ELECTRON SPIN RESONANCE STUDlES OF RADECALS EN [RRADIATED ACETATES Dissertation for the Degree of Ph. D. M‘EEEEGAN} STATE UNIVERSITY REHARD CLARK SCHOENING 1973 - 7.}? U €113“: LIB RA R Michigan Stat-t3 Universittj: This is to certify that the thesis entitled ELECTRON SPIN RESONANCE STUDIES OF RADICALS IN IRRADIA'IED. ACETATES presented by Richard Clark Schoening. has been accepted towards fulfillment of the requirements for Ph . D . degree in Chemi stry WM77E3’Q‘OV3 Major pro s6: Date 1973 December 11 0-7639 ? amomc av " NUAG 8x SUNS' 800K BINDERY INC. LIBRARY emoms SPRINSPORT. mcmm ABSTRACT ELECTRON SPIN RESONANCE STUDIES OF RADICALS IN IRRADIATED ACETATES By Richard Clark Schoening Electron spin resonance studies of radicals produced in irradiated single crystals of sodium acetate have been carried out. In CH3COONa'3D20 crystals three radicals are formed; the methyl radical predominates at 77° K, the 'CHZCOO- radical becomes the dominant species at 198° K and at room temperature the -CO2- radical is found. The ESR parameters for each radical were determined. In ad- dition, °CD3 was studied in irradiated single crystals of CD3COONa'3D20 at 77° K. Both the 'CH3 and °CD3 radicals are fl-electron radicals undergoing rotation about their threefold axes at 77° K. Changes in relative intensities of the lines of the spectra on cooling to u.2° K indicate some restriction of rotation at low temperatures. Anhydrous sodium acetate single crystals produce only °CH2COO- radicals at 77° K. In addition, a triplet spectrum was observed which was attributed to a radical pair with approximately 5.77 3 distance between the two unpaired electrons. Although the components of the pair appeared -w Richard Clark Schoening "\) if to be different the spectra did not permit their positive identification. The electron spin resonance spectra of radicals produced in irradiated fluorine-substituted ammonium acetates were also studied. Irradiation of single crystals of ammonium monofluoroacetate produced ~CH2COO- and °CFHCOO- in the approximate ratio of ”:1. On warming to room temperature only -CFHCOO- was present. This room-temperature radical was found to occupy two magnetically inequivalent sites. On cooling, two 'CFHCOO- radicals were observed each with different A(F) and A(H) tensors and each occupying two magnetically distinct sites. The g,and the fluorine and the hydrogen hyperfine splitting tensors,were evaluated for both radicals at -1H0° C and for the radical at room temperature. In addition, the 13 C hyperfine interaction tensor was also obtained for the room-temperature radical. An oscillation of the CHF group of the 'CFHCOO' radical about the C-C bond was postulated to account for the de- pendence of the ESR spectra on temperature. However, changes in crystal structure may also occur. Irradiated single crystals of ammonium difluoroacetate produced two radicals, 'CFHCOO- and 'CFZCOO-, in approxi- mately equal proportions at room temperature. Measurements 13C hyperfine splitting of the maximum components of the tensors gave estimates of the isotropic components. The -CFHcoo' radical was found to be planar, while the ~cr2coo‘ Richard Clark Schoening radical is bent approximately 8° from a planar structure. At 77° K,relatively low concentrations of 'CFZH are believed present in addition to more abundant 'CFHCOO- and possibly a third radical. A maximum fluorine coupling of 211 G was assigned to the Azz(F) principal component of 'CF2H by analysis of the powder spectrum. The 'CF3 radical has been tentatively identified as probably arising from irradiation of CF3COONDu. A long- range coupling of lu.7 G from a neighboring atom was not eliminated in the deuterated salt as would be expected from a previous assignment of the radical to [CF3COOH--NH3]-. Single crystals of ammonium chlorodifluoroacetate were irradiated,and the ESR spectra observed,at 77° K. These show the presence of at least two radicals, one showing chlorine and fluorine hyperfine splitting and the second fluorine hyperfine splitting only. The radicals were assigned the structures 'CFZCl and 'CF2C00-. ELECTRON SPIN RESONANCE STUDIES OF RADICALS IN IRRADIATED ACETATES By Richard Clark Schoening A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1973 To My wife, Margaret ACKNOWLEDGMENTS The author wishes to thank Dr. Max T. Rogers for his guidance, encouragement and patience during the course of this investigation. Special thanks also go to Dr. S. Subramanian of the Indian Institute of Technology, Madras, India and to Dr. Roger V. Lloyd of the University of Connecticut for assistance with some experimental parts of this work. To Dr. William G. Waller, appreciation is expressed for use of computer programs and for helpful discussions. Financial support provided by Michigan State University, Department of Chemistry and by the Atomic Energy Commis- sion is gratefully acknowledged. Thanks also go to my colleagues and in particular, to Robert F. Picone for helpful discussions and constant encouragement throughout this study. Finally, I am indebted to my parents for their under- standing and support and to my wife without whose encourage- ment and love this study could not have been possible. iii TABLE OF CONTENTS Page LIST OF TABLES . . . . . . . . . . . . . . . . . vi LIST OF FIGURES . . . . . . . . . . . . . . . . vii CHAPTER I. Introduction . . . . . . . . . . . . . . 