KINETICS or PHOTODIMERIZATION'

OF CERTAIN CYCLIC .ENONES

Thesis for theDegree of Ph. D.
MICHIGAN STATE UNIVERSITY
DAVID JOSEPH BUCHECK
1969

 

 

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University J
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This is to certify that the

thesis entitled

KINETICS OF PHOTODIMERIZA'I'ION
OF CERTAIN CYCLIC ENONES

presented by
David Joseph Bucheck

has been accepted towards fulfillment
of the requirements or

up A‘ D degree in_€__/.__..I.m. Lif-

   

  

/ofessor

Date .4 5/ (5)60

0-169

   

  
 

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ABSTRACT

KINETICS OF PHOTODIMERIZATION
OF CERTAIN CYCLIC ENONES

BY

David Joseph Bucheck

The kinetics of ultraviolet light induced dimerization
of four alicyclic 4,7-unsaturated ketones were studied in
this work. The enones thymine, uracil, cyclopent-Z—enone,
and cyclohex—Z—enone undergo a cycloaddition reaction to
form known cyclobutane adducts in each case. The dimeriza—
tion of the pyrimidines, thymine and uracil, is known to be
the primary cause of ultraviolet radiation damage to DNA
and RNA in cells (1;. In dilute acetonitrile solution the
reaction of the pyrimidines is solely from the excited
triplet state. The kinetics were studied by Stern—Volmer
quenching analysis and triplet counting. From thxe data
quantum yields and rate constants of the primary processes
were derived.

The simple enones, cyclopent—Z—enone and cyclohex—Z-
enone, react in a similar manner, but from the second ex—
cited triplet (T2). Their kinetics were studied by Stern—
Volmer quenching analysis and determination of quantum
yields of dimerization and intersystem crossing. Again the

rate constants were derived.

 

David Joseph Bucheck

The rates of addition, ka, of the excited enone to the
ground-state enone Hie in all four cases about 108M“1 sec.
The unimolecular decay rate, kd, of the triplet was low for
the pyrimidines at 105 sec, but three orders of magnitude
higher for the simple enones. The implications of the values
themselves are discussed fully.

Use of the derived rate constants and quantum yields
and the rate law for the generally postulated mechanism (2)
indicates that there is a further source of inefficiency
that is not accounted for in the mechanism. The ineffici—
ency is caused by reversible formation of an intermediate
photoadduct that can react further to form stable dimer or
decay back to two ground—state molecules. The intermediate
may be a complex or a triplet excimer that goes on to dimer
itself or collapses to a 1,4—biradical. This adduct may be
formed with nearly identical rates from either v, v* or n,
v* configuration of the excited enone.

The results indicate that only 2% of the original meta-
stable thymine dimers formed eventually yield stable ground
state dimers. The corresponding values are 6% for uracil,

36% for cyclopent~2~enone, and 74% for cycloheX—Z—enone.

REFERENCES

1. R. B. Setlow, Photochem. Photobiol., l, 643 (1968).

 

2. P. deMayo, J-P. Pete, and M. Tchir, Can. J. Chem., 46,
2535 (1968). ”V

 

 

 

 

 

 

 

 

KINETICS tF PHLTODIMERIZATION
(.F‘ CER'I'A I: CYCLIC ENONES

D r'; x .1 :stfi. iIutfl.tu:k

[a 'I‘IIES IS

:5 iI-El': ' I '_'\.l t .,

M: ‘!.i 21:. S'utt- Uhi'm‘xsity
'Idl tulfjllmunt qf the requirements

I .z *:.e_- degree -,I'
DUC'I‘UR OF PHILOSOPHY
Dupuximvni of Chemistry

19 (59

 

 

 

This thesis is dedicated to my parents who

helped and encouraged me to continue my studies,

 

and to my wife, Ann, who helped me to conclude

them.

ii

 

 

 

 

 

 

 

ACKNOWLEDGMENTS

It is hard for me to express fully my deep appreciation
to Professor Peter J. Wagner. He rescued me in troubled
times, encouraged me to the completion of this work and most
of all taught me some of his knowledge.

My thanks also to my fellow students whose friendship
made these years happier.

Special thanks to the Dow Chemical Company for a Summer
Fellowship in 1965 and to the National Institutes of Health
for a Predoctoral Fellowship from September, 1966 to January,

1969.

 

 

 

 

 

II.

III.

TABLE OF CONTENTS

INTRODUCTION . . . . . . . . . . . . .

RESULTS AND DISCUSSION . . . . . . . . .

A.

B.

C.

Kinetics of Thymine and Uracil .

Kinetics of Cyclopent—Z—enone and
Cyclohex-Z—enone . . . . . . . . .

Mechanistic Interpretations . . .

EXPERIMENTAL . . . .

A.

C.

I—AI—A

General . . . . . . . . . . . . . .
1. Ultraviolet . . . . . . . .
2. Vapor Phase Chromatography . . .
3. Irradiation Procedure . . . . .

Compound Preparation and Sflvent Purification

1. Acetonitrile . . . . . . . . . .
2. Thymine . . . . . . . . . . . .
3. Uracil . . . . . . . . . . . .
4. Cyclopent— —2— —enone . . . . . . .
5. Cyclopent— —2— —enone Dimers . . . .
6. Cyclohex-Z—enone . . . . . . . .
7. Cyclohex—2—enone Dimers . . . .
8. Isophthalonitrile . . . . . . . .
9. Ethyl Stearate . . . . . . . .
O. Piperylene . . . . . . . . . . .
1. 1, 3— —Cyclohexadiene . . . . . . . .
Kinetic Measurements . . . . . .
1. Thymine . . . . . . . .
a. Stern— —Volmer Quenching Studies
b. Determination of ®dim . . .

c. Completely Quenched Reaction

iv

 

Page
1

16

17

28
35
45
46
46
47
47
47

48
48

49
49

50
50
50
50
50

53
55

 

 

 

 

TABLE (JP CONTENTS (C innued)

Page

2. 'zucil . . . . . . . . . . . . . . . . 55
a. Stezn—T lmer Quenchjhg Studies . . 55
b. Duternznazi L f :1. . . . . . . 56

ulm
3. C'fvl 5.)L'.'.°.-2-UI. LL- . . . . . . . . . . 56
u. Szuxn—T irer QUeLching Studies . . 56
L. DULUTHALuil.L .f . . . . . . 58

'15c
C. Recip: Cal uuan’um Yield . . . . .

6‘;
O

4. CT:1 hex—Z—cn Lu . . . . . . . . . . 62
u. SterL—T lmer Quenching Studies . . 62
L. DULUTHILuLT L ‘ . . . . . 62

 

'isc
Ruflibl :ul guuLtum Yields . . . . 63

J. Sc s:t12ed F 1ma!i.n if Dimers . . 63
IV. LITERATURE CITED . . . . . . . . . . . . . . . 65

V. APPENDIX . . . . . . . . . . . . . . . . . . . 69

 

 

 

 

 

TABLE

II.

III.

IV.

VI.

VII.

VIII.

IX.

XI.

XII.

XIII.

LIST OF TABLES

Quenching of thymine and uracil photo—
dimerizations by 1,3—pentadiene in
acetonitrile . . . . . . . . . . . . . . .

Kinetic data for photodimerization of
thymine and uracil . . . . . . . . . . . .

Quantum yields for various concentrations
of cyclopent—Z—enune and cyclohex—Z—enone.

Quenching of cyclopent—Z—enone and cycle-
hex—2-enune photodimerizations by dienes
in acetonitrile . . . . . . . . . . . . .

Kinetic data fo r photodimerization of
cycl pent— —2— —enone and cyclohex— —2— —enone . .

Kinetic data for dimerization of enones
in acetonitrile .. . . . . . . . . . . . .

Preparation of samples for Stern—Volmer
quenching study of 7.32 x 10 4M thymine. .

Results of Stern—Volmer quenching study
of 7.32 x 10 4M thymine . . . . . . . . .

Results of completely quenched thymine
irradiation . . . .

Preparation of samples for Stern—Volmer
quenching study of 1.52M cyclopent—Z—enone

Concentrations of samples in Stern—Volmer
quenching study of 1.52M cyclopent—2—enone

Preparation of samples for determination of
t. of cyclopent-2—enone . . . . . . . .
isc

Composition of samples for determination of
Lise of cyclopent-Z—enone . . . . . . . .

vi

Page

24

27

 

30

33

33

40

51

52

55

57

57

59

59

 

 

 

 

 

LIST OF TABLES

TABLE

XIV.

XVIII.

XIX.

XXII.

XXIII.

XXIV.

XXVI.

XXVII.

Results for determination of @i

(Continued)

cyclopent— —2— —enone

 

 

of
sc

Preparation of samples for dependence of
quantum yield on concentration of cyclo—
pent-2-enone . . . . . . .

Results of dependence of quantum yield on
concentration of cyclopent-Z—enone . . . .

Preparation of samples for phenanthrene
sensitized photodimerization of cyclohex—Z—

enone

Dependence of quantum yield of sensitized
photodimerization of cyclohex-2—enone on
concentration of phenanthrene . . . . .

Results for determination of @.

cyclohex— —2— —enone

Results
of 2.99

Results
of 4.67

Results
of 7.30

Results
of 1.00

Results
of 1.05

Results
of 2.12

Results
of 2.22

Results
of 2.89

of Stern—Volmer
x 10 M thymine

of Stern—Volmer
x 10 4M thymine

of Stern-Volmer
x 10 4M thymine

of Stern—Volmer
x 10 3M thymine

of Stern—Volmer
x 10 3M thymine

of Stern—Volmer
x 10 4M uracil

of Stern-Volmer
x 10 4M uracil

of Stern—Volmer
x 10 4M uracil

for
isc

quenching study
quenching study
quenching study
quenching study
quenching study

quenching study
quenching study
quenching study

Page

59

61

62

64

64

71

71

73

73

73

74

74

74

75

 

LIST OF TABLES (Continued)

TABLE

XXVIII.

XXIX.

XXX.

XXXI.

XXXII.

XXXIII.

XXXIV.

XXXV.

XXXVI.

XXXVII.

XXXVIII.

XXXIX.

Results of Stern -Volmer quenching study
of 3.44 x 10 4M uracil . . . . .

Results of Stern— —Volmer quenching study
of 4.37 x 10 4M uracil . . . . . . . .

Results of Stern-Volmer quenching study
of 0.500M cyclopent—2—enone . . . . . .

Results of Stern-Volmer quenching study
of 0.750M cyclopent-Z-enone . . . . . . .

Results of Stern—Volmer quenching study
of 1.00M cyclopent—2—enone . . . . . .

Results of Stern— —Volmer quenching study
of 1. 52M cyclopent- -2— —enone . . . . .

Results of piperylene Stern—Volmer quench—
ing study of 0.25M cyclohex—Z—enone . .

Results of piperylene Stern—Volmer quench—
ing study of 0.50M cyclohex-Z-enone . . .

Results of piperylene Stern—Volmer quench—
ing study of 0.48M cyclohex—2—enone .

Results of piperylene Stern—Volmer quench—
ing study of 1.00M cyclohex—2—enone . .

Results of 1,3—cyclohexadiene Stern—Volmer
quenching study of 0.30M cyclohex—Z—enone.

Results of 1,3—cyclohexadiene Stern-Volmer
quenching study of 0.50M cyclohex—2-enone.

Results for reciprocal quantum yield deter—

mination of cyclohex—2—enones . . . . . .

viii

Page

75

75

76

76

76

77

77

77

78

78

78

79

79

 

Figure

I.

II.

III.

IV.

IVa.

VI.

VII.

VIII.

IX.

LIST OF FIGURES

Stern—Volmer plots for varying concentrations

of thymine in acetonitrile

Stern—Volmer plots for varying concentrations

of uracil in acetonitrile

Dependence of triplet-state lifetimes of
thymine and uracil on ground—state

concentrations . . . . . .

Dependence of quantum yields of concentration
of cyclopent-Z-enone and cyclohex-2—enone

Stern—Volmer plots for varying concentrations

0

 

of cyclopent—2—enone in acetonitrile

Stern-Volmer plots for varying concentrations
of cyclohex—Z—enone in acetonitrile

Dependence of triplet lifetimes on the con—
centrations of cyclopent—Z—enone and cyclohex—

2—enone . . . . . . . .

The ultraviolet spectra of thymine and uracil

in acetonitrile . . . . .

o

o

.

o

o

The ultraviolet spectra of cyclopent—Z—enone
and cyclohex—Z—enone in acetonitrile

Dependence of cyclopent—Z—enone and cyclohex—
2-enone sensitized isomerization of cis—
piperylene on the concentration of diene

Strip—scan of paper chromatogram of 14C-
labeled thymine and dimers from direct

photolysis . . . . . . . .

