the results of others,59:5°:
53'55 but taking into account also the effect of temperature on bond
rotation, and the possibility of biradical formation.

The radical formed in the irradiation of n-propanol seems particu-
larly to be in doubt. The need for more study on these simple compounds
is clearly indicated.

Ethylene glycol gives a triplet spectrum50355 as would be expected
if the C-C bond were broken but the spacing of this triplet is only

about half that observed for a methyl radical.

 

7 ..Q

3
5,
r

 

s
1
p n

...l. x» w. w a).

y-
I‘-
\
4..
5
l

I, o 9.. at. c: u I

37

The spectra of irradicated symmetric ethers follow much the same
pattern, indicating that one proton is removed from one of the alkyl
groups. lg-Propyl ether again shows a five—line Spectrum instead of the
seven lines which might be predicted. No interaction across the oxygen
is observed.

The Spectrum resulting from acetone irradiated by a small dose of
electrons is,surprisingly, a quintet.5° Under continued irradiation
and also under other conditions68 a triplet is observed, such as would
be expected if one C-H bond is broken. Assuming that the spin can only
interact with atoms on the alpha and beta carbons, a five-line spectrum
cannot readily be explained. Diethyl and methyl ethyl ketones both
Show five lines, indicative of proton removal from the ethyl group.

The spectrum of diisopropyl ketone appears to be a quintet, but there
is sufficient question about its form that the more likely seven-line
spectrum is quite possible.

Burrell66 used electrons to irradiate a series of amides. The
E.S.R. spectra he obtained also indicated radical formation by breakage
of a C-H bond. For N-substituted amides bond rupture preferentially
occurs in the group bonded to nitrogen. Burrell found that a proton
was usually removed from the CH2 group adjacent to the nitrogen. If
these positions are blocked, the proton is removed from the adjacent
carbon. No interaction is observed across the nitrogen, that is, the
other proton on the nitrogen does not interact. Only for N—(neopenhy1)-
propionamide was evidence of the presence of more than one radical found.
One radical is apparently formed by proton removal from the carbon
adjacent to the nitrogen, the other by proton removal from the carbon

adjacent to the carbonyl.

38

Burrell67 also studied EEEETE‘bUtYl ammonium halides. The seven-
line pattern obtainedikom these salts indicates removal of a proton from
the carbon in the beta position to nitrogen. This position for bond
rupture was attributed to the increase in stability of the resulting
radical due greater opportunities for hyperconjugation.

Single crystals of alphatic acids and their salts can readily be
grown and upon irradiation they form radicals stable at room temperature
in air. Therefore acids are usually studied in this form. Tsvetkov,
Rowlands, and Whiffen51 undertook a comprehensive study of these acids,
and several fine individual studies52‘55374"77 have appeared in the
literature. Parallel investigation of some of these acids in both
single crystal and polycrystalline forms have proven the fortunate
circumstance that radiation damage gives the same radicals in both
cases. This the more exact results from single crystal studies can be
used to verify the information from the study of polycrystalline samples.
In cases where the polycrystalline spectrum is a simple, fairly well
resolved pattern, the single crystal spectrum is found to be generally
quite similar and the radical identification from the former is correct.
The results of a few cases of this type are shown in Table III.

From the large number of acids considered in single crystal form
by Tsvetkov, Rowlands, and Whiffen, these authors were able to make a
number of generalizations concerning the site of the electon spin in the
stable radicals: (l) The a-carbon tends to be next to the Carboxylic
acid group and to have as few a-hydrogens as possible attached to it.
(2) In no case was a proton removed from CH3. (3) C-H bonds other

than those in CH3 break in preference to C-C bonds. (h) The C-C bond

39

in C-COOH is more readily broken than that in C-CH3. (5) Tertiary C-H
bonds in (C)3CH break in preference to secondary C-H bonds in (C)2CHZ
unless the CH2 is adjacent to CO0H. (6) In acids with more than two
methylene groups, the radical often observed from other long chain com-
pounds, R-CHZ-CH-CHZ-R', is not observed as the stable radical.

Abraham and Whiffen's61 study of irradiated Nylon showed a four-
line Spectrum. This is consistent with the generalizations above and
indicates that a proton is not removed from the center of a (CH2)n

chain.

EQUIPMENT

Introduction

 

The observation of electron paramagnetic resonance spectra may be
accomplished by the use of a spectrometer such as is shown in the block
diagram in Figure 2.

Such a spectrometer may conveniently be considered as four inter-
dependent systems: (1) a homogeneous magnetic field and appropriate
modulation of this field, (2) a system to measure the strength of the
static magnetic field, (3) a system to generate and propagate micro:
waves, and (h) a system for the detection and display of the change of
microwave power.

Two completely separate spectrometers were used in the experimental
work discussed herein. The spectra taken previous to June, 1962‘, by S.
K. Bolte and P. S. Rao were obtained using the spectrometer constructed
at Michigan State University under the direction of Dr. M. T. Rogers,
which is described in detail on pages h0—68. Spectra taken hy K. R.
way after June 1962, were obtained using a commercial spectrometer

described on pages 68-69.

The Spectrometer

 

The Magnetic Field

 

A magnetic field strength of roughly 3500 gauss is required to re-
move the degeneracy of the magnetic spin states in the irradiated com-

pounds used. In order to obtain this field, an electromagnet which had

b0

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gorgeous [I obwpmmCom uwufisa mason
women .

 

 

 

 

 

 

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ommwaaam hflnoam

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

cwosw oozon
nouns nonmaflfiomo
muHHIII mucosvouM . .m.u
-Haaao mocha Haoo
coyonfimw uuunnuunnunn m m mamaonob

 

 

 

 

W 1’ ilu
opmsm o>m3 IIIIIIII» » .

uoooopov - , oun50m o>mzouome
o>mzouuwa pocmma
>Jw>mo oaaamm nouuooao

 

 

h2
been constructed under the direction of M. T. Rogers, H. E. Thompson,
and J. R. Faber was used.78:79 The electromagnet consists essentially
of Six copper spools with approximately 1200 turns of Formvar insulated
#lh copper magnet wire on each spool. Three Spools are placed around
each 7 1/2 inch pole piece. The three coils on each Side are wired in
series and the leads brought out to the terminal box on the magnet mount.
The total from the two sides gives roughly 23,000 feet of wire, with a
resistance of 50 ohms. In order to increase the homogeneity of the
magnetic field, the original 7 1/2 inch pole caps were replaced by a~
pair of 12 inch pole caps removed from a varian? N.M.R. spectrometer.
Two sheets of 1/2 inch aluminum were suitably machined so as to fit
between the pole pieces and rest on the magnet mount. Circular areas
were machined out of the center of these sheets, so that the existing
pole pieces could extend through the aluminum sheets. To these sheets,
the 12 inch pole caps were bolted. At each of the four corners of the
aluminum sheets, turnbuckles were placed joining the two, thus holding
the structure rigid. These turnbuckles were adjusted until.the most
homogeneous field possible was obtained. At all times the pole pieces
were kept tight against the pole caps, by using the handwheels and
threaded steel rod extending into the pole pieces, as built into the
original design of the magnet. The homogeniety of the field was deter-
mined by recording the E.S.R. line of a solution of sodium dissolved in
liquid NH3. This line is reported by Hutchison80 to be 0.02 gauss wide.
The minimum.width recorded on this spectrometer was 0.2 gauss. The
setting of the turnbuckles used to obtain this minimal width was assumed

to give the best homogeneity and was thus used during succeeding experiments.

 

*Varian Associates, Palo Alto, California.

113

The power supply for the electromagnet is Shown in Figure 3. The
local 220 volt three-phase power supply iS converted into direct cur—
rent, using a three—phase bridge of selenium rectifiers as Shown in
Figure 3a. The rectifiers are cooled by means of a fan.

This bridge is able to deliver 250-300 volts at up to 25 amperes.
The pulsed, rectified current is next fed into a series of inductance-
capacitance filters which smooth out most of the ripple. However, the
remaining ripple is still too great for the desired steady field. The
current is therefore next fed into a current regulator which both mini-
mizes current fluctuation and provides the means by which the magnetic
field is swept.

The magnetic coils are in the cathode circuit of the 6L6 power
tetrodes. The 6L6's are placed in parallel on 5 chassis, holding l8
tubes each. The screen grid voltage for the 6L6's is obtained from a
separate fixed—voltage regulator circuit. (Figure 3c.)

Coarse selection of the current range for the magnet coils is pro-
vided through a choice of contact points on the dropping resistance
between the coils and the negative input. This dropping resistor is a
hand-wound, water-cooled coil of nichrome wire, providing a precision
resistor little affected hy temperature changes.

Sweep of the chosen magnetic field region is provided by a motor-
driven Helip083potentiometer which is in parallel with a portion of the
dropping coil (See Figure 3d.)

The luf and 16 uf capacitors smooth out current fluctuations across
the magnet. To eliminate ahy fluctuations due to another external power

source, batteries were used on the screen grid and cathode of the 6AU6.

 

*Beckman Instruments, Inc., Helipot Division, Fullerton, California.

1111
Any fluctuation in potential causing an increase in current through the
magnet causes a positive excursion of the control grid of the 6AU6, thus
increasing its conductance. This results in a negative excursion of the
grids of the 6L6'S, decreasing their conduction, and also the current
through the magnet.

In order to modulate the magnetic field, a modulation or 'wobbling'
coil was wound around one of the 12 inch pole caps. To decrease vibra-
tion, the pole cap was first covered with a layer of rubber mat, and
then wound with foam rubber. The coil was wound of lacquer-coated flat
copper wire having a cross sectional area equivalent to #18 wire. The
coil was wound in six layers, ten turns in each layer, the layers being
separated by a covering of masking tape. The leads to this coil were
run to the terminal board on the Side of the magnet yoke.