1 II. Theoretical Considerations . . . . . . . 20 A. Spin Hamiltonian . . . . . . . . . . 20 B. Hyperfine Interactions . . . . . . . 26 III. Experimental Aspects . . . . . . . . . . 32 Sample Preparation . . . . . . . . . 32 Crystal Morphology . . . . . . . . . 33 A B C. Sample Handling. . . . . . . . . . . 33 D Instrumentation. . . . . . . . . . . 36 E Studies Below 77° K. . . . . . . .-. 37 F Analysis of Data . . . . . . . . . . #1 IV. Irradiated Sodium Acetates . . . . . . . HM A. CH3COONa°3D20. . . . . . . . . . . . an 1. Analysis of Spectra at 77° K . . an 2. Analysis of Spectra at 198° K. . 50 3. Analysis of Room Temperature Spectra. . . . . . . . . . . . . 53 H. Analysis of Reaction Scheme. . . 59 B. CD COONa'BDZO. . . . . . . . . . . . 60 3 C. ENDOR Studies. . . . . . . . . . . . 61 iv Page D. Anhydrous Sodium Acetate. . . . . . 62 1. The Doublet ESR Spectrum. . . . 63 2. The Triplet ESR Spectrum. . . . 66 E. Methyl Radical Below 77° K. . . . . 73 V. Irradiated Fluoroacetates . . . . . . . 80 A. Ammonium Monofluoroacetate. . . . . 81 1. Analysis of Room Temperature Spectra . . . . . . . . . . . . 81 2. Analysis of Spectra at 77° K. . 9H 3. Variable Temperature Studies. . 95 B. Ammonium Difluoroacetate. . . . . . 11% 1. Room Temperature Radicals . . . ll” 2. Analysis of -CFHCOO' and °CF2COO' Spectra. . . . . . . . 119 3. Spectra at 77° K. . . . . . . . 122 C. Ammonium Trifluoroacetate . . . . . 123 D. Ammonium Chlorodifluoroacetate. . . 125 E. Radical Formation . . . . . . . . . 125 VI. Summary . . . . . . . . . . . . . . . . 129 REFERENCES. . . . . . . . . . . . . . . . . . . 133 Table 10 LIST OF TABLES A summary of motional averaging effects observed for radicals in irradiated organic single crystals by ESR methods . . . . . . . Summary of ESR results obtained for radicals trapped in irradiated single crystals of fluorine-containing organic compounds Isotropic hyperfine coupling constants and g values for fluorinated methyl radicals. . . . . . . . . . . . ESR parameters for radicals produced in y-irradiated single crystals of CH3COONa-3D20.. Summary of ESR results reported for methyl radical. . . . . . . . Summary of ESR results reported for -CH2COO' radical found in single- crystal matrices. . . . . . . . . Principal hyperfine splitting values _ and direction cosines for the -CH2COO radical produced at 77° K in irradiated single crystals of anhydrous sodium acetate ESR studies of radical pairs found in irradiated single crystals. . . Principal values and direction cosines for A(F), A(H), g and A(13C) for the -CFHCOO' radical produced by room temperature irradiation of ammonium monofluoroacetate . . . . . . . . Principal values and direction cosines for A(F) and A(H) for the two -CFHCOO' radicals produced at room temperature and observed at -1H0° C by irradiation of ammonium monofluoroacetate . vi Page 1U 18 H7 H8 SR 65 7O 87 108 Figure 5a 5b LIST OF FIGURES Page Liquid helium quartz Dewar for "flow— method" studies below 77° K. . . . . . . 39 Schematic diagram of the Andonian Associates variable-temperature throttle-system Dewar. . . . . . . . . . HO Second-derivative ESR spectra of °CH radical in y-irradiated single crystals of CH3COONa°3D20 at 77° K. . . . . . . . H6 Second-derivative ESR spectra for two orientations of the magnetic field with respect to the CHQCOO’ radical produced in irradiated CH3COONa 3D20 single crystals which have been warmed to ~198° K for five minutes. . . . . . . 52 Second-derivative ESR spectrum of -CO - radical produced in irradiated single crystals of CH3COONa 3D2O at room temperature. . . . . . . . . . . . . . 56 Second-derivative ESR spectrum of °CD radical produced in irradiated single crystals of CD3COONa-3D20 at 77° K . . . 56 Structure of the °C02- radical . . . . . 57 ESR spectra of radicals produced in irradiated anhydrous sodium acetate at 77° K . . . . . . . . . . . . . . . . 67 8(a-c) Second- derivative ESR spectra for CH3 and CD3 radicals at temperatures below 77° K. . . . . . . . . . . . . . . 76 Second-derivative ESR spectra for -CFHCOO' radical produced in irradiated single crystals of ammonium monofluoro- acetate at room temperature. . . . . . . 82 10(a-c) Variation with magnetic field orien— tation of the ESR hyperfine lines for the -CFHCOO' radical in irradiated single crystals of ammonium mono- fluoroacetate at room temperature. . . . 