Vapor phase chromatography traces of cycle—
pent-Z—enone dimers and cyclohex—2—enone di—

mers and internal standards

ix

o

 

o

o

o

Page

22

23

26

29

31

32

35

69

7O

72

80

81

 

 

 

 

I . INTRODUCTION

 

 

 

 

 

The light induced dimerization of o,5-unsaturated car—

bonyl compounds to form cyclobutane rings is a very old re—
action in organic chemistry. It was first reported over a
half century ago in the photoreaction of cinnamic acid to

form truxinic and truxillic acids (ll (Equation 1).

cans C5H5 HOOC C6H5

hv _ +

2 C6H5CH=CHCOOH (1)

HOOC COOH C6H5 COOH

From this time until the middle fifties, many other examples
of this reaction have been investigated (2). This earlier
era of photochemistry was accomplished mainly by an Edisonian
approach using sunlight as the energy source and yielded few
if any mechanistic implications (although very elegant struc-
tural determinations of the "photoproducts" were done in
this period). In the last decade, however, methods have
become available which, while not giving complete knowledge,
enable us to derive insights into the mechanisms and pathways
of,excited state chemistry.

In this thesis, a study of the kinetics of dimerization
of certain alicyclic, o,p-unsaturated ketones will enable us
to propose a mechanism for this reaction. The kinetic

measurements made (such as intersystem—crossing efficiencies

 

2

 

 

3m." .trielet lifetimes for these compounds) add to the pre—
viously published work and allow comparisons between struc-
tures and photochemical behavior to be drawn. Four compounds
have been studied in this work: thymine (1), uracil (g),

cyclopent-Z-enone (3), and cyclohex—Z—enone (4).

‘11? of? e i)

i

Z

(to :1:-

E. 2.

 

Because of both their impact on different areas of chemistry

and the different methods used to study them, the pyrimidines

(L and 2) and the simple enones (3 and 4) will be treated

separately.

When the enone chromophore is excited by incident

ultraviolet light, three reactions may result. The dimeri—

zation reaction (Equation 2) occurs when the excited enone

adds to a ground state enone and is the one studied in this

work. The cycloaddition reaction (Equation 3) is the addi-

tion of the excited enone to a simple olefin; it may be
considered the general reaction of which dimerization is

only the special case.

0

 

 

 

 

 

O

+ >'—OH_h'__> (4)

OR

The cycloaddition reaction is very useful for mechanistic
studies (3) and has also been the basis for the synthesis
of many natural products containing new ring systems such
as caryophyllene (4), bourbonene (5), atisine (6), stipito—
tanoic acid (7), y—himachalene (8), and others. The re—
duction reaction (Equation 4) can occur under certain cir—
cumstances with the addition of alcohol or water to the
unsaturated portion of the enone (9). These side reactions
are all unimportant when studying dimerization since the
adding molecule must be present in solution. The reverse
is not true, of course, for when studying olefin or alcohol
addition, the concentration of enone must be kept low in
order to minimize dimerization.

It should be noted that a five or six—membered cyclic

enone is necessary for the dimerization to take place.

 

 

 

5

RCYCljJ: enones undergo a photoinduced cis—trans isomeriza-
tiOYI (Equation 5). Apparently intermolecular cycloaddition

cannot compete with this reversible intramolecular process

 

(10a).
0
R C R R"
hv /
\c = c\/ \CH3 < > \/c = c\ /CH3 (5)
RI R“ RI C

The only known cases of intermolecular cycloaddition from
acyclic enones are with acetoacetone (11) dimethylmaleate
(12), and chalcone (12a). The former is held in a rigid
ring by H~bonding (Equation 6) and the second compound has a
transition state that is for some reason stabilized by the
carbonyls on each end (Equation 7). The third compound,
chalcone, isomerizes at high wavelengths (320 nm) but di—

merizes at low wavelengths (Equation 7a).

f0 h 0
+ ; -——X_>
O o/H OH (6)

COOMe COOMe
+ | g» (7)
COOMe COOMe

® ¢ _ .
\\z¢7\:U// light > Dimer (7a)

 

 

 

6

The EsiZe of the ring is also an important consideration be-
cause it has been shown that in cyclohept—Z-enone (13) and
cyclooct—Z—enone (14) the gig-trans isomerization occurs
exclusively and the cycloaddition reactions noticed proceed
entirely from trans-enones and take place readily in the
dark.

The last limiting factor in cycloaddition reactions is
the substitution of the C—4 position of 6-membered cyclic
enones. Irradiation of a 4,4—dialkyl compound will give
the lumirearrangement (15) to form a bicyclo[3.1.0]hexan-
2—one system (16) (Equation 8). Various substitutions at

other positions do not change the cycloaddition reaction.

0 o

(8)

So, in summary, properly substituted five— and six—
membered alicyclic q, —unsaturated ketones can be made to
dimerize under a variety of conditions by the action of
ultraviolet light.

In 1928 Gates (17) pointed out the probable relation—
ship between the bactericidal effectiveness of the various
wavelengths of uv light and the absorption of uv light by
DNA (18). Generally in these cases the death of bacteria
and other living organisms is caused by lesions which block

DNA or RNA synthesis. Investigation of these lesions

 

 

CODtJJnJed until 1958 when Beukers and Berends found an ir-
reVerSible reaction of the pyrimidine bases, thymine and
uracil, resulting from uv irradiation (19). Two years later,
they postulated that this reaction formed a dimer of uracil
or thymine (Equation 9) and that this dimeric structure was

responsible for uv damage to cells (20).

m: I HN NH (9)

 

R = H (uracil) gr CH3 (thymine)

In the following decade this theory was shown to be true
by a large number of researchers (2]). In fact Setlow has
estimated that cyclobutane dimers occur in irradiated
bacteria about a thousand times more often than the other
types of photodamage such as interstrand cross—links, chain
breaks or DNA—protein links (22).

There are two main approaches to determining the pro—
cess which leads to the formation of dimers; first, look—
ing at the isolated thymine and uracil moieties and apply—
ing these results to biological systems; and second to
investigate the photoinduced imerizations in DNA and RNA,
both in vitro and in vivo. The former method was much
easier to undertake, and the results quickly reported by

many workers. Irradiation of frozen aqueous solutions of

 

 

8
thyudJie gave a single dimeric compound (23). Since four
configurations about the cyclobutane ring are possible, the
isolation of a single product earned it the name "ice—dimer".
The structure of the "ice-dimer“ was shown to be syn-head—

to-head (g) (24).

c C O

        

E

The remarkable stereochemistry was explained by Wang who

showed that the pyrmidines freeze in crystallites where
regular arrays of molecules are stacked in a precise geom-
etry which enables formation of 2 by irradiation (25). The
ice reaction is very efficient (high quantum yield (26))
and was also shown to be a singlet reaction (27). It is
known that there is no fluorescence from frozen thymine
solutions and because of this, the reaction of dimer forma—
tion must be fast enough to quench fluorescence, a known
fast process. This fluorescence quenching and high dimer
quantum yield suggest excimer formation (28). This is
quite reasonable since there must be aggregation of some
sort to account for the stereospecific production of the
ice dimer.

In solution at room temperature, the pyrimidines can

react through both singlet and triplet manifolds. Thymine

 

 

 

9
and \lracil dimerization in dilute solutions (~1 x 10-3M)
is CDJenched by dienes and this indicates a triplet mech—
anism (29). On the other hand an analogous compound, di—
methyl thymine (E) is much more soluble and therefore

concentrated solutions can be prepared (0.1M). These react

0
CH3
CH3N

AN'

CH3

2

 

through both the triplet and singlet states (30). This A
behavior can be explained by a moderate rate of intersystem

crossing which promotes population of the triplet state in

dilute solutions, but allows singlet reaction to compete

with intersystem crossing at high concentrations. Although

irradiation of pyrimidines in ice gives only one product,

solution photolysis gives all possible products with gig

ring junctions (30,31):

0 O H
R R R H N 0
NH HN
05;\N N/gb 04L\N H R NH
I H H 0 '
H H H
syn-head—to—head syn—head—to-tail

 

 

 

anti-head—te-head anti—head—to—tail

In the simple enones (vide infra) there is a polarity effect
in the dimerization reactiJn. The use of different solvents
changes the relative yields .f isomeric products (10a).

This effect w,uld be expected in the pyrimidines but has

not been demonstrated as yet. Their limited solubility even

 

in very polar s lyents will probably preclude solvent studies
Since the solution dimerization has been shown to pro-
ceed by a triplet, it would be expected that the reaction
would take place if the triplet state was populated by
energy transfer from a suitable sensitizer. Krauch has
dimerized both uracil (32) and thymine (33) in water with
the use of acetone as a sensitizer. This work was repeated
by Johns (31) using other sensitizers. As would be expected,
all possible dimers are formed, in nearly the same ratio
as the direct photolysis.
The alternate approach to the determination of the
cause of biological damage by uv light was an investigation
of the dimeriZation of the pyrimidines in DNA and RNA them—
selves. Early in 1960 Wacker (34) recovered a dimer of
thymine identical in structure to Frankel's “ice-dimer" (23),
by hydrolysis of the DNA of irradiated bacteria. Similarly,

the syn-head-to—head dimer of uracil was found in the

 

 

 

11

phcnuolysis of RNA (35). It would be expected that a single
thYmine—dimer would be formed in native DNA because of the
rigidly ordered stereochemistry of the nucleic acids. That
the dimer found is the syn-HH dimer lends much support to
the fact that thymines attached to adjacent ribose moieties
are coupled (intrastrand dimerization (36)). It is postu—
lated that the interstrand dimerization (23) would give
anti-HR or 2£££‘HT and that reaction of the bases in compli—
mentary strands would require gross distortion of the helical
structure and therefore be unlikely.

It should be mentioned that cytosine dimers and mixed
cytosine-thymine dimers are formed in the irradiated DNA,
but these mixed bases have not been studied in much detail.

Very recently Lamola has reported the formation of
dimers in bacterial DNA by sensitized irradiation (37).

This indicates that reaction of DNA gag be through the
triplet state. But whether the direct irradiation of DNA
proceeds in this manner or through a singlet state mechanism

has not yet been determined.

Turning to simple unsaturated ketones, the literature
indicates that only four dimerizations have been studied
in detail- coumarin (1), isophorone (8), cyclopent—2—enone
(3), and cyclohex—Z—enone (1). Although much work has been
done on the coumarin dimerization (Equation 10) it is

hard to draw any definite conclusions.

 

 

 

EE'HT fl—HT

Direct irradiation of concentrated solutions (> 0.5M)

gives mostly the EXE products with some aaaa—HH dimer. Di-
lute solutions produce only EEEE products upon direct pho-
tolysis (38) and sensitized irradiation using benzophenone
also yields the aaaa—products. From these and many other
experimental results, Schenck (39) and Morrison (40) both
postulate a singlet excimer intermediate which goes on to
form only the aya products. The 32E; products arise from
the small amount of triplet formed by intersystem crossing
in direct irradiation and the complete triplet state popula-
tion in sensitized experiments. The variance of product

yields in different solvents reflect the polarity changes

 

 

 

13

of t*“3 Solvents evidently allowing changing population of
Singlet and triplet states. In addition different solva—
tion of the intermediates lead to changeable amounts of HE
or HT products. The complexity of the reaction indicates
the problem of deriving a mechanism to explain all the re—
sults and therefore the need of more work in this area.

All of the work on the dimerization of isophorone

(Equation 11) has been done by Chapman (41). The reaction
proceeds completely from the triplet state and gives varying

amounts of dimers, depending on the solvent used.

was

anti-HT

(11)

Based on sensitized and direct irradiation experiments,
Chapman postulates Egg distinct triplet states, each
giving rise to one of the dimers, HH or HT. This observa—
tion is interesting, but at this time insufficient informa—
tion is available to warrant further discussion.