The wobbling coil is energized by a square-wave signal obtained
from the non-sinusoidal oscillator, or astable multivibrator shown in
Figure h. A condition of instability is caused by Slight differences
in the plate currents of the two halves of the first 68N7. The positive
feedback network between these two sections supports this tendency
toward instability, giving a very rapid transition in which the grid of
one section iS driven below cutoff, the other half Simultaneously going
to full conduction. The transition occurs when one plate, say the one
on the left, tends to go positive, causing the right-hand grid also to
become more positive. Thus conduction in the right-hand side increases
causing that plate to become more negative and producing a negative ex-
cursion of the grid on the left. This results in the left-hand plate

becoming even more positive. This tendency rapidly increases until the

115
left Side reaches cutoff, the right Side conducting fully. When this
happens the charges on C and C' in the plate circuits leak off slowly
to ground. During this process the conduction of the right side Slowly
decreases, and conduction on the left side begins again when the grid
potential is sufficiently raised. When the two sections are conducting
equally again, the circuit is again triggered with the left Side going
to full conduction this time. This oscillation between cutoff and full
conduction alternately in the two halves continues. The rate at which
the charges leak off to ground is a function of the time constant of
the resistance-capacitance (R.C.) networks. Therefore the output fre-
quendy of the multivibrator can be adjusted by the insertion of various
values of C and C'. In use the multivibrator was adjusted to oscillate
at 112 cps. The signal from one cathode of the 6SN7 is amplified by a
6SJ7, and the amplified signal fed to two cathode follower outputs.

One signal from the multivibrator is fed into the audio amplifier
shown in Figure 5. A 6SL7 divides this input, providing proper signals
for push-pull operation of the two 6L6 tetrodes (180° out of phase),
the plates of which are connected to an output transformer. These lead
directly to the wobbling coil terminals. The proper amplitude and wave
form of the wobbling coil current are obtained by variable attenuation
of the Signal from the multivibrator as it enters the amplifier. The

maximum current obtainable in the coils iS 2.5 amperes.

Measurement of the Magnetic Field

 

An accurate and conveneint measurement of the magnetic field may
be made by the use of the N.M.R. resonance frequency of the proton,

which is givenelby

h6
Up = 17.257711 x 103 cps (25)

Use is made of the wide—range marginal oscillator described by Buss
and Bogart.82 This circuit was developed by those authors for use at
frequencies below 25 Mc/Sec, and is particularly suitable for the case
where it is necessary to insert several feet of coaxial cable between
the oscillator and the probe, as in this E.S.R. Spectrometer. The cir-
cuit used is shown in Figure 6. Power for the oscillator is obtained
from a Lambda* regulated power supply, Model 28.

A very dilute solution of manganous sulfate in distilled water
is placed in the probe coil of this oscillator. The probe is inserted
between the pole caps with its axis perpendicular to the magnetic field,
and as close as possible to the sample. The variable capacitor is used
to set the desired frequency of oscillation in the resonant circuit,
which includes the proton probe. The crystal in this circuit stabilizes
the frequency. Frequencies set on this oscillator are in the radio
frequency range. When the magnetic field, as modulated by the wobbling
coils, reaches the point of proton resonance, energy from the resonant
circuit in the oscillator is absorbed by the water sample in the probe,
decreasing the oscillator output amplitude. The oscillator output is
therefore periodically decreased at a frequency corresponding to the.
period of the wobbling coil current, i.e. 112 cps. The output of the
oscillator feeds into a low-pass filter, giving an input signal to the
amplifier, (V3), with amplitude variations at 112 cps. The output of
the amplifier is impressed on the vertical plates of an oscilloscope

which is synchronized to 112 cps., giving a positive—going pulse at the

 

*Lambda Electronics Corp., Huntington, Long Island, N.Y.

b7
point of proton resonance absorption. V2 acts as an impedance-matching
device between the oscillator and the frequency check point. The fre-
quency may be rapidly determined to an accuracy of five significant fig-
ures by the use of a type BC-22l-O United States Army Signal Corps

frequendy meter.

Microwave System

 

The microwave power is generated by a varian X-l3 or V260 reflex
klystron, which is capable of generating microwaves in the frequency
range of 8.2 - 12.h kmc. This type of klystron is normally operated
with the tube body (the anode) at ground potential. The cathode then
operates at -h50 volts, and the repeller at about h50 volts negative
with respect to the cathode. The beam current is about 50 m.a.

The cathode voltage is supplied by a voltage-regulated power sup-
ply. The circuit diagram of this supply, Figure 7, Shows it to be eS-
sentially composed of a D.C. supply, a filter network and a voltage
regulator. The input to this power supply is, in turn, from a Sorenson*
model 5008 voltage regulator.

In order to insure a constant voltage difference between the cathode
and the repeller, a battery high-voltage source was used between these
two, the voltage applied being variable by means of a series of switches
(Figure 8). This voltage source includes a 90-volt battery which is
always in the circuit, thus preventing the repeller from having a volt-

age positive with respect to the cathode.

 

J’—
I\

Sorenson and Co., Inc., South Norwalk, Connecticut.

h8

In order to reduce drift in the klystron frequency due to tempera-
ture variation, the tube was immersed in a circulating bath of oil.

Transmission of the microwaves iS accomplished by the use of type
RG-52/U waveguide. Coupling between the various components in the wave
guide train is made with type UG-39/U flanges and type UG-hO/U chokes.

Directly after the klystron in the microwave line, a Uniline* Micro-
wave Gyrator, model 88-968 is inserted. This isolator allows the radia-
tion to pass through in a forward direction, but attenuates any radia-
tion which might be reflected and which, if allowed to pass, would throw
the klystron off its set frequenqy.

A Waveline*% Attenuator, type 611, iS placed next in the microwave
line. This allows variation of the microwave power delivered to the
sample, to obtain a sufficiently large Spectrum for study, and to avoid
saturation.

Suitable lengths of waveguide then transmitthe microwaves to the
cavity within the magnetic field, and on to the detection systems on
the other end.

The microwave power transmitted through the microwave system is
indicated by an MA—h23A crystal diode, mounted in a tunable crystal
mount, Hewlett-Packard*** Model X-h85B.

Between the sample cavity and the detector crystal diode, a direc-

tional coupler is placed. This coupler leads to a Waveline 698 wavemeter,

 

*
Cascade Research Corp., Los Gatos, California.

**
Waveline Inc., Caldwell, New Jersey.

*** Hewlett—Packard Co., Palo Alto, California.

119
and then to a termination. This wavemeter was used to determine the
frequency of the generated microwaves. The wavemeter was calibrated
against another system for the measurement of klystron frequency, which
was known to have an accuracy of i 0.5 mc. This system.was composed of
a Gertsch$FM-hA and AM-lA beating a standard frequency against the kly-
stron frequency, with the frequency difference measured on a Hallicrafter**
5X-62A receiver. The error in the wavemeter was found to be of the
order of i 0.5 mc, with a maximum error of i 1 me. In the 10 kmc region
used, this error is about 1 part in 104. A further measurement was
made using diphenylpicrylhydrazyl (DPPH) as a standard having an absorp-
tion line of known frequency at a given field. The wave meter was

found to give a frequenry reading to an accuracy of 0.055%.

Signal Detection and Display

 

The electrical signal produced by the detector crystal diode as a
result of the microwave power impinging upon it, is immediately ampli-
fied by the use of a small preamplifier with a wide band pass. This
preamplifier is mounted directly on the crystal at the end of the wave
guide in order to reduce noise from a lead. This preamplifier is a two
stage amplifier followed by a cathode follower with a negative feedback
loop to the first stage. This feedback increases stability and decreases
noise. The circuit diagram is shown in Figure 9. Power for the pre-
amplifier is obtained from the power supply for the phase-senSitive de-
tector (Figure 10). The output from the preamplifier is fed into the

phase-sensitive detector.

 

*Gertsch Products Inc., Los Angeles, California.

**Hallicrafters Co., Chicago, Illinois.

50

Due to the use of 'wobbling coils' in this system, the magnetic
field is being modulated during its sweep across the absorption region.
The microwave power received at the crystal is at a constant level, ex-
cept when the magnetic field is such as to cause absorption. In that
region, the power is decreased and is modulated at the same frequendy
as the wobbling coil current, 112 cps. The amplitude of the modulations
as received at the crystal is proportional to the first derivative of
the absorption peak, going to zero at the point of maximum absorption
and reversing in phase on the return (upward) Slope of the line.

The phase—sensitive detector is used to amplify that part of the
received signal which is modulated at 112 cps., Shift the phase of this
signal to match that of a reference signal from the multivibrator, and
give an output the amplitude of which shows the amplitude and phase of
the 112 cps. signal from the preamplifier. The Circuit diagram of the
phase-sensitive detector is given in Figure 11.

The Signal from the preamplifier enters the phase—sensitive detec—
tor through an attenuator switch and onto one grid of V1, which acts
as an impedance-matching device. The Signal is next amplified by V2,
the plate of which is connected to the gain control potentiometer. The
signal then is fed into the grid of V33, used as cathode follower,
coupling the signal into a twin—tee amplifier circuit, which contains
V4, Vsb, and a twin-tee filter. The signal is amplified by V4 and then
appears on the grid of VSb' The Signal is further amplified at the
plate of Vsb and fed on to the next section of the circuit. At the same
time the Signal from the cathode of Vsb is fed back through the twin-tee

filter to the control grid of V4. The twin-tee is so constructed as to

51
present a low impedance to all frequencies except those in a narrow band
centered about the desired frequency, here 112 cps. Thus all signals
are cancelled out at V4, except those in the band about 112 cps. Design
of the twin—tee filter is given in Figure 12.

The output of the frequency selector section of the phase-sensitive
detector is fed into two phase-Shifting circuits, each capable of a
phase Shift of nearly 90°. The phase of the signal coming out of these
circuits is dependent upon the values of resistance and capacitance in
the R-C networks in the output circuits of V5a and V5b° Coarse phase
control is obtained by switching in capacitors of various values, fine
control by varying the resistance.

The output of the phase-shift section of the phase-sensitive de-
tector is coupled through an isolation amplifier, V6, to both cathodes
of V7. At the same time a 112 cps reference signal from die multivibra-
tor is fed through a buffer amplifier (V83) and direct-coupled to the
grid of a phase splitter (Vab). The two signals, equal in amplitude
and frequency, but opposite in phase, which are then obtained from the
plate and the cathode of Vbb are applied to the two grids of V7, so that
each half of V7 conducts alternately for one half cycle.