8” vii Figure Page 11 Plot of hyperfine splitting with respect to magnetic field direction for fluorine coupling in the -CFHCOO' radical in irradiated single crystals of ammonium monofluoroacetate at, room temperature . . . . . . . . . . . . 88 12 Plot of hyperfine splitting with respect to magnetic field direction for proton coupling in the °CFHCOO‘ radical in irradiated single crystals of ammonium monofluoroacetate at room temperature . . . . . . . . . . . . 89 13 Plot of g values with respect to magnetic field direction for the -CFHCOO' radical in irradiated single crystals of ammonium mono- fluoroacetate at room temperature. . . . 90 In Plot of hyperfine splitting with respect to magnetic field direction for carbon-13 coupling in the 'CFHCOO' radical in irradiated single crystals of ammonium monofluoroacetate at room temperature . . . . . . . . . . . . 92 15(a-i) Variation with temperature of the fluorine and proton hyperfine split- tings for the 'CFHCOO' radical . . . . . 97 16 Second-derivative ESR spectra for ~CFHCOO‘ radical produced in single crystals of ammonium monofluoro- acetate irradiated at room tempera- ture and observed at -1u0° C . . . . . . 102 17(a-c) Variation with magnetic field orientation of the ESR hyperfine lines for the -CFHCOO' radical in irradiated ammonium monofluoro- acetate at -1u0° C . . . . . . . . . . . 10H 18 Plot of hyperfine splitting with respect to magnetic field direction for fluorine coupling in the 'CFHCOO- radicals in irradiated single crystals of ammonium monofluoroacetate at -lu0° C. . . . . . . . . . . . . . . . . 109 viii Figure Page 19 Plot of hyperfine splitting with respect to magnetic field direc- tion for proton coupling in the °CFHCOO’ radicals in irradiated single crystals of ammonium mono- fluoroacetate at -lu0° C . . . .'. . . . 111 20 Variation with magnetic field orientation in the ac plane of the ESR hyperfine lines for the -CFHCOO' and -CF2COO‘ radicals produced in irradiated single crystals of ammonium difluoro- acetate at room temperature. . . . . . . 116 21 Second-derivative ESR spectra of the room-temperature radicals, -CFHCOO‘ and CF COO', produced in irradiated singIe crystals of am- monium difluoroacetate . . . . . . . . . 117 22a Second-derivative ESR spectrum of the radicals produced in single crystals of ammonium difluoroacetate at room tempera- ture . . . . . . . . . . . . . . . . . . 118 22b First-derivative ESR powder spectrum of ammonium difluoro- ~ acetate irradiated at 77° K. . . . . . . 118 23 Second-derivative ESR spectrum of radicals produced in a single crystal of ammonium chlorodifluoro- acetate by irradiation at 77° K. . . . . 126 ix CHAPTER I INTRODUCTION It has been almost thirty years since the discovery of electron spin resonance (ESR) spectroscopy, generally attributed to Zavoisky in 191+5.l In the intervening time studies in spin resonance have ranged from para- magnetic transition metals and other inorganic systems to organic radicals in solution and irradiated organic single crystals. Perhaps the most information to be gleaned from ESR can be obtained by studying single crys- tals. Geometric and electronic structure along with bond- ing and hybridization are among the types of information gathered through electron spin resonance studies. Much literature now exists in the area of ESR studies and several excellent reviews and books concerning inorganic 2,3,u 5,6 radicals, transition metal ions, and organic systems, including radicals in solution,7’8 single crys- tals,9’10 11,12 and substances of biological interest are available. Accumulation of data and information about radicals and structures has generated interest in applications of ESR to chemical, biological and pharmacological areas. For example, a new area of research in spin labelling 13 14 has been created by McConnell and others which now has general application to many investigations. Studies of polymers and macromolecules have entailed the use 15 of the spin resonance technique. Studies of photolytic 16,17 have reactions, aging processes and drug detection utilized the electron spin resonance phenomenon. With organic molecules it is usually necessary to generate the paramagnetic (free-radical) species. This can be accomplished in a number of ways. Various flow systems have been used for generating transient radicals in solution.18 UV photolysis is widely used and experi- ments with hydrogen atoms generated in a hydrogen discharge have been successful in producing radicals.lg Generally, for organic solids the most widely used method for pro- ducing unpaired spins has been irradiation. Perhaps the most convenient of these have been X-rays and y-rays, but high energy electrons or neutrons are also employed. This thesis will be concerned with only those aspects applicable to organic single crystal ESR studies. In early work,20 single crystal studies were con- ducted on