The dimerization of cyclopent-2—enone (Equation 12)
was first reported by Eaton (42) in 1962. The reaction

has been well studied since then and several important

  

14
Iceults are evident. After some opinions to the contrary
(notably from Leermakers (43)) Eaton showed that the re~
action proceeds from the triplet state exclusively and the

variance of HH/HT products could be easily explained by a

solvent effect (44). But the most significant result was
0 O
hv > + (12)
O
anti—HT anti-HH

that of deMayo (3a) who demonstrated that sensitizers with

triplet energy below that of benzophenone (E = 69.2 kcal/

T
mole) would gag sensitize the cycloaddition reaction. In-
stead these compounds sensitized the formation of a reduc—
tion product. This indicates the existence of £39 triplet
states: a higher state (ET z 74 kcal/mole) which leads to
cycloaddition, and a lower state (ET 2 61 kcal/mole) which
can give only reduction product. The reduction product in

isopropyl alcohol solution (3c) is g, and in cyclohexane

(45) is 1.9.-

 

OH

gm
)3

 

 

 

 

15

Haflumond in a single publication (46) has reported the
only Study of the dimerization of cyclohex-2-enone (Equa—

tion 13) .

h‘ \ trace
' > + + byproducts

(13)
0
anti-HT anti-RH

The reaction can be quenched by dienes and can be sensi-
tized. 'flus behavior indicates a triplet excited state inter—
mediate. The HH/HT dimer yields vary as before with concen—
tration and solvent polarity. Since naphthalene sensitized
the reaction, Hammond suggested that it proceeded by the
lowest triplet, but the possibility of singlet sensitization
makes this conclusion questionable (3c).

In the following sections of the thesis, the determina—
tion of the primary rate constants for the dimerization of
four enones (thymine, uracil, cyclopent—2-enone, and cyclo—
hex—2-enone) is reported. The values obtained enable us
to postulate the intermediacy of a metastable dimeric
species which can go on to dimer, or revert to two ground
state molecules in varying amounts depending on the struc—

ture of the enone.

 

 

 

II . RESULTS AND DISCUSSION

16

 

 

 

 

 

A. Thymine and Uracil

One of the general methods for the determination of
the mechanism of a reaction is to postulate a reasonable
mechanism, derive kinetic functions which describe that
mechanism, and do experiments to see whether the results
correSpond with it. A good correlation will lend support
to the postulated mechanism and a poor correlation will
indicate an erroneous mechanism. This approach was used
to study the dimerization reaction of unsaturated ketones.
The generally postulated mechanism for this reaction (3c,46)
is shown in Scheme I. Light is absorbed by the enone which
is excited vertically to the first singlet state (1E*).

The singlet can decay with a rate ki to the ground state or

intersystem cross with a rate k.

to the excited triplet
lSC

state (33’ . The triplet can decay (kd), transfer energy
with a rate kq to a quencher (Q) or add to a ground state

enone molecule with a rate ka to give a dimer (EE).

Scheme I
i
E + hv > 1E“
k.
lEd l " E
k.
1E! lSC > 3E*
k
3E” d > E‘
k
313" + Q ——q-——> E + 3Q*
ka
3E" + E ———-——> EB
17

 

 

 

 

 

18

These Precesses can also be represented by a rough energy

leVel diagram which is shown below:

 

13*

 

 

 

 

 

A k
sc
33*
+E
kl Ia kd +Q
kq ka
EE
7
E
In photochemical reactions, the quantum yield is a
very important and fundamental quantity. The primary quan—

tum yield of a process (48) is defined as:

 

o : le] dt : No. of molecules of X formed/cm3 sec (14)
f— No. quanta absorbed by reactant/cm3 sec
a
The quantity X can be a molecule, radical or ion. It is

also useful to consider quantum yields as probabilities.
The absorptions of light by a molecule is a one—quantum
process and the sum of the primary process quantum yields
w must be unity. Quantitatively, Z wi = 1.00, where ¢i is
the quantum yield of the ith primary process.

Referring to the energy level diagram, the total quan-
tum yield for dimerization can be written as a product of

all the process quantum yields:

m . = Q. ' ¢ . (15)

 

 

 

 

 

19
Each process quantum yield can be written as the rate for
that process divided by the sum of all possible processes

proceeding from that intermediate (49).

 

 

Q _ kiSC . ka[E] (16)
. - \
dim kisc + ki kd + kq[Q] + ka[E]
When no quencher is present, [Q] = 0 and
o kisc kalE] ( )
Q . = - —————————— 17
dim kiSC + ki kd + ka[E]

Taking a ratio of these two relations and defining T as the
lifetime of the triplet state in the absence of quenchers

(Equation 19) the following equations are derived:

 

m0 _ kd + kalEl + kqul — k [Q]
T * kd +ka[E] ‘ “‘kd +ka[E] (18)
= *1___ (19)
T kd + ka[E]
_%3 = 1 + kq T [Q] (20)

Equation 20 is the Stern—Volmer expression from which qu
can be determined by varying the quencher concentration and
measuring the unquenched to quenched ratio of quantum
yields.

The reciprocal quantum yield expression (Equation 22)

follows from the reciprocal of Equation 17.

 

 

 

 

 

 

 

 

 

 

20
1 = 1 . kd + ka[E] (21)
lbdim misc kaIE]
1 1 d
' 1 + 22)
(”dim <15isc ka [E] (

Using this relation, the change in ¢dim with varying enone
concentration can be measured and the ratio of rate constants
kd/ka can be determined (if misc is known).

Both of these expressions (Equations 20 and 22) can
also be derived by writing the rate laws and applying the
steady state approximation to the intermediates 1E* and
33*.

The kinetics of dimerization of the pyrimidine bases,
thymine and uracil, were determined by quenching studies
and triplet counting. Their low solubility precluded pro—
duct analysis in a quantitative manner and therefore disap—
pearance of pyrimidine was followed by uv spectroscopy. In
the large amount of literature on this reaction (2292:2333)
no report has mentioned any loss of pyrimidines in non—
prOtic medium except by dimerization. The solutions were
irradiated with the 2753—, 2804—, and 2894 8 lines of a

medium pressure mercury arc. The pyrimidines absorbed

varying amounts of this light through the 1 mm Pyrex wall

Of the sample tubes. This method was not optimal, but the

necessity of using a large number of samples for the kinetics

and the impossibility of obtaining Corex tubes left no al—

ternative. Because of these conditions, absolute quantum

Yields could not be determined directly. However, another

 

 

 

 

‘—

21
method was used by which the quantum yield at certain con—
centrations could be estimated (vide infra).

The products of the dimerization of thymine in aceto-
nitrile were analyzed by the use of 14C-labeled pyrimidine.
Irradiated samples were chromatographed on paper and
analyzed by a strip—scanner. The resulting trace is indi—
cated in Figure IX. Unfortunately, the resolution of the
dimers is only fair, but the following observations can be
made: first, all four dimers are present; second, the aya
dimers (HH and HT) account for about 55-60% of the mixture,
slightly less than Morrison has noted for dimethyl thymine
where he could measure the aya products accurately at 85%
in acetonitrile (30); third, there is much less formation
of aya dimers in solution than in the frozen state where
they approach 100%; and fourth, the result is in close agree—
ment with John's who finds 65% aya dimers in the direct
photolysis in water (31). These results are in agreement
With the fact that the reaction proceeds by the singlet
excited state in frozen medium, partially singlet in Morri—
son's work (high concentration), and triplet in dilute
Solution at room temperature. It is assumed that uracil
behaves in a similar manner.

Degassed acetonitrile solutions 2—10 x 10—4M in pyrimi—

dine and containing various concentrations of piperylene

were irradiated and analyzed. The Stern-Volmer plots were

linear out to large percentages 0f quenching (Figure I)'

indicating that at these concentrations the PhOtOdimerlzatlon

 

 

22

 

zvuofi
2 oH

z.-os

2 OH

w®.m
mh.®
mm.v
Ob.N

HNC‘OV‘

35:53:

to.m

 

 

 

 

 

 

 

.mafluuacoueom CH Haomuo wo mCOHumuueeocoo mcflwum> How muon umEHO>Icueum .MH euomflm

Svofi x Hecmawummflmg

 

 

oo.m om.a o.H om.o
J a — ‘
o.a
zvsoH x wo.e m
z -oH x mH.m a .
2v-oH x os.m m -o.m
2 (OH x mm.H m assuage“ . . .
zvlofi x mm.H H
m .nw
.0 69
Io.m
m.
a
m
N id.v
H

 

 

 

 

 

 

 

ll ‘0

A1,.

 

24
iis {predominantly a triplet—state reaction. The reactions,
in fact, could be quenched over 99% by the addition of
0.()1M piperylene. Table I contains values of the slopes
obtained at various base concentrations.

Table I. Quenching of thymine and uracil photodimeriza—
tions by 1,3—pentadiene in acetonitrile

 

 

-4 a —1 b -6

 

 

[Pyrimidine], 10 M qu, M r, 10 sec
Thymine F. t
2.70 26,000 3.36
4.33 20,800 1.89
6.75 15,700 1.43
9.68 . 12,200 1.12
Uracil
1.89 21,300 1.94
1.96 19,600 1.78
2.70 18,600 1.69
3.19 12,900 1.17
4.08 11,500 1.04
aAverage concentration. bSlopes of Stern—Volmer plots

reproducible to {5%.

The rate constant for energy transfer, kq, is dependent

on the viscosity of the solvent and has a value in aceto—

nitrile of 1.1 x 101°M_] sec_1 (50). This value was used

to determine the T values in Table I. The quantum yields

for the two pyrimidines were found by comparing the amount
of dimerization of a given concentration of base to the

amount of isomerization of 0.1M cis-1,3—pentadiene sensi—

tized by the same concentration of base. The pentadiene

 

 

 

 

 

25

is5C>merized gives a count of the total number of triplets
fCDrmed (51) and the dimer produced (assumed to be one—half
0f the pyrimidine that disappeared) gives the number of
triplets that actually went on to form dimer. A ratio of
these values for 6.2 x 10—4M thymine indicates that only
1.4% of the thymine triplets actually dimerize. In a 3.9 x
10—4M uracil solution, only 4.9% of the triplets dimerize.

Using Lamola's (29) values for mi (0.18 for thymine and

sc
0.40 for uracil) the quantum yields of dimerization at these
concentrations can be calculated as 0.0025 for thymine and
0.019 for uracil.

The triplet state lifetime, 1, is a function of the
concentration of enone as in Equation 19. The various 1—
values have been determined from the Stern—Volmer quenching
plots and when these are plotted (Figure II) according to
Equation 23 (which is the reciprocal of Equation 19) the
slope is the bimolecular rate constant of addition, ka.

and the intercept is the triplet decay rate constant, kd.

 

1
1 = kd +ka[E] . (23)

The values of the rate constants and quantum yields
are indicated in Table II. The quantum yield of addition,
¢a, can be calculated from the rate constants and the con—

centrations indicated, by using Equation 24.

kaIE]

¢ : ———-—-—-— 24)
kd +ka[E] (

The quantum yields for dimerization predicted by Equations 14

 

 

 

 

26

 

 

 

/
Uracil
10.0 ‘
o
8.0 r
l
T 0
x 105 Thymine
6.0 ’
4.0—
2.0-
1 I l I
2.0 4.0 6.0 8.0
[Pyrimidine] x 104
Figure II. Dependence of triplet—state lifetimes <

thymine and uracil on ground-state con-
centrations.

 

 

 

 

27
arufly 16 and rate constants are much greater than the observed
Viilues. That there is a further source of inefficiency in
tflie reaction is quite evident and this will be discussed at
length below.

Table II. Kinetic data for photodimerization of thymine
and uracil

 

 

 

 

Quantity Thymine Uracil
kd, seca 2.2 i .14 x 105 1.6 i .5 x 105
ka, M_lseca 0.70 i .02 x 109 2.0 i .17 x 109
m 0.65b 0.78C
a
o. d 0.18 0.40
lSC
b c
e . 0.0025 0.019
dim

 

aSlopes and intercepts analyzed by Least Squares. Standard
Deviation indicated.

b6.2 x 10_4M thymine. C3.9 x 10_4M Uracil. dValues from
reference 29.

B. Kinetics of Cyclopent—Z—enone and Cyclohex—Z—enone

The kinetics of the dimerization of the simple enones,
cyclopent—Z—enone and cyclohex-2—enone were determined by
measurement of quantum yields and Stern—Volmer quenching
slopes. Previous studies were complicated by the fact that
product ratios of head—to—head and head—to—tail products
were dependent on enone concentration. This behavior ap4
parently reflects a polar solvent effect which enhances the

formation of head—to—head dimer (10a). The use of

 

 

 

 

 

28

acetonitrile as solvent alleviates this problem. Only two

dimeliic product peaks appear in the vpc traces of irradiated
cYClopent—2-enone. The HH/HT ratio remains constant at 4:5
from 0.1M to 3.0M concentrations. With cyclohex-2—enone,
the HH/HT ratio is 2;1 and also independent of concentration.
A third product peak, amounting to 4% of the total, appears
just before the two major dimers on the vpc traces and is
probably a dimer with a trans—6/4 ring junction (3b). Thus,
the kinetics of dimerization were conveniently studied by
measuring product appearance by vpc relative to an internal
standard.