With no absorption signal, the two plates of V7 will be at the same
potential. Exact balance in the plate circuits of V8 is obtained by a
variable resistance between the two plates. This is the D.C. balance
control. The Signal applied to the cathode of V7 is adjusted to be
exactly in phase with the reference Signal. During absorption the sig—
nal on the cathode will be in phase with the Signal on one grid, and

180° out of phase with the Signal on the other grid. The half-tube with

52
Signals out of phase will increase in conductance, the other half will
decrease in conductance. Thus the two plates of V7 will be at differ-
ent potentials, the amount of difference being proportional to the
amplitude of the Signal, and the Sign of the difference being dependent
upon the phase of the Signal.

The output from the plates of V7 is fed through an R.C. filter net-
work to V9 which acts as a vacuum—tube voltmeter, the output of which
drives both a meter movement and a recording potentiometer. The time
constant of the R.C. network is adjusted by switching various values of
capacitance into the circuit. A large time constant reduces the noise
level of the output Signal. However, this creates the possibility of
distortion or loss of the fine structure in the spectra unless care is
taken to keep the sweep slow enough that the circuit is able to respond.

The Spectrometer has an alternate system of display, known as
crystal video detection. In this case the output of the preamplifier
is fed directly to the Y plates of an oscilliscope. The sweep of the
oscilliscope is synchronized with the modulation field sweep. At the
condition of resonance, the entire absorption Spectrum is traced out on
the oscilliscope screen. For this display system to be used, the ampli-
tude of the field modulation must be large compared to the Spread of the
spectrum, so that it sweeps through all of the absorption pattern twice
per qycle of modulation. This system of detection is of considerably
lower sensitivity than the phase-sensitive detection and pen-recorded
display. A polaroid camera can be used for a permanent record of the

oscilliscope spectrum.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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58

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Figure 13. Sample cavity and fittings, indicating the standing wave pat-
‘tern for oscillation in the T5102 mode.’

65
Sample Cavity

 

The sample cavity is a rectangular section of waveguide in which
the microwave radiation is reflected back and fourth many times before
emerging, and which is placed between the centers of the magnet pole
caps. The cavity is constructed so that it will resonate in the T5102
mode. (see Figure 13). Use of this mode of oscillation allows the
sample to be inserted into a region of maximum magnetic field intensity
and minimum electric field intensity. Since it is the magnetic compo-‘
nent of the radiation.which causes the magnetic dipole transition observ-
able in EJ3.R., such placement is desirable. When the sample is in the
position indicated in the figure, the polarization of the radio frequency
field is perpendicular to the static external magnetic field.

The Q, or figure of quality, of a cavity resonator has the same
significance as for any resonant circuit. For convenience, the Q of

a cavity is often based on the definition:83

 

Q = 2n energy stored
energy lost per cycle

The basic definition of Q is that in the measurement of the response
of the cavity as a function of frequency, when the response has dropped
to 70.7% of the response at resonance, the separation from resonant

frequenqy (measured in cps) is the resonant frequency divided by 20.

[Db 'lJl = éég (26)
or
_ U
Q - m9 (27)

where 110 is the frequency at resonance

66

L/is the frequency at 70.7% response

AL) is the change of frequency between points at
which response is 70.0% of that at ljo

Equation (27) was used to measure 0.

Several cavities are available for this spectrometer. A11 cavities
were made from sections of waveguide out to the proper length, with end
plates soldered to them. These end plates each have a circular iris in
the exact center. .A 5/16 inch hole is drilled in the center of the top
of the cavity for sample insertion, and a matching hole is drilled in
the center of the bottom.

The waveguide originally was silver—plated on the interior, but
much of this was stripped in soldering. Therefore, after construction,
the cavity interior was replated. Areas to be plated were first sanded,
then vapor-degreased using trichloroethylene, and further cleaned using
the copper bright dip. A blue dip was used to give a mercury immersion
plate for a basic coat. To get a white silver plate of satisfactory
coverage and adhesion, a silver strike bath was used with 99.9% silver
anodes, applying a strike at high current density and then a plate at
lower current density in the same bath.

The best cavity available was 1.701 inches in length with irises
of 3/16 i.d. This cavity gave a Q of 3h77 when empty, resonating at
9389.0 mc. With a sample of DPPH in a glass tube inserted in this
cavity the Q was found to be 2228, with resonance at 9359 mc.

The cavity is fitted with a sample tube. This is merely a two-
inch cylinder fashioned from Kel-F, with an internal diameter of l/L
inch, and an external diameter of 5/16 inch. There is a lip on the top

1/8 inch in thickness and 1/2 inch in diameter, which prevents the tube

H.
”1

67
from being pushed down and out of the cavity. This sample tube elimin-
ates the possibility of any sample being lost into the interior of the
cavity.

A small indentation and hole were made into one side of this tube,
just below the cavity. The indentation serves to prevent samples not
supported from above from slipping down through the cavity. A copper-
constantan thermocouple was inserted through the hole to measure cavity
temperature.

FrequentLy it is necessaty to observe spectra at low temperatures.
This is particularly true in the case of free radicals formed through
irradiation, such as are studied here. In many cases these free radicals
are destroyed or changed at room temperature. For spectra taken at low
temperatures, a flow of cold air was used to refrigerate the cavity.
The laboratory compressed air supply was first filtered through glass
wool, and then run through two traps. The first trap was immersed in
ice water, the second in liquid air. Thus all particles, and most of
the moisture was removed from the air. The air stream was then passed
through a coil of copper tubing immersed in a Dewar flask of liquid air
located directLy below the cavity, to cool the air. The stream of cold
air then flows directly into the cavity, the copper tubing being con-
nected to the bottom of the sample tube. Adquate room is left in the
sample tube, so that the air flow can readily pass around any sample,
and out of the top, escaping into the atmosphere.

The temperature of the cavity is adjusted by varying the rate of
flow of air. A small galvanometer which had been shunted to give a

greater sensitivity, was used to read the cavity temperature as indicated

68
by the thermocouple. In order to calibrate the reading of this galvan—
ometer in terms of true cavity temperature, simultaneous readings were
taken on it, and on a highLy accurate galvanometer, connected to a sec-
ond copper-constantan thermocouple which was inserted into the cavity
from the top, to the same depth as a normal sample. The lowest temper-
ature registered using this method of cooling was -162°C. Using this

method temperatures are easily read to within 1°.

The Commercial Spectrometer

 

The commercial spectrometer used after June, 1962, was.made up of
the following units. The magnetic field is produced by a rotating 12-
inch Varian Magnet, powered by a Varian Regulated Magnet Power Supply.
Control of the magnetic field is given by a varian B.P.R. Control Unit
Model VhSOO-lOA with a varian Precision Field4Scanning Unit Model
VL280A. For the power and control of wobbling coils in this spectro-
meter, a varian 100 kc/sec Field Modulation and Control Unit Model
Vh560 has been added.

Measurement of the magnetic field is accomplished in the same man-
ner as described above in the other spectrometer. The same radio-
frequenqy oscillator and probe are used. For the determination of the
radio-frequency a Hewlett-Packard Electronic Counter Model S2hC is
used. This counter has an accuracy of i 20 cps.

. The microwave system of the commercial spectrometer is as follows.
A varian reflex klystron VlS3C is powered by a varian Klystron Power
Supply model Vh500-20. The klystron is combined with a varian.X-band

superhetrodyne adaptor and a varian X-band microwave bridge. In this

69
Spectrometer a magic tee is used in the waveguide. On one leg of this
tee is a varian low temperature reflection cavity. On a second leg is
the same wavemeter as previously described with a bOBB crystal diode~
.detector and a ctystal current meter, used in conjuction with the wave-
meter for the determination of the klystron frequency. 0n the third
leg of the tee is the detector crystal diode, MAb23.

The detected signal from the crystal diode is fed into a Varian
Output Control Unit model Vh270A, and recorded on a Varian G-lO
Recorder.

The temperature of the cavity of this spectrometer is controlled
by a varian B.P.R. Heat Control Unit. In order to prevent the conden-
sation of water in the system, both the waveguide and the cavity itself
are swept by a stream of dny nitrogen. The temperature of the cavity
is read by the use of a copper-constantan thermocouple connected to a

Brown* Potentiometer model 27h5.

 

*
Minneapolis-Honeywell Regulator Co., Brown Instruments Division,

Phi lade lphia , Pennsylvania .

EXPERIMENTAL

Operation of the Spectrometers

 

The kLystron is allowed to warm.up for at least two hours before
aqy spectra are taken. ,This assures complete warm-up of the low voltage
power supply, and the establishment of a constant temperature in the
oil bath surrounding the ktystron tube.

The proper range of magnetic field is selected by the tap used on
the dropping coil in.the magnet current regulator and control circuit.

The resonant frequenqy of the cavity varies with temperature and
load, thus tuning of the klystron is necessary for each sample at each
temperature. With the sample in the cavity, and the temperature set as
desired, the repeller voltage is adjusted to give a maximum reading on
the crystal current meter. The ktystron is mechanicalty tuned using a
knob geared to the mechanical tuner on the ktystron tube, again adust-
ing to maximum ctystal current. Phrther fine tuning is accomplished
using the tunable onystal mount at the end of the microwave system.

The slight retuning necessary during a series of experiments, due to
the change in position of the sample or temperature variation, is done
by variation of the repeller voltage. The crystal current desired is
then set by use of the attenuator.

Along with each spectrum taken, the frequency of the ktystron is
measured using the wavemeter;. When this meter is set on the frequenqy
of the microwaves in the wave guide, a sharp decrease in ctystal current
is observed, since the microwaves are passed into the coupler, through
the wave meter, and are absorbed.

70

71

The wobbling coils are turned on, and the current is set between
l/L and 1 ampere. The line height of the recorded spectrum varies
directly with this current, making a high value desirable. However,
when phase sensitive detection and pen recording are used, a field
modulation sweep greater than one tenth of the absorption line width
results in distortion of the line shape.10 Selection of the current
used must be based on these two considerations.

Starting well beyond the region of absorption of the sample, the
magnetic field is slowly swept by rotating the Helipot potentiometer
in the magnet current control unit, driven by a reversible motor. The
resulting field sweep is nearly, but not quite, linear with time.