stable room—temperature radicals. However, to better understand radiation processes, a considerable amount of attention has been focused on radicals generated at low temperatures (usually 77° K and u.2° K) which tend to be unstable, yielding secondary stable room- temperature radicals by reactions in the solid. It is necessary, then, to understand the radiation damage processes resulting from ionizing radiations. Initial processes are relatively well-understood. The total absorption coefficient may be given as a sum of three coefficients due to photoelectric absorption, 21 The interac- Compton scattering and pair production. tion of the absorber with photoelectrons, Compton elec- trons or Compton photons has become of considerable interest to the chemist. Studies seem to show that the main primary process induced by ionizing radiation is the ejection of an electron from a molecule leaving a positive hole trapped in a preferred site (molecular cation). This is followed by capture of the electron by another molecule and the formation of a molecular anion. These species should be detectable by ESR methods at low temperatures. Neutral radicals may then be created by a series of reactions from these ionic primaries until a stabilized room-tem— perature species is formed or, in some instances, until a diamagnetic species is formed by recombination or dis- proportionation.’ Quite often, the secondary room-tem- perature radicals are short-lived and to study them they are quenched by cooling and data collected at this lower temperature. Studies of carboxylic acids and amino acids irradiated at low temperatures have produced examples of molecular cations and anions. Typical examples of each are given 22-2H by irradiation of succinic acid and its salts, and 25-27 by alanine, which seem to be rather general; the molecular anions produced are: 0- H 0‘ . l / HOOC-CH CH -c and H C-C-C 2 2 3 I OH NH; OH Observation of molecular cations, on the other hand, is more difficult. It appears that they react spontaneously by transfer of the acidic proton to a neighboring molecule to form the neutral species:22 9+ 0 / // HOOC-CHZCHZ-C .————>- HOOC-CHZCHz-C OH 0' There is still some confusion concerning the existence of a positive center in alanine.28 The radicals produced from these ionic primaries have been studied extensively in numerous carboxylic acidszg'au and amino acidsss"38 in attempts to elucidate the nature of the secondary radicals. Several reaction mechanisms have been proposed. However, there is considerable ambiguity and a general reaction sequence has not been found. The following process has been proposed, for example, in glycine39 using the positive primary: — 1 H O H 0 + + | /’ hv .+ I 47 NH3-C-C A: NHs-C-C + e' "——"’ I \ L I H O' H O' ‘ o I // o . +NH -c-c below 77 i +NH -CH + co 3 3 2 2 l \. H o . o ~110° K + NH + cl: 0/ 1 _ + - _. + v NH3 CH3 2 I \ NH3CH2COO' H 0‘ ~120°1< + // ; NHa-C-C l \ H o- (stable at room temperature) Similar decarboxylations have been observed in radicals from malonic acid (e.g. °CH2COOH),L‘0 but there is still some ambiguity in these proposed sequences. Similarly, negative primaries have been used to explain the produc- tion of secondary radicals through dissociative electron capture followed by hydrogen abstraction. Thus, in the malonic acid examplezul O‘ czo + OH+ -—>RCHCOOH R-CH -5: —> R-CH 2 2 OH Again, there has been considerable discussion as to whether ~C02- radicals play a role in these reactions. In any event, experimental results seem to indicate that radical formation is stereospecific and is strongly affected by the crystalline field, that is, molecular packing, orientation of surrounding molecules, hydrogen bonding and other weak interactions. One interesting phenomenon found in irradiated organic crystals has been pairwise trapping of radicals. If two neighboring molecules, or the decomposition of a single molecule, produce radicals in close proximity to one another (approx. 5 to 10 A) a triplet state is formed by the interaction between the two unpaired electrons. These pairs are stable only at low temperatures. In one recent study, a radical pair was believed to be formed within a single molecule, the two radicals being ”2 The mechanism #3 in different parts of the molecule. of pairwise trapping has been discussed. Three possible explanations are given: 1) hot hydrogen atom liberated from a molecule reacts with a neighboring molecule; 2) ion-molecule reaction followed by charge neutralization of a cation and an electron; and 3) charge neutraliza- tion of the anionic and cationic primaries to produce a pair of radicals by charge migration in the lattice. Support of the third mechanism arises from anion and cation pairs found in maleic anhydrideuu #5 and potassium hydrogen fumarate. Another interesting phenomenon has been stereoselec- tivity in the formation of the