The quantum yield of intersystem crossing was found by
measuring the amount of isomerization of different concen—
trations of gag-pentadiene sensitized by a constant concen—
tration of each enone (the method is described in detail in
the Experimental section). Extrapolation to infinite diene
concentration indicates that both enones have unit effici—
ency of intersystem crossing.

Equation 22 describes the dependence of quantum yield
on enone concentration. By measuring the amount of enone
found from parallel irradiation of various concentrations of
enone and the light flux by acetophenone-gaa—piperylene
actinometry, the absolute quantum yields were determined.
These values are in Table III. Plotting these according to
Equation 22 with the value of ¢isc equal to unity gives good
straight lines (Figure III). The quantum yields at infinite

concentration (intercept) are only .36 and .75 for

 

 

 

 

 

29

 

14.0

12.0

10.0

8.0
_1_

Ddim

6.0

4.0

0-ff—‘'fl~-fflIaA_aaIf_,iL..'~~-7-’"”"'7’”fi'fl
O
2.0

 

 

 

 

r 4|;

1.0 2.0 _1 _1 3.0 4.0
[Enone] M

Figure III. Dependence of quantum yields on concen—
tration of cyclopent-Z-enone and cycle-
hex—2-enone.

 

30

GYClOPEnt—2—enone and cyclohex—Z-enone, respectively. This
(gill lae discussed at length below. The ratio of rate con-
stants kd/ka is 0.06 and 2.7 for the two enones.

Table III. Quantum yields for various concentrations of
cyclopent—Z—enone and cyclohex—2—enone

 

 

[Enone], M m Cyclopent—Z-enonea m Cyclohex-Z—enonea

 

dim dim
1.00 0.342 0.204
0.50 0.324 0.115
0.375 0.308 0.091
0.25 0.292 0.064

 

aAverage of two runs; each value reproducible to i1%.

Stern—Volmer analysis of the relative quantum yields
as a function of quencher concentration was done for each
enone. Both 1,3—cyclohexadiene and 1,3—pentadiene have
been shown to be equally effective at quenching the photo—
dimerization of cyclopent—Z—enone (52). However, 1,3—penta—
diene is only 60% as efficient as 1,3-cyclohexadiene at
quenching the cyclohex—Z—enone reaction. The reason for
this is not known, but the same phenomenon has been observed
with other 6—membered, cyclic enones (52,53). Table IV
contains the qu values for Stern—Volmer analysis (Figure IV)
of both enones using pentadiene and that of the 6—membered
enone using 1,3~cyclohexadiene. The T—values are calculated

_1 _
using 1.0 x 1010M sec 1 for kq because the high concentration

 

 

 

 

 

 

 

31

.mcocmlmuucwmoHowo we mcofluwnucmocou mcflwum> How muon ueEHo>ICHmum .>H ensmflm

z .HocwahummHEH

 

 

ow. om. om. 0H.
o.H
Smm.H v .
Soo.H m .
20>.o N . . . o.N
Eom.o H .
)9
o9
. . o.m
. o.v
v
m N A . o.m

 

 

 

 

 

 

mucmwmnmmn

‘

.wcsu mcwflomxmsoaomolm.fi
I | l l “menu mcmflcmucmmlm.a mucwmwhmmu .
“egocelmnxeLOHoho mo mcoflumuucwocoo mcH>Hm> How muon HwEH0>Icueum .m>H wusmflm
S x s. 358.
oo.N om.H oo.H on.
II1II . . .

 

32

°|
4')

()

O
(‘0

 

 

 

 

 

 

33

¢ablfii IA]. Quenching of cyclopent-2—enone and cyclohex—2—
enone photodimerizations by dienes in aceto—

 

 

 

 

nitrile.
Enone Diene [Enone], qu, 1/T,
M M_1 108 sec-1
Cyclopent-2— 1,3—pentadiene 1.52 9.1a 10.5
enone 1.00 12.5 7.7
0.75 16.5 5.9
0.50 26.5 3.7
Cyclohex—2— 1,3—pentadiene 1.00 13.7 7.3
enone 0.50 17.5 5.8
0.48 17.5 5.8
0.25 18.5 5.5
1,3—cyclohexadiene 0.75 27.0 3.6
0.50 28.1 3.5
0.30 38.7 2.7

 

_1
aThe error for qu values is £0.2M

Table V. Kinetic data for photodimerization of cyclopent—2—
enone and cyclohex—2—enone.

 

 

 

Quantity Cyclopent—Z—enone cyclohex—Z—enoneu
a a
kd, sec 0.40 t .1 x 108 3.0 i .4 x 108
ka, M—lsec 6.6 i .4 x 108 1.1 i .2 x 108
¢. 1.0 1.0
lSC

 

aSlopes and intercepts analyzed by Least Squares. Standard
Deviation indicated.

 

 

34

Oi arugne changes the viscosity slightly. Evidently the

Valufii Of kq for the pentadiene quench of cyclohex—2-enone

must be less than this because it is not diffusion controlled

The 1, values are plotted for all three cases according to

Equation 23 in Figure V. All further kinetic treatment of

cyclohex—2—enone was based on the 1,3—cyclohexadiene quench-
ing results because these are more nearly diffusion control—
led.

The rate constant for addition, ka, is the slope of

the 1 : ya. [E] graph and although kd, the triplet decay

rate, is the intercept, its value was determined instead

from the rate ratio kd ka as found from the reciprocal

quantum yield plots. This was done because there is much

less error in the value of a slope than in that of an inter—

cept.

The rate constants and other data for the two enones

are listed in Table V.

C. Mechanistic Interpretations

The vahrs of the rate constants themselves are inter—

esting. The decay rate, kd’ for the pyrimidines is very

low at 105 sec. This value is comparable to the rate for

simple carbonyl triplets in solution. The simple enones

are about three orders of magnitude faster in their decay.

This great difference may partially reflect the greater

flexibility of the simple enones as compared to the rigidity

 

 

35

.mcon mcwflcmxezlm.fi monomeumen .0 can
“menu oceflomucwmlm.fi ucowoumou . D HO O ”AOV ecocwumlxeLOHomo can ADV

 

 

woocwlmnucwmoHowo we mcoflumuuceocoo one so eEflumeH Deamfluu we woceccemoo .> musmflm
Z .HmcocmH
om.H mN.H oo.H we. on. mm. o
i N
. I a
u 0: 0 I. m
.2
. H
H
I m
10H
NH

 

 

 

 

 

 

 

 

 

36

OE ting pyrimidine rings, for it is generally postulated
that_ the primary mode of radiationless decay is through
vibration of the molecule.

However, an even more important factor contributing to

the decay of enones is the fact that they dimerize from a

second triplet state T2 . The work of deMayo (3c) has

shown this to be true for cyclopentenone. Although Ham—

mond initially postulated cyclohex—2—enone dimerization
occurring from the lowest triplet on the basis of a naphtha-
lene sensitizing experiment, our own work has shown that

this too is a T2 reaction. Irradiating solutions of cycle—

hex—2—enone containing increasing amounts of the sensi—

tizer phenanthrene,which in all cases absorbed most of the
light, caused the quantum yields of dimerization to decrease.

Inspection of the energy level diagram shows that both

singlet V3c‘ and triplet energy transfers are likely from

phenanthrene to cyclohex—Z—enone.

 

 

 

1 "
P _—
“ \
1 .y
A R E
) 3P* 62 kcal
_1_/3E: z 60 kcal
P0 E0

 

 

 

 

 

 

 

37

'Phe only explanation for the decrease in sensitized
quantuniyield is that triplet energy transfer from Egg
intermediate which leads to dimer (3E2*) to the phenan-
threne occurs to form 3P'. Transfer from 3E1* to phenan—
threne is endothermic by at least 1 kcal and therefore un—
likely.

Because of these considerations, it is possible that
quenching of both enones done in the Stern—Volmer experi—
ments is quenching of the T2 —> T1 conversion. If this is
the case, it IS likely that the kd values are low. Liu has

1
for

estimated T2 —> T1 internal conversion at 5 x 101°sec_
anthracene (58). It was previously mentioned above that

the higher homologues, cycloheptenone and cyclooctenone, do
not dimerize upon photolysis. Rather, they undergo a gig —>
trans isomerization (10a). Zimmerman has postulated that
the difference between the two triplet states may be geo—
metric, the higher triplet T2 being more planar and lower
triplet being twisted at the y-carbon (53). The facile
twisting in the higher homologues may preclude dimerization,
whereas constraint in the smaller 6-membered enone and the
even smaller 5—membered ring compound may lower this rate

of internal conversion to about lossec_] and allow dimeri—
zation to occur at high enough enone concentration. If this

is true the rates, kd’ are the first measurement of internal

conversion in ketones.

 

 

 

 

 

38

'Phe rates of addition, ka, are all quite large at
about. 103M_1 sec. Lamola (37b) has postulated that the
pyrimidines dimerize from a r —> ~' triplet state on the
basis of the very long—lived phosphorescence emission. The
work of deMayo (3c has shown that the simple enones react
from the (second triplet state which is presumed to have

n -> *' configuration. It has been argued for some time

that rates of reaction could be used to predict the elec—

tronic state, the r —> ~‘ configuration reacting much
slower. This work shows at least one case of similar rates
arising from different electronic states. It is interesting

to note that triplet thymine adds to ground—state thymine
only one—third as fast as triplet uracil adds to ground—
state uracil. In a qualitative manner this effect is prob—
ably due to some steric hindrance by the methyl group of
thymine. But, it is still hard to draw any good conclusions
about the effect of structure on the rate of addition. The
fact is that this is a dimerization reaction and the excited
moiety is adding to itself. The ground—state enones must
act as olefins for this cycloaddition and therefore overall
effects are very complicated. The only way to resolve this
difficulty is to add a series of enones to a single olefin
under the same conditions and from this get a good idea of
the reactivities.

Using the measured data for all four enones and the
derived equation for the dimerization reaction (Equation 14),

0 can be calculated for each enone at any given

dim

 

 

 

 

 

39

COnCEHItration. It is obvious that these calculated values

of are too large and that there must be a further major

3) .
dim
source of inefficiency in the reaction. The data require

that some of the original photoadduct must be able to decay

 

back to two ground state molecules. The original mechanistic

 

scheme must be altered to include the formation of some
sort of intermediate \EE)‘ that can either decay or go on
to dimer. The modified scheme and rough energy level dia—

gram are shown below.

E + h. a > 1E.
k.
13‘ l > E
k.
1E» lSC > 33*
k
313' d > E
k
3 a (a
E + E 2 (EE)
k—a
(EE)‘ ; 2E
k

 

 

 

 

 

 

 

 

4O

'Phis new mechanism yields Equation 25 in place of
Equation 14 to describe the total quantum yield of dimeri-

zation.

9 (25)

: ____£L___ (26)

The quantity :p can be defined in terms of rate constants
(Equation 26) of coupling (kc) and uncoupling (k_a) and is
the probability that the intermediate will proceed on to

stable dimer. The quantity 9 is now the probability that

ad
triplet enone will react with ground—state enone. Using
Equation 25 and the measured data, the 3p values for each
enone can be calculated and are listed in Table VI. The
results indicate that only 23 of the original metastable
thymine dimers formed eventually yield stable ground state
dimers. The corresponding percentages are 6% for uracil,
36% for cyclopent—Z-enone, and 74% for cyclohex—2—enone.

Table VI, Kinetic data for photodimerization of enones in
acetonitrile

 

 

 

Quantity Thymine Uracil Cyclopent- Cyclohex—
2-enone 2—enone

[Enone],M 6.2 x 10'4 3.9 x 10‘4 1.00 1.00

t. 0 18 0.40 1.0 1.0

lSC

0a 0.65 0.78 0.94 0.27

T . 0.0025 0 019 0.34 0.20

dim

@pa 0 021 0 061 0.36 0.74

 

aCalculated from Equation 25.

 

 

 

41

Iri the plethora of literature on the reaction of

pYIiJnidines there have been no mechanisms advanced for the
triplet reaction in solution except the vague statement by
Johns (54) that "it is conceivable that the reaction [uracil
dimerization] might lead to an unstable product which would
not be detected." The only mechanism proposed for simple
enones that is largely different is that of Chapman who
postulates two triplets, each leading to different dimers

of isophorone, As of this writing his hypothesis has not
been further verified.