The determination of the magnetic field strength is accomplished
as follows. A series of readings on the frequency meter corresponding
to known harmonics of frequencies which give a proton resonance at 50
gauss intervals in the magnetic field region of interest was determined.
The frequenqy meter is set to a reading corresponding to a field value
just within the chosen sweep range of the magnet. The marginal oscil-
lator is set to the corresponding frequency by beating the two signals
together and adjusting to the null point as observed aurally with a
headset. The output of the marginal oscillator is then observed on the
oscilliscope. During the sweep of the field, at the point of proton
resonance, a positive-going pulse is observed on the oscilliscope screen.
At this point a mark is made by grounding the recorder instantaneously
with a key. This makes a line on the recorded spectrum, and the recorder
pen returns immediately to the recording of the spectrum, with very

little resultant distortion to the spectrum.

72

The operation of the commercial spectrometer is similar, but more
automatic. Rough tuning of the klystron is accomplished as above using
the ctystal current meter. Further refinement in the tuning to the cav-
ity frequenqy is made by putting a small to cps signal on the klystron
repeller and observing the resultant signal on the crystal using a
Hewlett Packard Oscilliscope Unit model l20AR. Fine tuning then consists
of centering the cavity resonance in the node of this signal.

The sweep of the magnetic field is set automatically on this spec-
trometer, and was assumed linear with time. Therefore, only two field
markers were placed on these spectra, at the beginning and end of each
Spectrum. The frequency for which the marginal oscillator is set to
give the proton resonance is read directly from a Hewlett Packard Elec-

tronic Counter.

Sample Preparation

 

Most of the samples were reagent grade Eastman Chemicals but sev-
eral were prepared in these laboratories for NMR studies. The purifica-
tion and properties of several have been described?4

Cylindrical containers small enough to slip easily into the cavity
tube were used to hold liquid and powder samples. These containers were
made by heat sealing trithene film to form a tube and closing both ends
by heat sealing. Cotton thread was tied through a hole in the sealed
area at one end for ease of handling the sample. The samples run by
K. R. Way were placed in Kimax capillary tubing, (1.5 - 2.0 mm in dia-
meter, 50 mm. in length) sealed at both ends, with a thread attached to

the tube with Goodyear Pliobond.

73

All samples were irradiated with l MEV electrons from the General
Electric*Resonant Transformer located in the Agricultural Engineering
Department of Michigan State University, operating at maximum current
(1 ma). Irradiation time was ten minutes, the samples being turned
over after half of the irradiation time.

All samples were irradiated at a low temperature using the follow-
ing method. A Thermos bottle was filled to within a few inches of the
top with liquid air on which was placed a float made of cork or of foam
glass, covered with aluminum foil. The volume of the float was such
that the upper surface was just above the liquid air surface. Grooves
were molded in the top of the float which were just at the liquid sur-
face, allowing some liquid to flow into the grooves. Sample tubes were
laid in these grooves during irradiation. The temperature at thi top
of the samples in such a position was found to be -l73°C.

After completion of irradiation, the attached thread was used to
slip each sample off the float into the liquid air, and to anchor the
sample to the Thermos**flask. With the float removed, the Thermos was
kept filled with liquid air for storage of the samples until spectra
were taken. In most cases storage time was only a matter of a few hours.
The cavity was always cooled to operating temperature before insertion
of the sample, and it is not felt that the time involved in.rapidly
transferring the sample from the storage Thermos to the cavity, using
the thread as a handle, allowed any significant warming. The cavity
temperature of the M.S.U.-built spectrometer was about 1380K, that of

the varian spectrometer varied from 110° to 1230K.

 

*General Electric Co., Schnectady, New York.

**American Thermos Products Co., Norwich, Connecticut.

7b

The sample cylinders were of such a diameter that the only adjust-
ment necessary within the cavity was for vertical placement and this
could be done easily with the attached thread.

All materials used as sample containers, which were irradiated and
present during the recording of spectra, were checked and those chosen

which gave no interferring spectra.

The Matching of Spectra

 

The lines observed in E.S.R. spectra approach either a Gaussian or
Lorentzian shape depending upon the broadening mechanism present. Di-
pole-dipole interactions are the chief source of Lorentzian broadening,
and with the low radical concentrations present in irradiated samples,
these may be neglected.57 The lines observed are thus expected to be
Gaussian in shape, and this is usually found to be the case in such
studies.

The theoretical curves to which the experimental spectra are matched
are Gaussian derivative curves obtained from MISTIC, the Michigan State
University computer, using a program devised by Dr. J. Roger Faber. The
input data consist of values for line width, spacing between.adjacent
peaks,and the center point of the spectrum, combined with specifications
for the number of lines, the equivalence of spacing between them, and
the relative intensities for each. To obtain the theoretical curves
used here series of three, five and seven equally spaced lines of inten-
sities corresponding to the binomial coefficients were calculated. var-
iations were made in the data input for line width between points of

inflection and peak spacing. Therefore, the MISTIC curve, or theoretical

75
curve is most easily specified by stating the ratio of the peak separa-
tion to the line width. Due to the overlapping of the lines in the
Spectra, the intensities and the peak separations in the theoretical
curves do not match those of the input data in appearance. In all
cases in which relative intensity is mentioned, it refers to the value
for the original theoretical Spectrum, not the apparent one.

Selection of the MISTIC spectrum to be used as the theoretical
curve to match the experimental curve was done by matching the values
at the points of inflection. The two variables are the multiplier of
the experimental values, and the theoretical curve used. A combination
of these two was sought which gives a minimum value for the sum of the
squares of the differences between experimental and theoretical values.
The one exception to this procedure was in the matching of the spectrum
Shown in Figure 30b, for which a five line spectrum and a seven line
spectrum fit equally well. Here the choice was made on the basis of
matching the peak separations.

To determine the numerical values for peak separation and line
width of those spectra which were measured, the spacing between ad-
jacent points of inflection on the experimental Spectrum was compared
to the Spacing between similar inflection points as they appeared on
the previously selected theoretical curve. A factor was determined
such that the product of the factor and the experimental spacing best
matched the apparent theoretical spacing. The criterion used for selec-
tion was again a minimal value for the sum of the squares of the dif-
ferences. This factor and the proton frequency markers were then used
to determine the values in gauss corresponding to the input data for

line width and peak separation of the theoretical spectrum.

76

Experimental Results

N-metbylamides

Eight amides having metbyl substituents on the nitrogen were irrad-
iated and the spectra were taken. These spectra are shown in Figures
lh-Zl. Table IV lists the parameters determined for the spectra.

All compounds studied of the this type give a three-line spectrum
with line intensity ratios of 1:211. In light of the previous discus-
sion of hyperfine Splitting patterns, the hyperfine spectra observed
must come from two hydrogen nuclei interacting equivalently with the
unpaired electron Spin.

From the diversity of the compounds used, it is apparent that the
radiation damage and the resulting odd electron spin must occur in the
metbyl group substituted on the nitrogen of the amides. The radical pro-

posed to account for these spectra is;

0 R: :
H i I I
R'—C—N-C«H :
I! I
11.“:
where R is either H or CH3
and R! 13 0'13, CHzms, (CHZ)2CH$, CH(G‘I$)2, CH=CH2, 01“

CHzCl
The atoms enclosed by the dotted lines are those interacting with the
unpaired electron.
Line widths and coupling constants listed in Tables IV and V have

an accuraqy of i l gauss or better.

77

 

experimental
o o 0 calculated

 

3

“‘38 “CH3 CHSEDAQSB %

 

 

 

 

 

 

Figure 1h. Experimental spectrum of
N-metbylacetamide and the matching
theoretical spectrum. 0

Figure 15. Experimental Spectrum
of N,N-dimethylacetamide and the
matching theoretical spectrum.

 

‘3 0—0 10 gauss

Figure 16. Experimental Spectra of N-metbylpropionamide and the matching
theoretical spectra. A. As recorded by K. R. way. B. As
recorded by P. S. Rao.

78

 

 

experimental
o G 9 calculated 0
o O 0
CH CH ghéH?’ , " ‘
5 2 H
3 t.
o 0 o
00 E3 0
0 h c
O U u g
o

r—4 10 gauss

Figure 17. EXperimental spectra of N,N-dimethylpropionamide and the match-
ing theoretical spectra. A. As recorded by K. R. Way. B. AS
recorded by P. S. Rao.

  
    
 

(tn/CH3
C H3CH2C H2 \CHS

 

p—4 10 gauss

Figure 18. Experimental Spectra of N,N-dimethylbutyramide and the match-
ing theoretical spectra. A. As recorded by K. R. Way. B. As
recorded by P. S. Rao.

79

 

  
 

experimental

@ 0 0 calculated

 

 

 

 

 

 

@|——1 10 gauss

Figure 19. Experimental spectrum of N-metbyl-
isobutyramide and the matching theoretical

Spectrum.
9

 

 

 

Figure 21. Experimental spectrum
of N,N-dimetbylchloroacetamide and
the matching theoretical spectrum.

 

 

Figure 20. Experimental spectrum of N,N,-
dimethylacrylamide and the matching theor-
etical spectrum.

80

N-etbylamides

 

Six amides having one or two etbyl groups substituted on the
nitrogen were studied. The spectra for these amides are shown in
Figures 22—29, and the parameters determined for them are listed in
Table IV.

This type of compound gives a five-line spectrum, the intensity
ratios of the lines being 1:b:6:h:l. 0f the nuclei present possessing
spin, only a combination of four protons interacting equivalently with
the electron Spin can produce the observed pattern. Again the variety
of amides used leads to the conclusion that the unpaired electron Spin
is located in the group bonded to the nitrogen. The radical proposed

to give this result is:

o R :H-H-"i
l 1:: l :
R'-C= -C-C-H.
I. ' I
:__g__j
where R is either H or CHZCH3
and R' is H, CH3, CHZCH3, (CH2)2CH3, CH(CH3)2, CH=CH2,

or CHZCl

81

 

experimental

9 9 0 calculated

 

h—alO gauss

Figure 22. Experimental spectrum of N-ethylformamide and the matching
theoretical spectrum.

i

 

Figure 23. Experimental spectra of N,N-diethylformamide and the match—
ing theoretical spectra. A. AS recorded by K. R. Way.
B. AS recorded by P. S. Rao.

82

 

experimental

0 0 0 calculated

o. 8.1;“th .