isolated radicals. It is possible to distinguish two chemically equivalent species if their orientations in the single crystal are different. Horsfield and coworkers“0 have shown that two distinguish- able -CH2COOH radicals are formed from malonic acid depending on which COOH group is lost. _ELDOR analysis H6 confirmed these results. One of these two radicals is predominantly favored. Further, it has been shown u7 by Rogers and Kispert that water of crystallization plays a dominant role in which type of radical is prefer- entially formed. And, in addition, succinic acid gives two stable radicals HOOCCHzCHCOOH and CH ch003, while 48 3 the disodium salt gives the latter predominantly. Considerable mobility appears to be possible in the solid state despite the close packing of the molecules in the crystal. For example, a hydrogen-deuterium ex- change reaction was first noted by Itoh and Miyagawau9 in the stable radical formed in alanine. When the nor— mally exchangeable NH3 protons are replaced with deuterons by growing the crystals in heavy water, it was observed that the protons bonded to carbon, which are normally unexchangeable, were successively replaced by deuterons to yield CD3CDCOO‘ radicals. Two other types of exchange reactions have recently been observed in the alanine system.50 However, when a similar radical formed in a-ureidopropionic acid was studied51 only the a-proton exchanged for a dueteron. Further evidence of crystal field effects was obtained in the case of radicals in irradiated aspartic acid,52 which exchanged protons for deuterons,but not in the similar radical found in succinic acid. And, in fact, the fumaric acid impurity found in succinic acid53 was shown to selectively exchange protons for deuterons. A number of studies have shown that internal molecu- lar motions may take place in the solid state. For example, the methyl group in the CH3CHCOO- radical found in alanine exhibits motional averaging of the protons at room temperature.5u On cooling to 77° K the motion is hindered and different couplings from the three dis— tinguishable protonsare observed. Interpretation of the motion with a modified Bloch treatment proved successful and an activation energy of 3.6 kcal/mole was determined. A number of examples of hindered motion have been found in single crystal ESR studies and Table l is presented as a comprehensive review. It may also be noted that motional averaging has been observed in methylene groups such as in the radical ~CH2COO- in glycine.55 Hayes and coworkers58 have pointed out that consideration must be made of changes with motion in the principal axes of the coupling tensors. Their analysis, based on the density matrix description, showed the spectral changes which take place in ~CH2COO' 83 in zinc acetate. Bogan and Kispert extended the analysis to motions in the-CFZCONH2 radical found in trifluoro- so oosuoma m .>.z m dresses asaaoaasas scrasnpmssxrm Arooouv mm oomua m .>.z me soamsaoaa ssh: -wmwmm “.mooonv as mmmlss m .>.z :.N Arms: sac sass dashed» Amooouv mm oaa-om m monm.a Ha.o rsrasssrsaoa 4 N sewerage Amoumouvnm m-o- mo- mCOPOLmIm mm oomnooa .. ..... .>.z sass sawsoEEmHssrmz +mmzummo. 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In some cases, motion cannot be quenched even at u.2° K and a quantum mechanical effect has been postulated. 8“ Rapid tunnelling was predicted by Freed theoretically and subsequently was observed in acetylalanine radicals73 at liquid helium temperatures. For the methyl group, an intensity ratio of 1:3:3:l is predicted for the motionally averaged system. However, tunnelling through a potential barrier would yield a l:l:l:2:l:l:l pattern as predicted by the theory. As can be seen from Table l, a large range of activation energies has been reported. In the past decade, considerable ESR data have been collected on fluorine-containing organic radicals. Most of the studies have centered on irradiated salts and amides of fluorinated carboxylic acids. The room-tem- perature systems usually involve loss of fluorine from the a-carbon atom. However, irradiation at low tempera- tures often gives different radicals. Table 2 is a com- prehensive list of fluoro-radicals found in organic single crystals. Considerable motional averaging is also exhibited in these systems in certain cases. The ~CF2CONH2 radical 86,87 has been studied by a number of workers. As pre- viously mentioned, Bogan and Kispert83 have applied a 1N OO has O.