Several groups have found results similar to ours in
related reactions, in that maximum quantum yields are signif—
icantly lower than unity. DeMayo's cycloadditions to cyclo—
pent—2—enone triplets proceed only 48% with cyclohexene
(Equation 27) and 21% with 3—hexene (Equation 28)° Tropone
dimerizes with 39% efficiency from the triplet state (55).
The cycloaddition of benzophenone and furan (Equation 29)

proceeds only 3% as reported by Sokurai (56)“

 

 

O o
h..
+ V ,. CEO (27)
O O
hu
+ (EC <2“
0 C6H5

O V CH5 29
ceHgKCGH: §\ /; h > 6 m .( >

O

 

 

 

 

42

DeMafikD suggests some sort of “complex“ which can fall
apart to two ground—state molecules or couple to produce
product; Sokurai postulates a 1,4—biradical (ii) that

behaves in an identical manner.

C6H5
Cells .1. ' \
O

11

So there are two possibilities for the reversible in—
termediate in enone dimerizations; (1) a triplet excimer;
or (2) a i—bonded biradical A singlet excimer is respons—
ible for singlet—state dimerizations of the pyrimidines in
frozen solutions (28), but it proceeds on to dimer with
100% efficiency (3p = 1.00). The high rate of addition,
lOsM-lsec, argues for the initial formation of some sort
of complex or excimer, This could either go directly to
dimer or collapse to a biradical, The intermediacy of bi—
radicals somewhere in the reaction scheme is supported by
much evidence, Corey showed (3b) that identical product
mixtures were found upon the photolysis of cycloheX—Z—enone
with either gis—Z—butene or trans—Z—butene. This requires
a two—step mechanism with an intermediate long lived enough
to allow isomerization of the double bond. Also 1,4—biradi—

cals are implicated in the photolysis of phenylalkyl ketones

(57)? The major result of this reaction is cleavage, but

 

 

 

 

43

C0“&Iination to cyclobutanols is also important. So it is
net possible to completely define the intermediate of the
photodimerization reaction at this time.

Since the pyrimidines give four dimers and the simple
enones two each, the rates (ka) and the probabilities (¢p)

measured for each system are undoubtedly composites of sets

of sets of such values. For thymine and uracil, for example,
kazp = k1:1 + kzzz + k3¢3 + k404 (30)
and

The quantity k1 is the rate of addition in a head-to—head
mode and :1 is the probability of the formed intermediate
closing to give gig—head~to—head product. The other quanti—
ties describe the formation of the other three dimers.

Until the actual amounts of the dimers in acetonitrile are
found further calculation is not possible. Even then the
series of £925 sets of unknowns make the problem immensely
complex, In the Simple enone cases only two dimers are

formed and the complete expression would be-

a

k"L-‘-k¢+k® (32)

k = k + k (33)

For cyclopent-2—enone the ratio of these dimer products is

known in acetonitrile and it can be calculated from Equations

32 and 34 that kHwH = 1-1 x 108M‘1 sec

 

 

 

 

 

= -— (34)

and kTaT = 1.4 x loam—lsec, where the H refers to head—to-
head product and the T to head—to-tail.
For cyclohex-Z-enone, the ratio of dimer products is

HT/HH = 36/64. Using Equations 32 and 35 it can be calcu—

 

. : 7 —l = 7 -1 ‘
lated that kHaH 5 2 x 10 M sec and kTQT 2.9 x 10 M sec.
kHiH = 23 (35)
kaT 36

The absolute values for k's or 0's cannot be found until

some other relationship is derived.

In conclusion, we have determined the primary rate
constants and quantum yields for the dimerization reaction
of a series of four alicyclic u,p—unsaturated ketones. Re—
quirements for the transition state of the reaction have
been determined. These include: (1) its formation is
reversible and the primary cause of low quantum efficienty;
(2) it may be a complex or triplet excimer that goes to
dimer or collapses to a 1,4—biradical; and (3) it may be

formed with nearly identical rates from either the v,v* or

n,w* confi uration of the excited enone.
g

 

 

 

 

III . EXPERIMENTAL

45

 

 

 

A. General Procedures

 

1. Ultraviolet Spectra. Ultraviolet spectra were

 

taken on a Unicam SP 800 recording spectrophotometer.
Matched quartz cells with 10.00 mm path length were used.
Beer's Law plots of thymine and uracil were obtained by use
of a Beckman DB spectrophotometer with a Gilford model 220
linear absorbance converter. Kinetic analyses of the two
pyrimidines were done using the latter instrument to read
absorbances which were changed to concentrations by use of

the linear Beer's Law plots.

2. Vapor Phase Chromatography. Two instruments were

 

used for all vapor phase chromatographic analyses: a)
Varian Aerograph HiFi III Series 1200, with a 6' x 1/8" col—

umn containing 5% QF—l and 1% Carbowax 20M on Chromosorb G;

 

and b) Aerograph HiFi Model GOO—D, with a 25' x 1/8" column
containing 25% 1,2,3—tris(2—cyanoethoxy)propane 60/80 on
Chromosorb P. Both instruments are equipped with flame
ionization detectors. An internal standard was used for all
quantitative work. Each standard was evaluated by use of

the following formula:

counts unknown =
K ' [standard] . m [unknown]

46

 

47

The Counts correspond to relative peak area as measured by
the disk integrators and K is a sensitivity factor re—

lating the standard to a specific unknown.

3. Irradiation Procedure. In a given run, all tubes
were irradiated in parallel for the same length of time on
a ”merry—go—round“ apparatus. This assured that each sample
absorbed the same intensity of light. For the pyrimidines,
a simple Vycor filter sleeve was used to screen the light
from a 450—w Hanovia medium—pressure mercury arc. This
filter allows only wavelengths longer than 2350 X to pass.
Cyclopent~2—enone and cyclohex—2—enone were irradiated with
the light from an identical arc, but the 3130 8 line was
isolated with a 1 cm path of 0.002 M potassium chromate in

1% aqueous solution of potassium carbonate.

B. Compound Preparation and Solvent Purification

1 Acetonitrile, Acetonitrile was used as solvent
for all runs and was purified by the method of O'Donnell
(60). This procedure lowered the ultraviolet cutoff to
about 200 nm and the liquid was completely transparent above

that value.

2. Thymine. Thymine (5—methyluracil) was purchased
from the Nutritional Biochemicals Corporation, Cleveland,

Ohio. It was recrystallized twice from water and sublimed

under vacuum.

 

 

 

48

3. Uracil. Uracil was purchased from Eastman Organic,
Rochester, New York. It was purified by recrystallization

from hot water and sublimation, 175°C at 0.50 mm Hg.

4. Cyclopent—2—enone. Cyclopent—2-enone was prepared

 

by the method of Garbisch (61 . Cyclopentanone was subjected
to bromo-ketalization, dehydrohalogenation and hydrolysis,
yielding 321 of colorless liquid, bp 51—530 at 18 mm; >99%
pure by vpc (column 3 Since photolyses were only carried
out to about 47, it was found that much of the cyclopentenone

could be recovered by simple extraction from salt water with

ether. Distillation gave —997 pure material again; ir
(CCl4) 1720 cm_1; uv reproduced in Figure VII.
5. Cyclopent—2—enone Dimers. A sample of the pure

 

dimers was needed in order to calibrate mole ratio;peak area
on the vpc. A 1.2 ml aliquot of cyclopentenone was placed
in a Pyrex tube, sealed at atmospheric pressure, and irradi—
ated for 16 hours strapped to the side of a quartz immersion
well. The light was from a 450—w Hanovia medium pressure
arc, filtered with a Pyrex sleeve. Addition of ether to

the crude product caused the dimers to precipitate. Three
crops were obtained in this manner. The off—white solid was
sublimed (900 at 0.1 mm) to give white crystals, mp 115—
118°, lit (42) 125—126 50, VpC analysis indicated 95% head—

to—tail and 5% head—to—head dimer. Each pure dimer was not

needed because it is assumed that their vpc response would be

 

 

 

49

identxical on the flame detector. Ir (CHCla) 1730 cm-1-

l

uv (cascm) 208 nm (e = 350), 298 nm (e = 59).

6. Cyclohex-Z—enone. Cyclohex—2—enone was prepared
from cyclohexanone using the Garbisch (61) procedure in 66%
overall yield. The product distilled cleanly at 70-710 at

30 mm Hg to give a colorless liquid which was >99% pure by

vpc (column 2;: ir (CCl4f 1685 cm_1; uv max (CH3CN) 222 nm
(a = 11,500), 327 nm (a = 30). Uv reproduced in Figure VII.
7. Cyclohex-2—enone Dimers. Again a sample of pure

 

dimers was needed for vpc calibration. A 4.3 g sample of
pure cyclohex—2—enone was sealed in a Pyrex tube at atmospheric
pressure, strapped to the side of the immersion well and ir—
radiated through a Pyrex sleeve for 24 hours. The orange
oil which resulted was distilled on a short path apparatus
at 128—1320 at 0.4 mm Hg. The product was 3.8 g of yellow
oil. The entire sample was chromatographed on a silica gel
column and the fractions eluted with methylene chloride were
combined and caused to crystallize from pentane ( which
contained a small amount of ethyl acetate) at dry ice—iso—
propanol temperature. Recrystallization from hexane and
sublimation at 50—600 at 0.4 mm Hg gave a white solid whose
composition was 85% HT and 15% HH by vpc (column 3) mp 40—

45°, lit. (62) 53-550 (for pure HT). Ir (CHC13) 1700 cm_1;

uv (CH3CN) 212 nm (t = 344), 287 nm (e = 52).

 

 

 

 

50

8. Isophthalonitrile- Isophthalonitrile (Eastman
Organic Chemicals, Rochester, New York) was used without
further purification as an internal standard for the cyclo—
pentenone—dimer analysis. Acetonitrile solutions are uv

transparent above 2950 R.

9. Ethyl Stearate. Ethyl stearate (Eastman Organic
Chemicals, Rochester, New York) was recrystallized twice
from carbon tetrachloride before use in vpc analysis of
cyclohexenone-dimers. The resulting compound was 98% pure
by vpc (column a at 180°) and showed only a small absorbance

above 2500 X in the uv (relative max at 2950 R, e = 10).

10. Piperylene. Piperylene (1,3—pentadiene) as a mix-
ture of isomers was used for quenching studies (Aldrich
Chemical Company, Milwaukee, Wisconsin). It is reason-
ably stable if kept at refrigerator temperature, but was
redistilled every two months to remove dimers. Pure gis—
piperylene (299%) was obtained from the Chemical Samples Co.,

Columbus, Ohio and was used without further purification.

11. 1,3—Cyclohexadiene. 1,3—Cyclohexadiene (Chemical

Samples Co., Columbus, Ohio) was used after one distillation
at 80.00.
C. Kinetic Measurements

1. Thymine. Thymine dissolves poorly in Acetonitrile.

Typically the weighed amount of thymine was placed in a

 

 

51

100 Tnl volumetric flaSk with ~80 ml acetonitrile and a small
stirring bar was used to stir the mixture overnight. The
stirrer was removed by use of a large magnet on the outside
of the flask. After washing the stir-bar with fresh aceto-

nitrile, the flask was brought up to the mark with solvent.

a. Stern-Volmer Quenching Studies. The thymine
stock solution was prepared by weighing 18.5 mg of thymine
into a 100 ml volumetric flask with the addition of purified
acetonitrile as described above. After the solid dissolved
the flask was filled to the mark which resulted in a
1.465 x 10-3M solution. To prepare piperylene 86.0 mg were
weighed into a 25 ml volumetric flask and solvent added to
the mark, which resulted in a 5.05 x 10_2M solution. One ml
of this solution was diluted to 10 ml to give a 5.05 x 10_3M
solution which was further diluted 1 to 25 to leave a stock
solution 2.02 x 10_4M in piperylene. The solutions for the

run were prepared as in Table VII.

Table VII. Preparation of samples_for Stern—Volmer quench—
ing study of 7.32 x 10 4M thymine.

 

 

 

Samplea How Made [Piperylene]
10 M
1,1a 5ml Thy stock + 1ml pip stock/to 10ml 2.0
2,2a 5ml Thy stock + 2ml pip stock/to 10ml 4.0
3,3a 5ml Thy stock + 3ml pip stock/to 10ml 6.1
4,4a 5ml Thy stock + 4ml pip stock/to 10ml 8.1
5,6,7 5ml Thy stock + 00 pip stock/to 10ml 0.0
8 5ml Thy stock + 00 pip stock/to 10ml 0.0

 

aAll samples contained 7.32 x 10_4M thymine.

 

 

 

52

Two 3-(3 ml aliquots of each SOlution were added by syringe
to Separate 13 x 100 mm Pyrex tubes, constricted about 10 mm
from the open end, After all the samples were prepared

in this manner, they were degassed three times by freeze—
thaw at less than 0.05 torr using liquid nitrogen. The
sample tubes were sealed under vacuum and irradiated for

6.6 hours in the previously mentioned apparatus. Sample 8
was analyzed after 4 hours to check progress of the reaction.
The tubes were opened and each sample diluted 2/10 for uv
analysis. The absorbances were converted to concentrations
(by use of the Beer's Law plot‘ and these were corrected to
values before dilution by multiplying by five. Each concen—
tration was subtracted from the starting material concentra—
tion in order to determine amount of dimer formed. Ratios
were then taken between the unquenched sample [5,6] and each
quenched sample [1,2,3,4] to give the 00/0 values. The
values for this run are in Table VIII.