3

 

 

Figure 21;. Experimental Spectrum
of N-ethylacetamide and the ratch-
ing theoretical spectrum.

 

 

9
CH3CH2CH 81:92“
. 2 CHZCH ,
O
o O A
0
L—I 10 gauss
Figure 26.

C pHZCHS
”t H 2CH

0 F J J 7
C -

o '7 0
J

Figure 25. Experimental spectrum
of N,N—diethylacetamide and the
matching theoretical spectrum.

 
    

[CH CH
20

were 1H,

3

Experimental Spectra of N,N-diethyl-p—butyramide and the
matching theoretical spectra. A.

As recorded by K. R. Way.

B. As recorded by P. S. Rao.

83

 

experimental

o o a calculated

   

 

 

.CH CH
.. ,2 3
opt CH
0
H10 gauss
o o
Figure 27. Experimental spectrum Figure 28. Experimental Spectrum '
of N-ethylisobutyramide and the of N ,N—diethylacrylamide and the
natching theoretical spectrum. matching theoretical Spectrum

C HZC l8 (: 2:3
O

3

Figure 29. Experimental spectrum of N ,N-diethylchloroacetamide and the
matching theoretical spectrum.

8h

 

Neprgpylamides

Spectra were also taken of three amides having propyl groups sub-
stituted on the nitrogen. Both normal propyl and isopropyl substitu-
tions were used. These spectra are Shown in Figures 30-32, and the re-
lated parameters are given in Table IV.

These amides all given spectra of seven lines, with intensity ratios
l:6:lS:20:lS:6:l. Such a pattern is possible here only from the equi-
valent interaction of six hydrogen nuclei. Since all of these compounds
are substituted acetamides, the requisite number of hydrogen nuclei is
present only in the propyl groups on the nitrogen, leading to the con-
clusion that this is the location in which radiation damage occurs. The

proposed radicals are;

l H I
: ' : I -------- I
0 RI HCHI O R'H H '
II I: I : II ti! . i :
H3C-C—N1—C' : and Hsc-c-N-p-c-c-H;
: Hc'm : :I'i H H i
: . I ' ------- 4

I H '

where R is H or C3H7.

85

— experimental

0 0 0 calculated

H CHZCH

CH 2 3

 

\
CHZCHZCH3 +

  

 

k—i 10 gauss

A

 

 

Figure 30. Experimental spectra of N, N-di-p-propylacetamide and the
matching theoretical spectra. A. AS recorded by K. R. Way.
B. AS recorded by P. S. Rao.

86

 

experimental

o o o calculated

H 0
8H3
CH N"CH
3\H 3

 

 

 

 

 

 

 

 

 

 

 

Figure 31. Experimental Spectrum of Nyisopropylacetamide and the’ 1
matching theoretical Spectrum.

8?

mmsmm 9.. T1—

 

 

 

 

 

 

 

.omm .m .m an poppooou m<
Sm: .m .g an poppooou ma.

 

o mupooam Hmbcoawuogxm

 

 

 

 

 

poumaaoHMo o o o

Hancoamuomxo IIIIII

odd ooaeflooflsdoodoomoéé mo

+ .950on “mowuouoonp @5338 o5. .4.
.mm 9.53m

 

mmsmm 0H1

.m
39

 

 

 

 

 

 

 

 

 

 

 

88

Table IV. Parameters of the E.S.R. Spectra of irradiated N-Substituted

 

 

amides.
No. Line a Shown
Compound of width peak ratio rec'd in
Irradiated lines (gauss) spacing a/lw by figure
(gauss)

 

N-Methylacetamide --- -—- - 1.72 Rao 11

N,N-Dimethylacetamide --- --- 1.175 Rao 15
N-Metbylpropionamide

11.5 17.1 1.18 Way 16A
N-Methylpropionamide --- _

-- 1.20 Rao 16B

N,N—Dimethylpropionamide

11.2 17.5 1.21 Way 17A
N,N-Dimethylpropionamide --

--- - 1.31 Rao l7B

N,N-Dimetbylbutyramide

,16.8 1.20 Way 18A
N,N-Dimetbylbutyramide --

N-Metbylisobutyramide 11.7 19.6 1.33 way 19

N,N-Dimetbylacrylamide --- . --- "1;52 Rao 20

N,N-Dimethylchloroacetamide --e --- 1.17 Rao 21

N-Etbylformamide --- --- 1.h0 Rao 22
N,N-Dietbylformamide

16.7 20.1 1.225 Way 23A
N,N-Dietbylformamide - -

- --- 1.175 Rao 23B

N-Etbylacetamide --- --- ’ 1.h5 Rao 2h

N,N-DiethylaCetamide ——- -+- 1.15 Rao 25
N,N-Diethyl-n-butyramide

_ 15.2 20.1 1.325 Way 26A
N,N-Diethylfg-butyramide -

- - --- 1.h5 Rao 26B

N-Etbylisobutyramide --- --- 1.27 Rao 27

N,N-Diethylacrylamide 15.5 20.6 1.33 Way 28

N,N-Dietbylchloroacetamide -—- ——- - 1.33 Rao 29

N,N-Di-n-propylacetamide

-— 17-5 18.1 1.05 Way * 301
N,N-Dijg-propylacetamide __- __

- 1.17 Rao 308

a 4N m m m\mn\n\n\MH\nu:w w mu um mu m w

N-Isopropylacetamide —-— —-— 1.55 Rao 31
2.7 1.12 Way 321

N,N-Diisopropylacetamide 2
11.7 22.1 ‘1.50 Rao 32B

N,N-Diisopropylacetamide

xix]

 

89
Amides, Carngylic Acids and Others

A series of amides not substituted on the nitrogen was also run.
The spectra of these amides are shown in Figures 33—11, and the related
parameters are given in Table V.

Acetamide gives a three-line spectrum with line intensities in the
ratio 1:2:1. AS in the above mentioned spectra, this pattern must be
due to two proton nuclear spins interacting equivalently with the un-
paired electron Spin. In this case however, radiation damage must oc-
cur in the alkyl part of the molecule bonded to the carbonyl group. The

reasonable radical is:

Propionamide gives a five line spectrum with line intensity ratios
1:1:6:h:l. This suggests a similar type of radical with four equivalent

hydrogen interactions:

The Spectrum of g-butyramide is not that which might be expected
from the next amide in the series. The Spectral pattern obtained is
complex, having nine or perhaps ten lines. Apparently more than one
Species of radical is present. The dominant lines of the spectrum fall
into a pattern of five lines with intensity ratios l:1:6:1:l. The
theoretical points and parameters for these five lines only are Shown

in Figure 35 and Table V, respectively. The equivalent interaction of

90
four hydrogen nuclei must be used to interpret these results. A pos-

sible radical would be:

If this is the radical present it would necessitate the assumption
of proton removal from a methyl group which is not usually observed if
ahy other choice is possible.

Another possible explanation in this case is the breaking of a C-C
bond and the loss of a methyl group, either by the initial radiation or

as a secondary reaction. The radical would then be:

It was not possible to resolve the remaining smaller lines of this
Spectrum, thus the identity of the other radical species which apparent-
ly is present cannot be determined. It can be said,however, that the
other lines do not appear to be due to the interaction of one or both

of the hydrogen atoms of the carbon next to the carbonyl group inter-
acting as non-equivalent nuclei. Such an interaction would result in

the formation of a doublet, or a 1:2:1 triplet from each of the five
fominant lines of the spectrum. Neither the intensities nor the place-
ment of the 'extra' lines observed suggest such a Splitting. From the
results observed for carboxylic acids,75 radical formation by the removal

of the methyl group seems more likely.

91

Irradiated isobutyramide gives a complex spectrum when irradiated.
The two Spectra shown in Figures 36A and 37A, while not identical, both
give similar results. Using the four outer lines of the Spectra, each
has been fitted with the seven-line theoretical.spectrum.shown in Figures
36A and 37A, and having the related parameters given in Table VI. 'These
spectra have line intensity ratios 1:6:15:20:15:6:l, and the radical
indicated below having six equivalent hydrogen nuclei is suggested as

that responsible for these lines.

Figures 36B and 378 Show the remaining spectrum after subtraction
of the appropriate theoretical seven-line spectra from 36A and 37A
respectively. These are patterns of four lines with intensity ratios
' of l:3:3:l, such as would be predicted for a °CH3 radical. These lines
were matched as well as possible using a combination of four individual
peaks of the appropriate height. Parameters for these peaks are given
in.Tab1e V also, and the curves are indicated on the spectra.

The isobutyramide sample which gave the spectrum in 36A was ac-
cidently warmed. It was then kept in liquid air for three days, and
rerun at -183°C. The resulting spectrum is shown in Figure 38. This
is an almost perfect, resolved seven-line spectrum.

It would appear from these results that the radical with Six equi—
valent hydrogens is the more stable radical. The other species, pre-
sumably 'CH3, is perhaps formed immediately upon irradiation, but

readily recombines in some manner so as to leave only the more stable

92

Species. The breaking of a C-C bond leaves the further question of what
has become of the extra unpaired spin remaining on the larger fragment
after separation of the methyl. Such a fragment would be expected to
give a further five-line spectrum but this is not observed.

Trimethylacetamide also gives a complex spectrum. 0f the mahy lines
present, eight of the main lines have been identified as the central
lines of a ten-line Spectrum having intensity ratios 1:9:36:81:126:81:
36:9:1. The spectrum is shown in Figure 39, and the parameters measured
are in Table V. The first and tenth lines do not appear, but as their
intensities are of the same order of magnitude as the noise level present,
this is not surprising.

The observed lines could only be produced by a radical having nine
equivalent hydrogen nuclei. To obtain such a radical, radiation damage
must break the C-C bond to the carbonyl group in the molecule. The

suggested radical is:

r““?""1
:HsC‘?‘CI'I3:
L___-§8§__J

The remaining lines of the spectrum could not be resolved, and
therefore it is not possible to Conjuecture what other type of radical
is present. Several sources of further radicals are possible; the
initial breaking of a C-H bond or of a different C-C bond, the break-
down of the C4H9 radical mentioned above, and the interaction of the
electron remaining on the amide fragment with the H or N nuclei therein.
The relative intensities of the two sets of lines does appear to vary
with both the conditions of irradiation and the temperature at which

the spectrum is recorded.