=s O.ON a.mOs :.Na some a.O .m.m.a.Ousmmv< st O.Os a.Os O.NOs a.OO News aNsns0ssOxsa< O.a O.Os O.NOs O.OO seam IOOONNO. OOO .mzuooooNNsO OO as as OOs .Na IOOONNO. OOO +amz-OOOmao as O NON 0.0a Na OO O Os NON O.Oa sw NmzooNao. aa pa O.:s N.ON sOs a.Na szooNso. OOO aO O.assusOOsOxsa< as Os NON O.aa NxzooNao. aa Nmzoumao aO s.OO O.sO OmsusOmscw as O.ON OOs O.Na Nxzoono. OOm NszOmwO OO 3N 3N Oas O.ma NszONaO. OOO NmsOOOsO XME OO OOusOmsO a s.ss- O.NNI O.OOI "Amos Os- O.OI OOs O.Om Nmzoomao. OOO Nmzoonao .wom muma gozuo Ammsmwv acmcoe 0mwu Hmowomm AXoV ocsoaeoo ocswpoqzm .anH .omupH mcwposamIo .mocsomeoo owcmmpo mcwcwoucooIocwpossm mo msmumaso mswcwm oopdwpdppw cw ooaampu mamosomp pom nocwmuno wuaamop «mu no zpmessm .N oHAMH 15 OO Os as OOHOAOOOOO O Ns sON :a NOZOONOOOO OOO szooNaoOaO I Ni mav u a ma :N u ow I s: OOV u ON :3 aOsu as sN as NON OOe >s N a.O u N- s- :s u as , s Os u s: O: ONsn OO O O ONN aa ss N O.s u NI O- O u asses OO s OO u s: s: NNsn Ossva N as asN Oa s xoaa or ooaooo O: u ON aN sauNOsOOs s OO OO u Os ON OOH asses 0.0 OO O.OOs N.sa IOOOOONOOOOO- OOO OZOOONOONNOOOOOx NO s- Os- OON OO szooamOONmz OOO NmzoonOOONmz sO as aN OON OO IOOOOOOOO- OOO xOOONaoooox a Os OOs Norm OO N N ss Os sOs seam IOOO so. OOO xooo aoooox .wmm upon honpo Ammzmov howsoh Oms Hmofiomm AXov ocsoasou mcwwpmaxm .u .mEmh mcwwossmua .omapm .voscOpcou N capo“ 16 OO O.OI N.OI Os" ssOOOO< . on . . . N m N O OO" so a O O. a Os- OOs s OO mzouwsoo. OOO OZOOsO so OO OO OO sON O.N:s aO OO aON O» OO OOs OON NO a.O .O.O .s.:su arcs OO Oss OON sa sNOOmszOOOOO as OmzoooOsO aO saN u sON OON OsOu sOsOOO OO NO OON O.:Os OOO. as NmzooOaO .mom muma ponuo Awmzowv nomads OmOu HMOfiomm Axov oczomsoo ocwmpoazm .aaoh oCMQOSHmId .cmwuH .Osaaspaoo N asses 17 density matrix treatment to the ESR data to analyze the motions found in the system over a temperature range of 77°-290° K. An activation energy of 0.H1 kcal/mole was determined. In addition to the -CF2CONH2 radical found in tri- fluoroacetamide, Rogers and Kispert87 have reported the -CF3 radical at 77° K. The isotropic splittings of fluorine and carbon—13 are reported to be luu.6 G and 271 G, respectively, in good agreement with those reported 97 The for the same radical formed in liquid C2F6. radical exhibits nearly cylindrical symmetry, with equi- valent coupling by the three fluorine atoms indicating rotational averaging about the threefold symmetry axis. The radical exhibits nearly tetrahedral geometry. It would be of considerable interest to study the related fluorine-substituted methyl radicals, °CFH2 and °CF2H, to gain more information about structure and bonding in 97 the series. Fessenden and Schuler have determined isotropic coupling values from rapidly tumbling -CH2F and ~CF2H in fluorinated methanes (Table 3). Schrader 101 and Karplus have derived an equation for degree of bending in the methyl radical: 2 ac(e) = ac(0) + 1190(2tan 6) (1) where 8 is the angle of bend from planarity (the angle between the CH bond and the plane normal to the symmetry 18 m o m: II :.NNI mmoo.m s.sm II m.N~I mmoo.m m.mm II o.mmI mo. o.m a.Nm m.ss m.sI II II II II m:oo.m m.:m m.:m H.HNI ammo. a.Na H.m:H H.sm m.sm II II II II H:oo.m m.m:s m.:m N.mm+ ammo. m.sa m.:ma m.mms II omoo.m HON m.::a II Hmoo.m m.Hsm :.m:H II mmo. m Oswss sOOsVs sacs Amos O sOOsOs sacs Amos O sOOsvs sacs secs smossdm . UmpMHSOHmU samvmhho mamcwmv ACOHpSHOmV OOH mm mmssmHMHCoEOpomxm smadpcoEwpomxm .mHMOHoms szpoe Uo#MCHposam pom modam> m can menopmcoo mcwsmdoo ocwmsong UHQOQPOmH .O rssss 19 axis). Partial substitution of fluorine indicates an increase in 8 character in the bond. These results are 99’100 (Table 3). in accord with calculations by Pople The present study was undertaken to further under- stand the irradiation processes in organic, particularly fluoro-organic, single crystals. This thesis is presented in two parts. The first concerns studies of radiation damage in sodium acetate and related compounds and motion- al averaging of one of the resulting species, the methyl radical. The second part concerns formation of radicals in substituted fluoroacetate salts with particular em- phasis on ammonium fluoroacetate and motions of the radi- cals produced at room temperature. CHAPTER II THEORETICAL CONSIDERATIONS There have been a number of excellent books recently published which deal rather extensively with electron spin 102 resonance spectroscopy. A book by Poole deals com- prehensively with the instrumentation involved. Several 103 10” books such as those by Alger and Ayscough have thorough treatments of experimental techniques, analysis of data and results. Perhaps Abragam and Bleaney's105 book is the most comprehensive on transition metals, while 2 that by Atkins and Symons thoroughly investigates other inorganic systems. And finally, several reviews are avail- 106 able which periodically survey the literature. Two exCellent books on theoretical aspects of electron spin resonance are those by Carrington and Pkflachlan107 and 108 by Poole and Farach. This presentation will be a sum- mary of the latter two sources. A. The Spin Hamiltonian For a single crystal ESR problem, the usual spin Hamil- tonian for an electron spin interacting with an atom with a nuclear spin is given by: IC = 33 .f. - gNsNE I (2) <13" ‘1- (WM Ian 20 21 The first term on