Table VIII. Results of Stern—Volmer quenching study of
7.32 x 10 4 thymine.

 

 

 

[Thy]
sample A270nm 10‘5M x 5 [Thy]o-[Thy]x 00/0
1 .864 1.36 _5
1a .862 1.36 6.80 x 10 .050 2.20
2 .856 1.345
2a .854 1 345 6 725 .0575 1.91
3 .844 1.33
3a .844 1.33 6.65 .065 1.70
4 .828 1.30
4a .821 1.29 6.475 .0825 1.33
5 .789 1.24
6 .792 1.24 6 20 110 $0
[Thy]o .924 1.45
[Thy]o .928 1.46 7.30 —-— -—-

 

 

 

 

 

53

The ratio 00/0 was plOtted versus the corresponding
piperylene concentration and the resulting straight line
graph intercepted at unity and had a slope (equal to qu)
of 1.57 x 104 (Figure I). This procedure was repeated for
several runs at different thymine concentrations (Tables
XX to XXIV). The piperylene concentration was set to keep
¢0/® ratios less than 3.0 (at this value over 66% of the
reaction is quenched and very low conversions result in
larger error in single points). The qu values at each

concentration are recorded in Table I.

b. Determination of ©dim' Thymine stock solution
was prepared as before to make 1.54 x 10—3M solution. To
prepare pure pig—piperylene solution 283.0 mg were dissolved
in 5 ml of acetonitrile to give a 0.830M solution. Samples

were made up as.

Quenched Samples

3 ml piperylene stock plus 10 ml thymine stock diluted
to 25 ml.

Unguenched Samples
10 ml thymine stock diluted to 25 ml.

The quenched samples contained 0 10M pip—piperylene and

6.15 x 10_4M thymine. The unquenched samples contained only
6.15 x 10—4M thymine. A total of three quenched and three
unquenched tubes were made using exactly 3.0 ml of sample
solution and were degassed, sealed, and irradiated for nine

hours. The tubes were opened and analyzed. The unquenched

tubes (diluted 1/5 for uv measurement) indicated a final

 

 

 

 

54

thYHUJie concentration of 5-50 x 10-4M, showing that 0.65 x
-4
10 M thymine disappeared. Half of this value is the amount

of excited thymine that went on to dimer: 0.325 x 10—4M.

 

The quenched samples were analyzed by vpc (column b for

the amount of trans—piperylene formed: 1.27 i 0.10%.

 

Since the excited piperylene decays to both gis— and trans-
forms in a known ratio (63' calculations for the total
amount of piperylene excited (and thereby the total amount
of thymine excited‘ can be made;

[trans piperylene]

 

= 1.22 at the steady state
[Cis—piperylene]

[cis—piperylene]x % trans = moles trans (.101)(.0127) = .00128
moles trans : 1.22 2 moles cis (.00128)/(1.22) = .00105
moles trans + moles cis = moles excited .00233

The above calculation neglects back conversion for moles of
cis formed and re—excited, but for extremely low conversion
this is not necessary Finally, taking the ratio of moles

of excited thymine which dimerized to the total moles of

 

thymine that reached triplet gives-

 

3,25 X 10_5 : 1.40%

2.33 x 10‘3
Therefore 1.40% of excited triplet thymine molecules eventu—
ally dimerize. Since wisc is equal to 0.18 (29), ¢dim under

these conditions is only 0 0025.

(®iSC)(excited thymines that dimerize) = ®dim

(0.18) (0.0140) - 0.0025 .

 

 

 

 

55

c. Completely Quenched Reaction. A thymine stock

 

Solution (9.85 x 10_4M\ and a piperylene stock solution
(5.00 x 10‘2M\ were prepared and diluted to make quenched
and unquenched samples of thymine. The 0.01M quencher con—
centration was sufficient to stop over 99.5% of the triplet
reaction The starting concentrations and results are in

Table IX.

Table IX Results of completely quenched thymine irradia—

 

 

 

 

tion.
5 1 a [Th 1 [p‘ 1 I d b
amp e y 0 1p rra . a
-4 M Time [ThY]fin ” RX”
10 M
3 U 2 95 —0— 2.0 hr 2.62 11%
3 Q 2.95 0 010 20.1 3.07 0% (0%)
5 U 4.92 —0— 4.0 4.05 18%
5 Q 4 92 0.010 39.7 4.85 1% (.1%)
7 U 6.89 —0- 6.0 5.95 14%
7 Q 6.89 0 010 60 0 6.60 4% (.4%)
aU, represents unquenched; Q, represents quenched. bThe

% Rx. of quenched samples is divided by 10 to account for
that much longer irradiation time,
2 Uracil Uracil dissolved even less readily than
did thymine in acetonitrile. Consequently the same technique

of stock solution preparation was followed for uracil.

a. Stern-Volmer Quenching Studies. The method of
determination of kqm values for uracil at different concen—

trations was exactly analogous to that for thymine. The

initial uracil concentrations were somewhat lower

 

 

 

 

 

56

i0-40 to 0.20 x 10-3M) and this enabled uv analysis of un—
diluted samples to be measured at 270 nm, thus simplifying
the procedure. The Stern-Volmer data for each concentration
of uracil are found in Tables XXV to XXIX and the correspond—
ing plots in Figure I. The resulting slopes (qu) are

listed in Table I.

b Determination of The determination of

a . .
dim

o for uracil was again completely analogous to that for

dim
thymine. It was found that 0.042 x 10-3M uracil dimerized
and that 0.484% trans—piperylene was formed, which indicated

0.880 x 10-3M uracil molecules were excited. A ratio of

these values indicated 4 84% of excited uracil molecules

 

actually dimerized. Lamola‘s value (29) of 0.40 for ¢isc
yields 0.019 for tdim‘
(1. )'(excited uracils that dimerized) - 0 .
isc dim
(0.40) - (0 0484) = 0.019.
3, Cyclopent—Z—enone
a. Stern-Volmer Quenching Studies. To prepare a

 

stock solution of cyclopentenone 6.233 g were weighed into

a 10 ml volumetric flask: this was diluted to the mark with
acetonitrile, which resulted in a 7.59M solution. Iso—
phthalonitrile (IPN) was used as the internal standard.
Solution Q‘of IPN (0 0556M) was prepared as follows: 71.6 mg

Vwere weighed into a 10 ml volumetric flaskIthen diluted to

the mark. This solution was diluted 5/10 to give stock

 

 

 

 

 

57

solution 8 (0.0278M). Piperylene was similarly made in two
stock solutions, Dilution of 687.2 mg to 10 ml gave solu-
tion A (1.01M) and 3 ml of A was further diluted to 10 ml
to yield solution 8 (0.30M). The solutions for the run
were prepared as in Table X.

Table X. Preparation of samples for Stern—Volmer quenching
study of 1.52M cyclopent—2—enone.

 

 

 

Sample Enone IPN Pip Diluted
Stock Stock Stock to
1 1 ml + 1 ml + 2 ml A 5 ml
2 1 ml + 1 ml + 1 ml A 5 ml
3 1 ml + 1 ml + 2 ml B 5 ml
4 1 ml + 1 ml + 1 ml B 5 ml
5,6 2 ml + 2 ml + —0— 10 ml

 

The concentrations for this run are found in Table XI.

Table XI. Concentrations of samples in Stern—Volmer quench—
ing study of 1.52M cyclopent-Z—enone

 

 

 

Sample [Enone], M [IPN], M [Piperylene], M
1 1.52 0.00556 0.40
2 1,52 0 00556 0.20
3 1.52 0.00556 0.12
4 1.52 0.00556 0.06
5,6 1 52 0.00556 0.00

 

Exactly 3.0 ml of each sample was placed in the con—

stricted Pyrex tubes, degassed, sealed and irradiated for

 

 

   

 

58

5.3 hours. The samples were opened and analyzed by vpc
using column a. The IPN had a retention time of 2.3 min.;
the head—to—tail dimer, 5.5 min.; and the head-to—head dimer,
7.5 min., at 170°. Table XXXIII contains the dimer to
standard ratios and :0 : calculations. The vpc trace is
reproduced in Figure X.

Plotting of these results according to the Stern-
Volmer expression gives an excellent straight—line graph
(Figure IV), with a slope (kqt) of 9.1. The ratio of HTrHH
dimer of 55:45 in this run is typical to that found for all
cyclopentenone kinetic experiments. Several other (Tables
XXX to XXXII) quenching runs were done (at enone concentra—
tions down to 0.50M) and the slopes (qu) are found in

Table IV.

b Determination of Qi c‘ To prepare a stock solu—

S

 

tion of cyclopentenone (2.48M) 2 030 g were diluted to 10 ml.
The quantity 0 681 g of gig—piperylene weighed into a 25 ml
volumetric flask foDowed by addition of solvent to the mark
gives a 0.400M stock solution. Acetone (purified by dis—
tillation from potassium permanganate) was used to make the
actinometer solution. The sample solutions were prepared

as in Table X11 and the concentrations which resulted are
listed in Table XIII. The samples were placed in tubes as
before and irradiated for two hours. The tubes were opened

and analysed by vpc. Column 2 was used to determine the

percent trans—piperylene formed. By use of the relationship:

 

 

 

A.

 

59

Table XII. Preparation of samples for determination of ¢

of cyclopent-2—enone. iSC

 

 

 

 

 

 

 

 

 

 

 

 

Enone cis—Pip Diluted
Sample Stock Stock to

1 2 ml + 4 ml 10 ml

2 2 ml + 3 ml 10 ml

3 2 ml + 2 ml 10 ml

4 2 ml + 1 ml 10 ml
Actinometer 2 mla + 6 ml 25 ml
aPure acetone.

Table XIII. omposition of samples for determination of
; of cyclopent—2—enone.
lSC
Sample [EnoneJ [Cis—Piperylene] [Acetone]
M M

1 0 495 0 160 ——

2 0 495 0.120 ——

3 0 495 0 080 ——

4 0.495 0 040 ——
Actinometer —— 0.096 1 06M
Table XIV Results of determination of wisc of cyclo—2—

enone

Sam 1e 4 tran“ lr za 3lPi 1* w 5 ‘1

p ” ———L3 I p c—>t c—>t
Actinometer 11.2 0 224 0 0388 —— ——

1 6 01 0 114 0.0328 0.846 1.18

2 7 68 0.148 0.0319 0.823 1.21

3 9.63 0.182 0.0252 0.675 1.48

4 13.36 0 274 0 0198 0.510 1.96

4a 13 15 0 277 0.0203 0.503 2.01

 

aZ represents (0.555)/(0 555 — % trans).

 

 

 

 

60

. . .555
[Cis-piperylene]O ln 7555'?_%"E?§H§

= [excited triplet piperylene]
The amount of triplet piperylene formed in each sample can
be calculated. This value divided by the amount of triplet
piperylene formed in the actinometer samples determined

the These results are listed in Table XIV. The

'r»t.-

1 :c--t plotted versus the reciprocal gig—piperylene con—
centration gives a straight line whose intercept is 1/0isc
(Figure VIII) The value for the intercept is 0.95. This
makes ;isc = 1 05 which is within experimental error of
unity.

c Regiprocal Quantum Yield. Cyclopentenone

 

(2 5549 g) was diluted to 25 ml to give a 1.250M stock solu—
tion. Isophthalonitrile (IPN) was again used as the internal
standard Exactly 40 3 mg was dissolved in acetonitrile in

a 10 ml volumetric flask, which gave a 0 0314M stock solu—
tion. The actinometer was acetophenone sensitized isomeri—
zation of gig—piperylene. To prepare this solution both

the acetophenone (0 310 g) and the gig—piperylene (0.2564 g)
were weighed into a 25 ml volumetric flask and diluted with
solvent. This resulted in a 0.1508M gig—piperylene solution
which was used directly to make the samples. The samples

which contained varying enone concentrations were prepared

as in Table XV.

 

 

 

 

 

 

61

Table XV. Preparation of samples for dependence of quantum
yield on concentration of cyclopent—Z—enone.