93

Isobutyric acid gives a complex Spectrum, which contains a domin-
ant series of seven lines of intensity ratios 1:6:15z20:15:6:1. The
spectrum is shown in Figure 10 and the related parameters in Table V.
In this case the radiation damage must break a C-H bond and form a
radical similar to the more stable radical formed from isobutyramide,
which has six equivalent hydrogen nuclei.

Irradiated pivalic acid (trimethylacetic acid) gives a complex
spectrum similar to that of trimethylacetamide. Again the eight domin-
ant 1ines are best matched by a ten-line theoretical spectrum.with in-
tensity ratios 1:9:36:81:126:l26:81:36:9:1, as shown in Figure hl.

This pattern is presumably due to the same radical as suggested for
trimethylacetamide. The remaining lines of the Spectrum could not be
resolved.

Only one compound having two different substituents on the nitro-
gen, N-butyl-N-methylformamide, was studied. The resulting Spectrum is
Shown in Figure 12. It is quite apparent from the form of this spectrum
that more than one radical is present. A theoretical spectrum was not

determined to match this experimental result.

9h

 

 

 

experimental

o o 0 calculated

Figure 33. Experimental spectrum of acetamide and the matching

theoretical spectrum.

 

 

 

 

 

o
o
o
0
11
o
I—I 10 gauss

CH

3

CHZgNHZ

0

 

 

 

 

Figure 31. Experimental spectra of propionamide and the matching
theoretical Spectra. A. As recorded by K. R. Way.

B. As recorded by P. S. Rao.

95

 

experimental
o o 9 calculated

CHSC HZCHngHz

 

 

 

 

 

I—-4 10 gauss

Figure 35. Experimental spectrum of p—butyramide and the matching
theoretical spectrum.

Figure 36.

 

96

 

experimental

0 <9 <9 calculated

 

 

 

 

o

 

 

 

TUB

o 0 r——4 10 gauss

 

 

 

Experimental Spectrum of isobutyramide recorded by K. R. Way
and the matching theoretical Spectra. A. Total experimental
spectrum.as recorded by K. R. Way, and the theoretical seven-
line spectrum. B. Difference spectrum obtained from 36A

and the theoretical four-line Spectrum.

97

 

experimental

o o ocalculated

 

Figure 37. Experimental spectrum of isobutyramide recorded by P. S. Rao
and the theoretical matching spectra. A. Total experimental
Spectrum as recorded by P. S. Rao, and the theoretical
seven-line Spectrum. B. Difference spectrum obtained from
37A, and the theoretical four—line spectrum.

 

experimental
o o ocalculated

CH

\3
0 CFi/aCHgNHZ

 

 

 

+—-I 10 gauss,

Figure 38. Experimental spectrum of aged isobutyramide and the matching
theoretical spectrum.

 

 

 

Figure 39. EXperimental spectrum of-trimethylacetamide and the match-
ing theoretical spectrum.

99

popmaaoawo o o o

HmpcoamnoQXo

 

 

 

 

Q

G

 

 

 

 

 

 

 

 

 

 

.8530on Hmofipouoonp 9:
Income o5 pom Bum 3831503 .Ho
58:16on Hmpcoewponxm .0: 8de

 

 

00

l

.8536on Hmombouoofi
@55me on» pew Bow 3.35m Ho 6
83.3.0on Hmpcoafiuonxm 23 shaman

 

00

 

 

 

 

 

 

 

 

  

 

US$338 o o o

Hmpcoamuog o o

 

101

‘LH
H ”t 3
HZCI-IZC.HZC.I~I:5

 

 

 

 

Figure 12. Experimental spectrum of N-butyl-N-methylfornamide.

102

Table V. Parameters of the E.S.R. spectra of irradiated amides and
Carboxylic acids.

 

 

 

No. Line a Shown

Compound of width peak ratio rec'd in
Irradiated lines (gauss) spacing a/1w by Figure

(gauss)

Acetamide 3 —-- ~-- 1.36 Rao 33
Propionamide 5 17.25 21.7 1.13 ‘Way 31A
Propionamide 5 —-- --- 1.57 Rao 31B

lg-Butyramide 5 15.8 22.6 1.125 Way 35
Isobutyramide 7 15.3 23.6 1.51 Way 36A
‘ 1 9.6 21.6 2.25 Way 368
Isobutyramide 7 --~ --- 2.00 Rao 37A
1 --- --- 2.00 Rao 37B

Isobutyramide (aged)* 7 6. S 19.6 3 .00 Way 38

Trimethylacetamide 10 --- --- 5.51 Rao 39

Isobutyric acid 7 --- --- 1.90 Rao 10

Pivalic acid 10 --- --- 5.25 Rao 11

 

*Error for this spectrum only is estimated at i 2 gauss.

DISCUSSION

The radical produced by the irradiation of an N-alkylamide is found
in all cases to be the result of the removal of a hydrogen from the
substituent on the nitrogen atom. A carbon-hydrogen bond of CH2 or CH
is broken in preference to one of CH3. The alpha-carbon also tends to
have as many beta-hydrogens as possible.

The identification of the radical produced from irradiated N-methyl-
amides may be based on three factors. (i) Since all compounds of this
type studied give similar Spectra, it is reasonable to assume that all
radicals formed are similar. (ii) Except for the substituted acetamides,
damage in the hydrocarbon group bonded to the carbonyl group would have
to break a carbon-carbon or a carbon-chlorine bond to produce a radical
which would give a three-line spectrum. By comparison to the previous
results on irradiated organic compounds, we find this to be less likely
than the breaking of a carbon-hydrogen bond in most cases. (iii) Luck
and Gordy48 have observed a coupling constant of 23 gauss for irradiated
acetamide, which gives a radical similar to the other possible ones for
N-methylamides. This value is considerably larger than those found here,
which vary from 16.8 to 19.6 gauss. Thus radical formation for N-methyl-
amides by removal of a hydrogen from the carbon atom bonded to the nitro-
gen seems definite.

N-Ethylamides produce a radical giving a five-line E.S.R. spectrum.
Only in the case of the substituted butyramides in this series could
such a spectrum arise from damage at any site other than in the N-ethyl
group. This would again necessitate the less likely removal of a methyl

103

101
group, and would leave a radical Similar, but not identical to that be-
lieved to be produced from irradiated propionamide. The coupling constant
observed, 20.1, for N,N-diethylbutyramide, again is considerably smaller
than that for the radical from propionamide which is 21.7 gauss. The
validity of this argument is reduced by the fact that for the two radi-
cals compared the unpaired electron is assumed to be on different carbon
atoms. From the similarity of the appearance of the spectra and the
constancy of the observed coupling constant in radicals obtained from
N-ethylamides, (20.1 to 20.6 gauss), it seems likely that the same radq.
ical is produced in each case.

We have assumed that the proton is removed from the CH2 rather than
the CH3 in the ethyl group. This assumption is based on the observed
difficulty of proton removal from methyl groups in general, but we can-
not say with certainty that this is the site of radiation damage. To
verify the selection, studies of the spectra of irradiated Single ctys-
talS or studies of deuterated compounds are required.

Amides having as nitrogen substituents either normal propyl or iso-
prOpyl groups all give seven-line E.S.R. spectra after irradiation.

All of these radicals must result from the removal.of a proton from the
central carbon atom of the propyl group if interaction with the odd
electron is assumed to be limited to alpha- and beta-protons. As men-
tioned above, gamma-proton interaction has been observed, but is much
too small to be observable with the large line widths found here. Once
again the coupling constants observed, 18.1 for an n—propyl substituent
and about 22.5 gauss for an isopropyl substituent are smaller than that

for the seven-line spectra for radical sites on the carbonyl side,

105
(22.6 and 23.6 gauss), but the difference is not as great in this case.

Burrell66 recorded a four-line spectrum for Nip-propylamides and
also for Nag-hexylamides, which indicates that a proton is removed from
the CH2 group adjacent to nitrogen. He also irradiated a sample of N-p—
propylpropionamide which had deuterium substituted for both protons on
the central carbon of the nitrogen substituent. The radical thus formed
gave an EtS.R. spectrum of two broad lines consistent with his results.

Our seven-line spectrum indicates removal of a proton from the CH2
group next to the terminal CH3 group. Burrell57 did find a seven-line
spectrum for a radical in irradiated.tgtga-apebutyl ammonium.salts,
and Smaller and Matheson59 observed seven lines in the spectra of irrad-
iated long, straight-chain alkanes, as did Alger, Anderson and Webb50
fer g-propyl ether. Burrell has suggested that in these cases the pro-
ton was removed from the CH2 next to the methyl group because of the
additional 'resonance' stability which is to be gained through the added
hyperco‘n‘pgation possible in this form. This reasoning does indeed seem
to give a valid argument for the formation of the 'seven-line' radical,
but the explanation of the differing results obtained for N-p-propyl-
amides is not clear.

Another possibility for the seven lines observed in our study for
N12: propylamides is molecular rearrangement after initial radical form-
ation to form a secondary radical identical to that formed by the irradi-
ation of N-isopropylamides. We feel,however, that the comparatively large
difference in observed coupling constants (18.1 gauss for Nag-propylacet-‘
amide and 22.1-22.7 gauss for N,N-diisopropylacetamide) makes it unlikely

that these two form the same radical. Single cbystal studies would help

106
to elucidate the structure of the radical formed, since one could dif-
ferentiate between alpha- and beta-proton interactions in this way.

The presence of a second substituent on the nitrogen apparently
has no effect on the radical formed, or on the observed coupling cons-
tant as long as both substituents are the same. As seen for N-methyl-N-
butylformamide, mixed substituents give mixed radicals. A study of amides
with more widely varied mixed substituents might produce interesting
data on preferred damage sites.

In no case is any interaction observed either with the nitrogen
or with any nucleus 'across' the nitrogen bridge from the radical site.
This seems to be somewhat surprising, especially for N-methylamides,
where one might expect to observe interaction of the hydrogen bonded to
the nitrogen with the electron spin.

Most of the amides without nitrogen substituents form radicals by
the removal of a hydrogen atom from the alkyl group; in one case a car-
bon-carbon bond is broken in the alkyl radical. Once again a primary
carbon is a less likely point for attack than a secondary or tertiary
carbon.