the right is the electronic Zeeman term which produces the main ESR transition, the second is the hyperfine interaction term which splits the observed spec— trum and the third is the nuclear Zeeman term which is often small compared to the second term and so may be neglected. The:; tensor and the hyperfine coupling tensor, T are second-order tensors with orientation dependent 7:, matrix elements. In degenerate perturbation theory the usual Hamiltonian JC=J-C0+J-C', (3) is employed, where 1g): BS-g-R is the zero-order Hamilton- ian whose solution is known and the perturbation JC' = ZS-Ti-Ii removes the degeneracy in first order. t For organic radicals, thegy factor is nearly isotropic, hence the Hamiltonian simplifies to JC = BgH-S + S-T-I . (u) The hyperfine interaction tensor, T, includes an isotropic Fermi contact interaction term and an anisotropic dipolar interaction term. In the principal axis coordinate system (x, y, 2), which must be deduced from experimental angular rotation data, the solution of the eigenvalue problem leads to the hyperfine term t .. 1C — ASxIx + 135ny + CSzIz, (5) 22 where A, B and C include both dipolar (Ad, Bd’ Cd) and isotropic (a) parts: A = Ad + a Q ll Cd + a Now the contact hyperfine term is given explicitly as 9 II (8328) givetv °(”N)’ (7) where h is Planck's constant, 9 is the isotropic g factor, 9” is the nuclear g factor, 8 and B” are the Bohr magneton and nuclear magneton, and p(rN) is the electron spin density at the nucleus whose radius vector is r”. For the spatial distribution of the electron over the p orbital wave func- tion, the dipolar components are given by: Ad -_- 959N317] p”) (1-3 sing a cosgg) Ch 3 h s Ir-rNI _ Ch (8) ngB _ .2 .2 B N N Jr p(r) (1 31:3:N1g szn ¢) 989 8 _ 2 C' ___ ”N N] p(r) (1 3 co; 8) 611' Ir-rNI where (r - r”) is the vector denoted by the polar and 23 azimuthal coordinates (aand 4;between the electron and the nucleus in the principal coordinate system (x, y, 2) wherein T is diagonal. Since the principal directions are not usually known at the outset, this dipolar term can be written in the non-diagonal coordinate system as: 2 I r2-3x -3xa ~3xz 5 5 5 r r r Td = T- -3xy 1" -3H ~3yz (9) r5 r5 r5 3 2 - xz -3yz r -32 5 5 5 r r r There exists a real unitary transformation which can trans- form Td into a diagonal matrix with eigenvalues Ad, Ed and Cd for which the trace vanishes Hence the isotropic hyperfine interaction constant is given by (A + B + C). (11) a ll mus One experimental difficulty in analyzing the ESR spectra is the presence of the off-diagonal elements of the Td matrix, since the principal directions are unknown. The Hamiltonian may be represented by a matrix in which 32 is diagonal. This hyperfine matrix has nine 2H terms: [hm 3y 5;] FTxx Txy sz (ix = TxxSxIx + Txy(Sny + syrx) TyX Tyy Tyz Iy + 1133(8sz + SzIx) + _sz sz TzzJ _Izj Tnyny + Tyz(SyIz + szry) TzzSzIz' (12) With the simplification Sx=S =0, this may be written in y the much simpler form 1C = gBHSz + Sz(szIx + szIy.+ TzzIz) (13) and the secular matrix becomes: 1 1 1 . 2[gBH + ZTzz] 4':sz - 7’sz 0 0 1 . 1 1 4[sz + thy] §[98H ' ETzz] 0 g 0 1 1 1 . 0 0 ‘§[93H * 2Tzz '4 [Tax ' way 1 . 1 1 o 0 —4[sz + trzy -2[gBH -2Tzz] (1”) from which, the eigenvalues are readily calculated as _ .2. 1 2 2 2 , Ei — i {ngH t 4 [Tax + sz + Tzz ] } (15) for i = 1,2,3,u. Accordingly, the ESR selection rules, AM8 = :1 and AMI = 0, give two allowed transitions with the energies ls¢ 2 2 2 98H :4 Tax + ray + Tzz . (l6) 25 It will be necessary, then, to take a radical whose principal axes x, y, z are unknown at the outset relative to the crystal axes q,b,c and.find the rotation R which transforms the principal axis system (xyz) to the crystal axis system (abc): T = R T R. (17) abc x: y) 3, The usual procedure is to rotate a crystal in the magnetic field in three orthogonal directions, determining the elements of the Td matrix by one of several methods,and then diagonalizing the matrix to obtain the squares of the eigenvalues. The transformation matrix then becomes the direction cosines of the principal axes x, y, 3 relative to the crystallographic axes a, b, c. That is, cos Bax cos Gay cos eaz R = cos ebx cos eby cos ebz (18) cos 80x cos 80y cos ecz . In a completely analogous manner, the 9 factors may also be determined. The spin Hamiltonian is written (without the hyperfine term) as JC : [Ex Hy Hz] gxx gxy gxz Sx g S (19) yx yy gyz y gzx gay 923 z 26 Again, measurements of g values for three orthogonal rota- tions give the elements of the g tensor in the laboratory axis system. Diagonalization of the tensor will give the principal g values and the direction cosines which trans- form the laboratory axes to the principal axis system. B. Hyperfine Interactions For many organic free radicals, the unpaired electron is localized on a carbon p3 orbital. Hyperfine structure arises, then, from nuclei with