 

 

 

Enone IPN Diluted [Enone] [IPN]
Sample Stock Stock to M 10_3M
2 2 ml + 1 ml 10 ml 0.25 3.14
3 3 ml + 1 ml 10 ml 0.375 3.14
4 4 ml + 1 ml 10 ml 0.50 3.14
8 8 ml + 1 ml 10 ml 1.00 3.14

 

The samples were placed in tubes and prepared as before.
They were irradiated for one hour, opened and analyzed.
The amount of trans—piperylene (actinometer) was measured
on vpc column b and the ratio of counts of standard versus
dimers were determined on column a at 170°. The total num—
ber of piperylene triplets was determined as before by use

of the equation-

555

[Cis—piperylenelo 1“ 7335‘:7%‘EE§H§

0.1508 ln = 0.0157M

.555
.555 - .055

Finally, to calculate the actual quantum yields the total
amount of dimers formed is found and divided by the total
amount of phonms absorbed (0.0157M). These values are
found in Table XVI. A plot of reciprocal quantum yield
versus reciprocal enone concentration (Figure III) gives
a straight line which intercepts at 1/¢>CO and has a slope

of (1/¢OO)(kd/ka). The quantum yield at infinite enone

concentration is 0.36 and the ratio of rate constants is 0.06.

 

 

 
 

 

 

 

62

Table XVI_ Results of dependence of quantum yield on con—
centration of cyclopent—Z—enone.

 

 

 

 

. a Moles —1
Sample Dim/IPN S F, [IPN] Dimer 0 0
2 2 01 x 0 853 x _00314 = .00538 .342 2-93
3 1 90 x 0 853 x 00314 = 00510 .324 3.09
4 1 81 x 0 853 x 00314 = .00485 .308 3.25
8 1 72 x 0 853 x 00314 = .00460 .292 3.42
a

S F represents the vpc standardization factor.

4. Cyclohex—Z—enone

a. Stern—Volmer Quenching Studies. The quenching

 

studies for cyclohexenone were done in an analogous manner
to those described in detail for cyclopentenone. Ethyl
Stearate was used as the internal standard and had a reten—
tion time (Vpc column a at 190°) of 3.0 min. The head—to—
tail dimer (r? = 4 8 min, and the head-to—head dimer (rt =
6.2 min, were found in a ratio of 36/64 in all the kinetic
experiments. A typical vpc trace is reproduced in Figure X.
Both 1 3—pentadiene and 1,3—cyclohexadiene were used as
quenchers The results for the quenching runs at various
enone concentrations are indicated in Tables XXXIV to
XXXVIII, the kqa values in Table IV, and the plots given by

the Stern—Volmer expression are in Figure IV.

b. Determination of m. , The determination of 0.
159 15c

 

was done in the same manner as for cyclopentenone. The

 

 

 

 

 

 

 

63

cyclohexenone concentration was held constant (0.500M) and
the gig—piperylene was varied from 0.04 to 0.16M. Aceto—

phenone was the sensitizer for the actinometer. A plot of
the results is in Figure VIII and the straight line graph

gives a quantum yield for intersystem crossing of 1.05,

which can be taken as unity.

c. Reciprocal Quantum Yield. Again the reciprocal

 

quantum yield determination was done the same way as that
of cyclopentenone, except for the use of ethyl Stearate as
the internal standard. The quantum yield at infinite enone
concentration was 0.75 (average of two runs) and the ratio
of rate constants (kd ka) was 2.67. The results are listed

in Table XL and plotted in Figure III.

d. Sensitized Formation of Dimers. Stock solutions of

 

cyclohex—2-enone (1.25M‘ and phenanthrene (0.276M) were
prepared in acetonitrile. These were diluted to make three
samples which had the compositions as indicated in Table
XVII. These concentrations were expressly chosen to vary

the light absorption of the enone (also noted in Table XVII).
The samples were placed in tubes, degassed and sealed as
usual. After irradiation for five hours, the tubes were
opened. The amount of dimer formation relative to added

ethyl Stearate standard was measured on vpc column a at

190°. The results are in Table XVIII.

 

 

 

 

 

.
11;. . 1.... .._..._§Es¢nm. 4.11, , ....mn..fi..,.....-..la i. | 7.... 1

 

64

 

 

 

Table XVII. Preparation of samples for phenanthrene
sensitized photodimerization of cyclohex—
2—enone.

[Enone] [Phenanthrene] % light absorbed

Sample M M by enone

0P 0.125 -0— 100
1P 0.125 0.027 32
SP 0.125 0.220 5

 

Table XVIII. Dependence of quantum yield of sensitized
dimerization of cyclohex—Z—enone on concen—
tration of phenanthrene.

 

 

 

. , a
Sample Moles Dimer wdim
0P 0.00228 0.033
1P 0.00131 0.019
8P 0.00049 0.007

 

aQuantum yield of dimerization without added sensitizer is
calculated from Figure III.

 

 

 

 

 

 

 

 

IV . LITERATURE CITED

 

 

 

 

 

 

 

 

 

11.

12.

IV. LITERATURE CITED

G. Ciamician and p. Silber, Ber., 35, 4129 (1902).

 

a; A. Mustafa, Chem. Rev., 51, 1 (1952).
b A. Schonberg, ”Praparative Organische Photochemie“,
Springer—Verlag, Berlin, 1958.

a» P. E. Eaton, J. Am. Chem. Soc., 84, 2454 (1962).

b) E. J. Corey, J. D. Bass, R. LeMahieu and R. B.
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c) P. deMayo, J—P. Pete, and M. Tchir, Can. J. Chem.,
46“ 2535 (1968).

 

E. J. Corey, R. B. Mitra, H. Uda, J. Am. Chem. Soc.,
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J. D. white and o. N. Gupta, ibid., §§u 5364 (1966).

R. w. Guthrie, Z. VaLenta, and K. Wiesner, Tet. Letters,
4645 (1966).

G. L. Lange and P. deMayo, Chem. Comm., 704 (1967).

B. D. Challand, G. Kornis, G. L. Lange and P. deMayo,
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a) M. Pfau, R. Dulou, and M. Vilkas, Compt. Rend.,
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b) A. M. Moore, Can. J. Chem., 36, 281 (1958).

c) B. J. Ramey and P. D. Gardner, J. Am. Chem. Soc.,
89“ 3949 (1967).

P. E. Eaton, Accounts Chem. Res., 1, 50 (1968).
R. A. Schneider and J. Meinwald, J. Am. Chem. Soc.,

§2, 2023 (1967).

0‘9)

P. deMayo and H. Takeshita, Can. J. Chem., 41, 440
(1963).

P. deMayo, S. T. Reid and R. W. Yip, ibid., 42“ 2828
(1964).

65

 

 

 

13.

14.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

66

E. J. Corey, M. Tada R. LeMahieu and L.
. Libit, J. .
Chem. Soc., 81, 2051 (1965). Am

 

P. E. Eaton and K. Lin, ibid., 86, 2087 (1964).

O. L. Chapman, Advan. Photochem., 1, 323 (1963).

 

W. G. Dauben, G. w. Shaffer and N. D. Vietmeyer, J.
Org. Chem., EB» 4060 (1968). —

 

F. L. Gates, Science, §§/ 479 (1928).

Abbreviations used throughout the paper: DNA, deoxy—

ribonucleic acid; RNA, ribonucleic acid: T or Thy,
thymine: U or Ura, uracil.
A. Rorsch, R. Beukers, J. I] lst and w. Berends,

,—

Rec. Trav. Chim., 11, 423 (1958).

R. Beukers and w. Berends, Biochim. Biophys. Acta,
44’, 550 (1960

Proceedings International Symposium on ”Basic Mechanisms
in Photochemistry and Photobiology" contained in en—
tire issue x6 of Photochem. Photobiol., Z (1968). It
contains references to virtually all aspects of the
photochemistry of biological systems.

R. B. Setlow, Photochem. Photobiol., Z, 643 (1968).

D. L. Wulff and G. Fraenkel, Biochim. Biophys. Acta,
51, 332 (1961

"N

a; G. M. Blackburn and R. J. H. Davies, Chem. Comm.,

215 (1965).
b; T. Kunieda and B. Witkop. J. Am. Chem. Soc., 89,

4232 (1967).
S. Y. Wang, Nature, 190, 690 (1961).

w. FUchtbauer and P. Mazur, Photochem. Photobiol., 2,

323 (1966).

J. Eisinger and R. G. Shulman, Proc. Nat. Acad. Sci.

U.S., 58“ 895 (1967).

A. A. Lamola and J. Eisinger, ibid., 59, 46 (1968).

A. A. Lamola and J. P. Mittal, Science, 154, 1560

(1966).

 

 

 

 

 

 

Ill .3.» 1.5.... LI

30.

31.

32.
33.

34.

36.

37.

38.

39.

40.

41.

42.

43.

44.
45.

46.

47.

48.

 

67

H. Morrison and R. Kleopfer, J. Am. Chem. Soc., 90,
5037 (1968). “”

C. L. Greenstock and H. E. Johns, Biochem. Biophys.
Res. Comm., 19“ 21 (1968).

C. H. Krauch, Ang. Chemie (Int. Ed.), 6, 956 (1967).

 

 

C. H. Krauch, Photochem. Photobiol., 6, 497 (1967).

A. Wacker, H. Dellweg, and D. Weinblum, Naturwissen—
shaften, 41, 477 (1960).

a H. Schuster, Z. Naturforsch, 19b, 815 (1964).
b) V. Merriam and M. P. Gordon, Photochem. Photobiol.,
6, 309 (1967).

 

0. L. Wulff, J. Mol. Biol., 1, 431 (1963).

a) A. A. Lamola and T. Yamane, Proc. Nat. Acad. Sci.
U.S., 58, 443 (1967).
b) A. A. Lamola, Photochem. Photobiol., Z, 619 (1968).

 

G. 8. Hammond, C. A. Stout, and A. A. Lamola, J. Am.
Chem. Soc., §§u 3103 (1964).

G. O. Schenck, S. Farid, and C. H. Krauch, Chem. Ber.,
22, 625 (1966).

H. Morrison, H. Curtis, and T. McDowell, J. Am. Chem.
Soc., 88“ 5415 (1966).

O. L. Chapman, P. J. Nelson, R. W. King, D. J. Trecker,
and A. A. Griswold, Rec. Chem. Prog., 18, 167 (1967).

P. E. Eaton, J. Am. Chem. Soc., 84, 2344 (1962).

P. A. Leermakers and J. L. Ruhlen, ibid., 88, 5672
(1966).

P. E. Eaton and W. S. Hurt, ibid., p. 5038.
J. L. Ruhlen and P. A. Leermakers, ibid., 82/ 4944 (1967).

E. Y. Y. Lam, D. Valentine, and G. S. Hammond, ibid.,
p. 3482.

Private communication from P. deMayo.

J. G. Calvert and J. N. Pitts, Jr., "Photochemistry”,
John Wiley and Sons, Inc., New York, N.Y., 1966, p. 587.

 

 

 

61.

62.

63.

68

P. J. Wagner and D. J. Bucheck, J. A .
6532 £1968). m Chem. Soc., 2Q

I

 

P. J. Wagner and I. Kochevar, ibid., p. 2232.

A. A. Lamola and G. S. Hammond, J. Chem. Phys., 43,

 

2129 (1965).
D. I. Schuster, A. C. Fabian, N. P. Kong, W. C. Bar—
ringer, W. V. Curran, and D. H. Sussman, J. Am. Chem.
Soc., 90/ 5027 (1968).
H. E. Zimmerman and K. G. Hancock, ibid., p. 3749.

I. H. Brown and H. E. Johns, Photochem. Photobiol.,
8, 273 (1968).

Private communication from A. S. Kende and P. J. Wagner.

S. Toki and H. Sokurai, Bull. Chem. Soc. Japan, 40,
2885 (1967). “”

Wagner, J. Am. Chem. Soc., 82“ 5898 (1967).
Wagner and A. E. Kemppainen, ibid., 22x
5898 (1968).

a)
b)

UI'U'U
(DC—«L4

89

D. Weinblum and H. E. Johns, Biochim. Biophys. Acta,
114, 450 (1966).

R. S. H. Liu and J. R. Edman, J. Am. Chem. Soc., 90,
213 (1968).

J. F. O'Donnell, J. T. Ayres, and C. K. Mann, Anal.
Chem., 37, 1161 (1965).

E. W. Garbisch, Jr., J. Org; Chem., 30, 2109 (1965).

D. Valentine, N. J. Turro, and G. S. Hammond, J. Am.
Chem. Soc., 86, 5202 (1964).

 

Saltiel, A. A. Lamola, N. J. Turro,

G. S. Hammond, J.
v. Vogt

J. S. Bradshaw, D. O. Cowan, R. C. Counsell,
and C. Dalton, ibid., p. 3197.

 

 

 

 

 

 

 

 

 

 

V . APPENDIX

 

 

 

69

 

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71

Table XIX. Results for determination of ¢'sc for cyclo—
hex~2—enone. l

 

 

 

Samplea [cis—Piperylene] % Trans ac—>t ©c—>t 1
Actinometerb 0.096M 9.71
Actinometer 0.096 9.61
Actinometer 0.096 9.61
1 0.040 10.58 0.467 2.14
2 0.080 7.15 0.610 1.695
3 0.120 5.61 0.701 1.43
4 0.160 4.53 0.753 1.33

 

 

aNumbered samples contained 0.500M cyclohex-2—enone.

bActinometer contained 0.98M acetophenone.