Acetamide gives the three-line Spectrum previously observed by Luck
and Gordy,48 due to the radical formed by the removal of a proton from
the CH3 group. The five-line spectrum from irradiated propionamide also
agrees with that found by Luck and Gordy. The coupling constant, 21.5
gauss, recorded by them agrees well with our value of 21.7 gauss.

The Spectrum observed for irradiated g-butyramide, a quintet, is
most puzzling. Considering the results of Tsvetkov, Rowlands, and

Whiffen51 for irradiated acids, we tend to believe that a CH3 group is

107
removed from this compound by irradiation, since they found that in no
case was a proton removed from a primaby carbon. Luck and Gordy also
studied prbutyramide, but were unable to explain the quintet they re-
corded'which they identified as a pair of overlapping triplets.

It is interesting to observe that among irradiated alconols studied
by EQS.R.,‘2-pr0panol is the one for which the radical is most in doubt.
The same alkyl group is available for attack in g-butyramide and p—prop-
anol. This same alkyl group is also the one for which we observed a
seven—line Spectrum when it was on the nitrogen, but Burrell found only
four lines. It would appear that this particular grouping may have
several points of attack which are veby nearly equivalent. Perhaps
small amounts of differing impurities or veby small differences in ex-
perimental conditions among various studies could be the determining
factor in such a case.

The isobutyramide spectra Show clearly the preferential removal of
a proton from a tertiary rather than a primary carbon. Even the carbon-
carbon bond of the methyl group is more readily broken than any carbon-
hydrogen bond on it. Since the radical responsible for the seven-line
Spectrum has no alpha-protons interacting, it is somewhat surprising
that the observed line widths are not narrower in the combined spectrum.
It appears that a methyl radical can move through the lattice readily,
and thus its rapid decay, probably by combination, at a Slightly higher
temperature is not surprising. The change in the coupling constant
observed in the aged sample is not readily explained. It may be the

result of the large experimental error in this particular spectrum.

108

The ten-line spectrum observed for irradiated trimethylacetamide
again shows the difficulty of removing a proton from a methyl group,
with the preferential breaking of the carbon-carbon bond to the carbonyl
group. The remaining amide fragment would surely seem unstable, and
some recombination or rearrangement is likely to occur soon after its
formation.

The two acids studied give spectra and apparently also radicals
Similar to the comparable amides. This is usually found to be the case
for analogous amides and acids. The ammonium salt of pivalic acid has
been studied in polycrystalline and single crystal form by Tsvetkov,
Rowlands and Whiffen. Their results are similar to those presented
here, and they have determined a coupling constant of 23 gauss, which
is typical of methyl-proton interaction with the electron spin. They
also observed Some further complexity in single crystal spectra which
they could not explain.

AS was observed in the review of results from the study of other
irradiated hydrocarbon compounds, branching increases the complexity
of the Spectra recorded. We have found the same to be true for amides.
This undoubtedly results from a greater number of possible points of
attack, thereby producing small amounts of radicals other than the main
one. Only in one case, that of isobutyramide, were we able to distin--
inSh two separate radicals.

The general trend observed of a smaller coupling constant for rad—
icals of alkyl groups on the nitrogen than for the same groups on car-
bonyl seems to indicate a greater delocalization of the unpaired elec-

tron by the nitrogen than by the carbonyl group. Presumably some of the

109
spin density is moved from the alpha-carbon toward the nitrogen. The
fact that the effect is less noticeable as the size of the group in-
creases is consistent with this idea, and we would not expect the dif-
ference to be noticeable in a chain of four or more carbons, assuming
the removal of a proton from the group adjacent to the terminal methyl
group. If the difference in the coupling constants observed here is
significant, it will be most valuable in further identification of rad-
ical sites.

All of the proposed sites of spin concentration are consistent with
the generalizations set forth by Tsvetkov, Rowlands and Whiffen51 for
acids, except that they always found a proton to be removed from the CH2
next to the carbon double bonded to oxygen. ‘p-Butyramide once again does
not conform to this. Since similar results are always found for anal-
ogous amides and acids, such correlation of results is excellent verifica-
tion of our results. .

In all of the suggested radicals, the molecule is Shown as remain-
ing intact except for the specific bond broken to form the free radical.
No interaction has been observed for any part of the molecule other than
the alkyl group attacked. Therefore we cannot state with certainty that
fUrther disintegration of the molecule has not occurred. There is, how-’
ever, chemical evidence59'73 to assert that the breaking of a Single
bond is the usual result in radiation damage. We have no reason to be-
lieve that more than one bond in any given molecule should be broken,
although the energy involved in l Mev. electron irradiation is certainly
sufficient to break any chemical bonds. If other damage does occur, it

'would seem.unlikely that the large fragments which would have to remain

110
in most cases would be able to move through the lattice readily, and
therefore recombination would probably be immediate.
The value of g was not specifically measured, but was close to the

free-electron value of 2.0023 in all cases.

SUMMARY

A series of amides and Carboxylic acids have been irradiated with
l Mev. electrons at liquid air temperature, and their E.S.R. spectra
recorded. The spectra observed are used to identify the radicals formed
and to suggest a Scheme for the prediction of the site of radiation
damage in such compounds. Hyperfine coupling constants for the protons
of these radicals have been measured.

In most cases a hydrogen is removed from one of the alkyl groups
by the radiation. An alkyl group on the nitrogen is always attacked
in preference to a group attached to the carbonyl carbon. In some
cases a carbon—carbon bond is broken rather than a carbon-hydrogen bond.
We have observed no other type of radiation damage.

The particular site of hydrogen removal within the alkyl group is
found to be consistent with the following generalization: (l) a methyl
carbon hydrogen bond is broken only if no other carbon-hydrogen or
carbon-carbon bond is present in the preferred alkyl group, (2) a pro-
ton is removed from a CH2 group next to a methyl group in preference
to a similar group adjacent to the nitrogen, (3) for a tertiary carbon
the carbon-carbon bond to the carbonyl group is broken in preference to
the carbon-carbon bond to a methyl group.

Radical formation can be accurately described by these generaliza-
tions for N-substituted amides, but some anomalies appear when the
only alkyl group present is adjacent to carbonyl. The radical formed
from g-butyramide cannot be identified with certainty, but carbon-
carbon bond breakage and loss of the terminal methyl group is suggested.

lll

112

For isobutyramide significant amounts of two radicals appear to be pre—
sent, with either a methyl group of a hydrogen atom being removed from
the tertiary carbon adjacent to the carbonyl group. The radical formed
by the loss of hydrogen is the more stable. Irradiated isobutyric and
trimethylacetic acids give the same radical as the analogous amide.

Substitution of two different types of alkyl groups on the nitrogen
gives the only other example observed of the formation of sibnificant
amount of more than one radical. Branched alkyl groups give some extra
Spectral lines which could not be identified accurately, but which
probably indicate the presence of small amounts of radicals other than
the predominent species identified. The presence of two substituents
of the same kind on nitrogen has no Significant effect on the site of
radiation damage or on the spectrum observed. Interaction with the un—
paired electron spin is limited to alpha- and beta—protons in the alkyl
group attacked.

In allczases the coupling constant observed for an alkyl radical
on nitrogen is smaller than that for a similar alkyl radical bonded
to the carbonyl. Thus N-methylamides give a value of 16.8 to 19.6 gauss,
while the generally accepted coupling constant for acetamide is about
23 gauss. N-ethylamides give a coupling constant from 20.1 to 20.6,
while that of propionamide is 21.7 gauss. This difference indicates
that some of the unpaired electron spin is localized on the nitrogen
atom, even though no interaction from this nucleus is observed.

We feel that these results will give Significant assistance in the
identification of radicals in future studies of irradiated organic

molecules.

10.

11.

12.

13.
11.

LIST OF REFERENCES

Zavoisky
(1916)

Cummerow, R. L., and D. Halliday, Phys. Rev;, 79, 133 (1916).

K., J. Phys. USSR, 9, 211, (1915); c. A. 19, 1072,

Baggiley, D. M. S. and J. H. E. Griffiths, Nature, _1_6_g, 532 (1917).

Holden, A. N., C. Kittel, F. R. Merritt, and W. A. Yager, Phys.
Rev., 15, 1611 (1919); II. 117 (1950).

Hutchison, C. A., Jr., Phys. Rev;, 75, 1769 (1919).
B

Schneider, ., M. J. Day, and G. Stein, Nature, 168, 611 (1951).

E
Symons, M. C. R., Ann. Repts. Chem. Soc., 51, 68 (1960).

Hutchison, c. A., Jr., Ann. Rev. Phys. Chem., 7, 359 (1956).
McConne11,H. M., Ann. Rev. Phys. Chem., 6,105 (1957).

Wertz, J. E., Ann. Rev. Phys. Chem., 9, 93 (1958).

Fraenkel, G. K. and B. Segal, Ann. Rev. Phys. Chem., 10,135 (1959).
Weissman, S. 1., Ann. Rev. Phys. Chem., 12,151 (1961)-

Shulman, R. G., Ann. Rev. Phys. Chem., 13, 325 (1962).

Ingram, D. J. E., "Spectroscopy at Radio and Microwave Frequencies,"

Butterworths Sci. Pub., London, 1955.

Ingram, D. J. E., "Free Radicals as Studied by Electron Spin Resoa.
nance," Academic Press Inc., New York, 1958.

Wertz, J. E., Chem. Rev;, 55, 829 (1955).

Griffiths, J. H. E., Disc. Faraday Soc., 19, 106 (1955).

Bleaney, B., Disc. Faraday Soc., 19, 112 (I955).

van Wieringen, J. 8., Disc. Faraday Soc., 19, 118 (1955).

Owen, J., Disc. Faraday Soc., 19, 127 (1955).

Kozgrev, B. M., Disc. Faraday Soc., 19, 135 (1955).

Ingram, D. J. E., and J. E. Bennett, Disc. Faraday Soc., 19,110 (1955).

Pake, G. E., S. I. Weissman, and J. Townsend, Disc. Faraday Soc.,
19,117 (1955)-

Schneider, E. E., Disc. Faraday Soc., 19,158 (1955). -

Livingstone, R., H. Zeldes, and E. H. Taylor, Disc. Faraday Soc.,

Ramsey, N. F., "Nuclear Moments," Wiley, New York, 1953.