Table XX. Results ofi Stern—Volmer quenching study of
2.99 x 10 4M thyminea.

 

 

 

. —5 —4
Sample [Piperylene], 10 M [ThY]disap’ 10 ®O/®
1 1.25 O 43 1.43
2 2.50 0 39 1.57
3 3.75 0 31 1.99
4 5.00 0.27 2.28
to —0— 0.61 1.00

 

aPlotted in Figure I. Irradiated 2.2 hrs. A270nm of 1/5
diluted Thyo - 0.760.

 

 

 

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72

 

 

 

 

 

 

 

 

73

Table XXI. Results o§ Stern—Volger quenching study of
4.67 x 10 4M thymine

 

 

5 4

 

Sample [Piperylene], 10 M [Thy]disap,10 M ¢o/¢
1 1.55 0.345 1.39
2 3.10 0.29 1.65
3 4.65 0.245 1.96
4 6.20 0.21 2.29
9o —0— 0.48 1.00

 

aPlotted in Figure I. Irradiated 6.0 hrs. A270nm of 1/5
diluted Thyo = 0.835.

Table XXII. Results o§ Stern—Volger quenching study of
7.30 x 10 4M Thymine .

 

 

 

. —5 —4 /
Sample [Piperylene], 10 M [Thy]disap,10 M 00/0
1 2.02 0.825 1.33
2 4.04 0.65 1.70
3 6.06 0.575 1.91
4 8.08 0.50 2.20
:0 —0- 1.10 1.00

 

aPlotted in Figure I. Irradiated 6.6 hrs. A270nm of 1/5
diluted Thy0 = 0.926.

Table XXIII. Results o§ Stern—Volger quenching study of
1.00 x 10 3M thymine

 

 

 

. —4 —4
Sample [Piperylene], 10 M [Thy]disap,10 M ¢0/®
1 0.256 0.47 1.48
2 0.512 0.42 1.68
3 0.768 0.34 2.06
4 1.02 0.315 2.22
00 —0— 0.70 1.00

 

aPlotted in Figure I. Irradiated 7.2 hrs. A270nm of 1/5
diluted Thy0 = 1.283.

 

 

 

 

74

Table XXIV. Results of Stern—Volmer quenching study of
1.05 x 10 3M thymine

 

 

 

Sample [Piperylene], 10 4M [Thy]disap’10_4M 00/0
1 0.254 1.33 1.35
2 0.508 1.14 1.58
3 0.762 1.01 1.79
4 1.016 0.88 2.04
30 —0— 1.80 1.00

 

aPlotted in Figure I. Irradiated 5.0 hrs. A27onm of 1/5
diluted Thyo = 1.351.

Table XXV. Results o§ Stern—Voémer quenching study of
2.12 x 10 4M uracil .

 

 

 

 

Sample [Piperylene], 10 5 [Ura]disap,10 4 80/8
1 2.5 0.24 1.79
2 5.1 0.19 2.25
3 7.6 0.18 2.35
4 10.1 0.165 2.64
00 —O- 0.43 1.00

 

aPlotted in Figure I. Irradiated 0.5 hr. A27onm for
Urao = 0.756.

Table XXVI. Results o§ Stern—Vo1mer quenching study of
2.22 x 10 4M uracil .

 

 

 

 

. -5 —4
Sample [Piperylene], 10 M [Ura]disap,10 M ¢0/¢
1 2.5 0.275 1.50
2 5.0 0.195 2.1()
3 7.5 0.16 2.54
4 10.0 0.15 2.69
00 -0— 0.41 1.0()
aPlotted in Figure I. Irradiates 0.5 hr. A270“m for

Urao = 0.788.

—_—

 

 

 

75

Table XXVII. Results o§ Stern-Vo1mer quenching study of

2.89 x 10 4M uracil .

 

 

 

 

 

 

 

. —5 —4

Sample [Piperylene], 10 M [uraldisap’lo M ¢0/¢
1 3.1 0.24 1.50
2 6.2 0.22 1.64
3 9.3 0.125 2.88
4 12.4 0.11 3.32
00 —0— 0.36 1.00

aPlotted in Figure I. Irradiated 4.0 hrs. A270nm for

Ura0 = 1.017.

Table XXVIII. Results o§ Stern—Voémer quenching study of

3.44 X 10 4M uracil .
. -4 -4

Sample [Piperylene], 10 M [Ura]disap,10 M ¢o/¢
1 0.40 0.28 1.72
2 0.80 0.24 2.00
3 1.20 0.18 2.65
4 1.60 0.17 2.78
90 —0— 0.48 1.00

 

aPlotted in Figure I. Irradiated 0.5 hr. A270nm for
Urao = 1.208.

Table XXIX. Results of Stern—Vo1mer quenching study of

4.37 x 10—4M uracil .

 

 

—4

 

Sample . [Piperylene], 10 M [Ura]disap.10 4M ¢O/¢
1 0.51 0.34 1.64
2 1.03 0.265 2.11
3 1.55 0.19 2.94
4 2.06 0.175 3.24
00 —0— 0.56 1.00

 

aPlotted in Figure I. Irradiated 0.5 hr. A270nm for
Urao : 1.513.

 

 

 

 

    
  
 
    
   
   
   
   
  
  
   
  
     
   
    
    
  
    
 
     
 
    
   
   

76

Table XXX. Results of Stern-Volmer guenching study of
0.500M cyclopent-Z—enone

 

 

 

Sample [Piperylene], M Dim/Stndb ¢o/¢
1 0.040 0.921 2.02
2 0.080 0.628 2.96
3 0.133 0.421 4.41
4 0.266 0.227 8.20
:0 —0— 1.86 1.00

 

aPlotted in Figure IV._ Irradiated 2.0 hrs. bIPN, internal
standard, at 5.9 x 10 3M analyzed on vpc column a at 170°.

Table XXXI. Results of Stern-Volmer guenching study of
0.750M cyclopent—2-enone

 

 

 

Sample [Piperylene], M Dim/Stndb ¢o/¢
1 0.046 1.46 1.73
2 0.092 1.06 2.46
3 0.135 0.767 3.30
4 0.270 0.407 6.22
90 —O— 2.53 1.00

 

aPlotted in Figure IV._ Irradiated 4.3 hrs. bIPN, internal
standard, at 6.4 x 10 3M, analyzed on vpc column 3 at 170°.

Table XXXII. Results of Stern—Volmer quenching study of
a
1.00M cyclopent—2-enone

 

 

 

Sample [Piperylene], M Dim/Stndb ¢O/¢
1 0.082 3.86 2.00
2 0.164 2.55 3.01
3 0.270 1.69 4.53
4 0-544 —— —-
$0 -0- 7.71 1.0()

 

aPlotted in Figure IV. _Irradiated 2.3 hrs. bIPN, interna]_
standard, at 1.88 x 10 3M, analyzed on vpc column a'at 17C”).

 

 

77

Table XXXIII. Results of Stern—Volmeraquenching study of
1.52M cyclopent—Z-enone

 

 

 

Sample [Piperylene], M Dim/Stndb 80/8
1 0.06 3.37 1.55
2 0.12 2.42 2.16
3 0.20 1.83 2.86
4 0.40 1.12 4.67
.0 —0— 5.24 1.00

 

aPlotted in Figure IV. _Irradiated 5.3 hrs. bIPN, internal
standard, at 5.56 x 10 3M; analyzed on vpc column a at 170°.

Table XXXIV. Results of piperylene Stern-Volger quenching
study of 0.25M cyclohex—2-enone .

 

 

 

b

 

Sample [Piperylene], M Dim/Stnd ¢o/¢
1 0.0274 0.3815 1.47
2 0.0548 0.295 1.90
3 0.0915 0.208 2.69
4 0 183 0.121 4.61
to -0- 0.5595

 

b
aPlotted in Figure IV. Irradiaged 4.3 hrs. Ethyl stearate,
internal standard, at 4.4 x 10 3M7 analyzed on vpc column g
at 190°.

Table XXXV. Results of piperylene Stern—Volger quenching
study of 0.50M cyclohex—2—enone

 

 

 

Sample [Piperylene], M Dim/Stndb ¢O/¢
1 0 .0294 0 .540 1 .48
2 0.0586 —— ——
3 0.0980 0.286 2.8()
4 0.196 0.173 4.62
(D0 —0- 0.799 1.00

 

aPlotted in Figure IV. Irradiated 3.2 hrs.

b

 

Ethyl stearate ,

internal standard, at 4.16 x 10_3M; analyzed on vpc columl1 a

at 190°.

 

 

 

 

 

78

Table XXXVI. Results of piperylene Stern—Volger quenching
study of 0.48M cyclohex-Z—enone

 

 

 

 

Sample [Piperylene], M Dim/Stndb ¢O/¢
1 0.015 0.967 1.25
2 0.030 0.794 1.52
3 0.049 0.639 1.89
4 0.098 0.435 2.76
60 -0- 1.205 1.00
aPlotted in Figure IV. Irradiated 4.5 hrs. bEthyl stearate, 2,1
internal standard, at 4.42 x 10 3M; analyzed on vpc column a .3.
at 190°. N 5.;

Table XXXVII. Results of piperylene Stern—Volger quenching
study of 1.00M cyclohex—2—enone

 

 

 

Sample [Piperylene], M Dim/Stndb 40/0
1 0.029 0.719 1.43
2 0.058 0.582 1.76 ‘
3 0.097 0.446 2.30 V
4 0.194 0.279 3.68
50 —0— 1.026 1.00

 

aPlotted in Figure IV. Irradiated 2.7 hrs. bEthyl stearate,
internal standard, at 4.98 x 10 3M; analyzed on vpc column 3
at 190°.

Table XXXVIII. Results of 1,3—cyclohexadiene Stern—Volme
quenching study of 0.30M cyclohex—2—enone .

 

 

 

Sample [1,3—Cyclohexadiene], M Dim/Stndb ¢O,/¢
1 0 0238 0.395 1.’74
2 0.0476 0.248 2.77
3 0.0796 0.159 4.32
4 0 .159 0 .0824 8 .34
<30 —0— 0.687 1.00

 

aPlotted in Figure IV. Irradiated 3.5 hrs. bEthyl stea111te
internal standard, at 4.12 x 10 3M; analyzed on vpc colunm A
at 1900. ~

 

 

 

 

79

Table XXXIX. Results of 1,3—cyclohexadiene Stern—Volme
quenching study of 0.50M cyclohex—2-enone .

 

 

 

Sample [1,3—Cyclohexadiene], M Dim/Stndb 80/3
1 0.0088 0.944 1.20
2 0.0176 0.796 1.43
3 0.0293 0.660 1.72
4 0.0585 0.420 2.70
30 —0- 1.135 1.00

 

aPlotted in Figure IV. Irradiated 3.5 hrs. bEthyl stearate,
internal standard, at 4.16 x 10 3M; analyzed on vpc column 3
at 190°.

Table XL. Results for reciprogal quantum yield determination
of cyclohex—2—enone

 

 

 

Sample [Enone]0,M Dim/Stndb [Dim],M 4C ¢_1
2 0.25 0.383 0.00149 0.0646 15.48
3 O 375 0.541 0.00210 0.0913 10.92
4 0-50 0.681 0.00264 0.115 8.72
8 1.00 1.18 0.00458 0.200 5.00

 

aPlotted in Figure III. bBased on actinometer (0.10M aceto—
phenone end 0.104M cis—piperylene) isomerization to 11.360
trans. Ethyl stearate, internal standard, at 2.11 x 10 3M»

 

 

 

Sample [Enone]O,M Dim/Stnda [Dim],M @b ¢—1
2 0.25 0.443 0.00162 0.063 15.8()
3 0.375 0.638 0.00238 0.091 10.935
4 0.50 0.801 0.00298 0.114 8.374
8 1.00 1.445 0.00537 0.207 4.84

 

aEthyl stearate, internal standard, at 2.02 x 10_3M. bBased
on actinometer (0.10M acetophenone and 0.112M cis—piperyfiLerua)
isomerization to 11.48% trans.

 

 

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