Gordy, W., W. V. Smith, and R. F. Tambarulo, "Microwave Spectroscopy,"
John Wiley and Sons, New York, 1953.

113

15.
16.

17.

18.

19.
20.

21.

22.

23 .
21.
25.

26.
27.
28.
29.
30.
31.
32.
33.
31.
35.
36.
37.
38.

111
IBowers,K.B., and J. Owen, Rep. Prog. Phys., 18, 301 (1955).
Bleaney, B., Phil. Mag., 13, 111 (1951).

Bleaney, B. and B. Bleaney, "Electricty and Magnetism," Clarendon
Press, Oxford, 1957, Chap. 20.

Owen, J., J. Inorg. and Nuc. Chem., 8, 130 (1958).
Carrington, A., Endeavour, 21, 51 (1962).

Weissman, S. I., T. R. Tuttle, and E. de Boer, J. Phys. Chem.,

Abragam,.A. and M. H. L. Pryce, Proc. Roy. Soc., A206, 161 (1951).

Pake, G. E., Free Radicals in Biol. Systems., Proc. Symp. Stanford,
Calif., 1960, 91.

Whiffen, D. H., Pure Appl. Chem., 1, 185 (1962).
Whiffen, D. H., Quart. Rev., lg, 250 (1958).

Whiffen, D. H., Free Radicals in Biol. Systems. Proc. Symp. Stan-
ford, Calif. 1960, 227

Carrington, A., Quart. Rev., 11, 67 (1963).

Bersohn, R., Ann. Rev. Phys. Chem., 12, 151 (1961).

Weissmn, S. I., and D. Banfill, J. Am. Chem. Soc., 7_5, 2531 (1953).
Fermi, E., 2. Phys., 63, 320 (1930).

Townes, C. H. and J. Turkevich, Phys. Rev., 11, 118 (1950).
McConnell, H. M., J. Chem. Phys., £1, 761 (1956).

Weissman, S. I., J. Chem. Phys., 25, 890 (1956).

Jarrett, H. S., J. Chem. Phys., 25, 1289 (1956).

Bersohn, R., J. Chem. Phys., 21, 1066 (1956).

McConnell, H. M., J. Chem. Phys., 21, 632 (1956).

McConnell, H. M., Proc. Nat'l. Acad. Sci. UQS.,I15, 721 (1957).
McConnell, H. M. and D. B. Chesnut, J. Chem. Phys., fig, 107 (1958).

McConnell, H. M. and H. H. Dearman, J. Chem. Phys., gfi, 51 (1958).

15.
16.

17.

18.

19.
20.

21.
22.
23.
21.
25.

26.
27.
28.
29.
30.
31.
32.
33.
31.
35.
36.
37.
38.

111
TBowers,K.B., and J. Owen, Rep. Prog. Phys., 18, 301 (1955).
Bleaney, 8., Phil. Mag., 82, 111 (1951).

Bleaney, B. and B. Bleaney, "Electricty and Magnetism," Clarendon
Press, Oxford, 1957, Chap. 20.

Owen, J., J. Inorg. and Nuc. Chem., 8, 130 (1958).
Carrington, A., Endeavour, _2_l, 51 (1962).

Weissman, S. I., T. R. Tattle, and E. de Boer, J. Phys. Chem.,

Abragam, A. and M. H. L. Pryce, Proc. Roy. Soc., A206, 161 (1951).

Pake, G. E., Free Radicals in Biol. Systems., Proc. Symp. Stanford,
Calif., 1960, 91.

Whiffen, D. H., Pure Appl. Chem., 8, 185 (1962).
Whiffen, D. H., Quart. Rev., lg, 250 (1958).

Whiffen, D. H., Free Radicals in Biol. Systems. Proc. Symp. Stan-
ford, Calif. 1960, 227

Carrington, A., Quart. Revy, ll, 67 (1963).

Bersohn, R., Ann. Rev. Phys. Chem., 12, 151 (1961).

Weissmn, S. I., and D. Banfill, J. Am. Chem. Soc., 75, 2531 (1953).
Fermi, E., Z. Phys., 89, 320 (1930).

Townes, C. H. and J. Turkevich, Phys. Rev., 71, 118 (1950).
McConnell, H. M., J. Chem. Phys., 28, 761 (1956).

Weissman, S. I., J. Chem. Phys., 25, 890 (1956).

Jarrett, H. S., J. Chem. Phys., 25, 1289 (1956).

Bersohn, R., J. Chem. Phys., 28, 1066 (1956).

McConnell, H. M., J. Chem. Phys., 2_1, 632 (1956).

McConnell, H. M., Proc. Nat'l. Acad. Sci. U.S., 1_3_, 721 (1957).
McConnell, H. M. and D. B. Chesnut, J'. Chem. Phys., g, 107 (1958).

McConnell, H. M. and H. H. Dearman, J. Chem. Phys., 28, 51 (1958).

39.
10.
11.
12.
13.
11.
15.

16.
17.

18.
19.
50.
51.

52.
53.
S1.
55.

56.
S7.

58.
S9.

61.

115
McConnell, H. M. and D. B. Chesnut, J. Chem. Phys., gz, 981 (1957).
McConnell, H. M. and J. Strathdee, M01. Phys., 5, 129 (1959).
MacLaChlan, A. D., M01. Phys., 5, 233 (1958).
Coulson, C. A., and V. A. Crawford, J. Chem. Soc., 8255, 2052.
Chesnut, D. B., J. Chem. Phys., 2_9_, 13 (1958).
Coulson, C. A., "valence," Oxford University Press, New York, 1952.

Mulliken, R. 5., J. Chim. phys., 88, 197 (1919); C. A., 88, 2296e.
Mulliken, R. 5., Phys. Rev., 11. 736 (1918).

Wheland, G. W., J. Am. Chem. Soc., 8;, 2025 (1911).

Chirgwin, B. H. and C. A. Coulson, Proc. qu. Soc. (London), A201,
196 (1950).

Luck, C. F., and W. Gordy, J. Am. Chem. Soc., 18, 3210 (1956).
deBoer, E. and S. I. Weissman, J. Am. Chem, Soc., 89, 1519 (1958).
Mulliken, R. 8., J. Chem. Phys., 55, 1833 (1955).

Tsvetkov, Y. D., J. R. Rowlands, and D. H. Whiffen, M01. Phys., to
be published.

Heller, C., J. Chem. Phys., 58, 175 (1961).
Heller, C. and H. M. McConnell, J. Chem. Phys., 58, 1535 (1960).
Pooley, D. and D. H. Whiffen, M01. Phys., 8, 81 (1961).

Horsfield, A., J. R. Morton, and D. H. Whiffen, M01. Phys., 8, 125
(1961).

Stone, E. w. and A. H. Maki, J. Chem. Phys., 57, 1326 (1962).

Cochran, E. L., F. J. Adrian, and V. A. Bowers, J. Chem. Phys.,
58, 1161 (1961).

Hirota, N. and S. I. Weissman, J. Am. Chem. Soc., 85, 1121 (1960).
Smaller, B. and M. S. Matheson, J. Chem. Phys., 88, 1169 (1958).

Alger, R. S., T. H. Anderson, and L. A. Webb, J. Chem. Phys., 59,
69S (1959).

Abraham, R. J. and D. H. Whiffen, Trans. Faraday Soc., 58, 1291 (1958).

116
62. Fujimoto, M., D. J. E. Ingram, Trans. Faraday Soc., 51, 1301 (1958).
63. Symons, M. C. R. and M. G. Townsend, J. Chem. Soc., 1959, 263.

61. Gibson, J. F-, M. C. R. Symons, and M. G. Townsend, J. Chem. Soc.,
1959, 269.

65. Gibson, J. F., D. J. E. Ingram, M. C. R. Symons, and M. 0. Townsend,
Trans. Faraday Soc., 55, 911 (1957).,

66. Burrell, E. J., Jr., J. Am. Chem. Soc., §_3_, 571 (1961).
67. Burrell, E. J., Jr., J. Chem. Phys., 55, 955 (1960).

68. Gordy W., W. B. Ard, and H. Shields, Proc. Nat'l. Acad. Sci. U.S.,
11, 996 (1955).

69. Meisels, G. G., W. H. Hamil, and R. R. Williams, Jr., J. Chem.
Phys., 2_5, 790 (1956).

70. Dewhurst, H. A., J. Chem. Phys., 21, 1251 (1956).
71. McDonell, W. R. and S. Gordon, J. Chem. Phys., 35, 208 (1955).

72. Skraba, W. J., J. G. Burr, Jr., and D. N. Hess, J. Chem. Phys.,
21, 1296 (1953).~

73. Collinson, E., and A. J. Swallow, Chem. Rev., 55, 171 (1956).

71. McConnell, H. M., C. Heller, T. Cole, and R. W. Fessenden, J. Am.
Chem. Soc., §g, 766 (1960).

75. Rowlands, J. R., and D. H. Whiffen, M01. Phys., 1, 319 (1961).

76. Horsfield, A., J. R. Morton, and D. H. Whiffen, M01. Phys., 1, 169
(1961). "

77. Morton, J. R. and A. Horsfield, M01. Phys., 1, 219 (1961).

78. Faber, R. J., "Paramagnetic Resonance Absorption Spectra of some
Adsorbed Transition Metal Ions," (unpublished Ph.D. disserta-
tion, Dept. of Chemistry, Michigan State University, 1957).

79. Rogers, M. T;, H. B. Thompson, R. E. Vander vennen, and M. J. Boehm,
Final Technical Report, Contract no. 02300, NR358-232, 1952-51.

80. Hutchison, C. A., Jr., and R. C. Pastor, J. Chem. Phys., 55, 1959
' (1953).

81. Bagguley, D. M. S., and J. H. E. Griffiths, Proc. Roy. Soc. (London),
A201, 366 (1950) .

117
82. Buss, L. and L. Bogart, Rev. Sci. Instr., 51, 201 (1960).

83. Terman, F. E., "Electronics and Radio Engineering," McGraw—Hill,
New York, 1955, p. 162.

81. Woodbrey, J. C. and M. T. Rogers, J. Am. Chem. Soc., 81, 13 (1962).

 

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