magnetic moments (1H1/2, lgFl/z, 15N1, etc.) bonded directly to that carbon (called a) or to an adjacent carbon (called 8). Long-range coup- lings extending beyond the 8 position are usually negligibly small unless brought about by delocalization of the odd electron in a n system as in aromatic rings. In a qualitative manner, hyperfine coupling of protons may be accounted for in terms of valence-bond structures. The dominant structure which gives rise to anisotropic hyperfine interactions at the a and 8 protons through di- pole coupling may be represented as H H IBl/ “1 R-C-C° (l) l H H 82 0.2 The isotropic proton hyperfine interactions may be ex- plained as arising from a spin polarization of the elec- trons in the C-H bond. That is, the spins of the two 27 electrons forming the C-H 0 bond can be represented with respect to the unpaired electron in the sz orbital of carbon, as indicated in diagrams (a) and (b): In the approximation of perfect pairing, both structures would be equally important. However, the exchange inter- action between the n electron and the carbon 0 electron slightly favors structure (a) for which the spins on car- bon are parallel. If the odd electron has a spin a, there is then a slight excess of a spin in the carbon 0 orbital and hence excess 8 spin in the hydrogen 13 orbital, which gives rise to the isotropic proton splitting. It has been 109 110 shown both experimentally and theoretically that the a-proton coupling constant is negative while the B-proton coupling is positive.lll’112 The anisotropic contribution to the hyperfine coupling arises from dipole-dipole interactions of the electron with the magnetic nuclei. Its magnitude may be estimated from the approximation A1: 2 (ngNBN) ((3 00.926 - II/r3>, (20) where 8 is the angle between the electron-nucleus vector 28 and the magnetic field direction; for protons 2 A, = 28 (3 008 e ‘ 1) 0 (21) 1.3 where r is in angstroms. For an unpaired electron in a pa orbital which is perpendicular to the C-H bond (i.e., a p,T orbital), the principal directions are along the C-H bond, parallel to the P." orbital and perpendicular to both the PTr orbital and the C-H bond. The (3 cos2 6 - 1) term leads to a dependence of the sign of the coupling on direction. The positive regions may be designated by cones as shown below for the external field oriented along each principal direction. \ x \\ I \‘ ’I \ + / \ I \\ / \\*|1’/ \ I -, \_ ’1 \ / ___‘ — _...\ ” §\ /" ‘\\ I \ \\\\\ \\ l ‘—.___a’ I ./ + I I X7. plzmc XY plane XA plane (a) (b) (C) Hence, for 3 along the bond direction (a), the pa orbital is in the positive region of the cone and the anisotropic hyperfine component is positive. For 5 along the pz or- bital direction (c), thelpz orbital is in both positive and negative regions of the cone so the coupling is small. And when the magnetic field is along the principal direction (b), thelpz orbital is in the negative region, so the anisotropic hyperfine component is negative. Typical a-proton coupling constants are:69 Ad = -10 G (along 1 to 0-H and in plane). A = -30 G Bd = +10 G (along 0-H) B = -10 G Cd = 0 G (along p-orbital) C = -20 G and also = ~20. The isotropic interaction for 8 protons is comparable to that for a protons, except that it is positive. For the general case of an electron in the pz orbital on Ca,and the CB'HB bond at an arbitrary angle,the contact hyperfine interaction will vary according to the equation69 a: Bo + Bgcos2 8, where 8 is the angle between the projection of the C-HB bond on a plane perpendicular to the Ca-CB bond and the direction of thep1T orbital in that plane: . ‘s‘ /‘2PI o'bl'Ol on ‘ 'Hoonol carbon 30 Typically, Bo and 82 have values approximately 2 G and no G, respectively. On the other hand, the dipolar inter- action is only about one-eighth as large as for an a proton because of the increased distance R. It might be thought that fluorine hyperfine couplings would be similar to those of hydrogen, since the spins are the same and the magnitudes of the magnetic moments are similar. However, due to the mixing of the fluorine 2p orbital with that of the unpaired spin, the actual hyper— fine couplings are entirely different. Both the isotropic and anisotropic couplings of a fluorines are much larger than those of a hydrogens. Also, the principal directions of the hyperfine tensor components are different. The predominant contribution to the anisotropic fluo- rine coupling arises from spin density in the fluorine 2191T orbital. The origin of this spin density is attributed to the contribution from the excited spin configuration (b) to the ground state (a): :§%%: 2 : iiii> :5 g a b The interaction between the 62p spin density and the n fluorine nucleus gives only a minor contribution and causes the slight observed deviation from axial symmetry. 31 The large isotropic coupling is due to spin density in the fluorine 18 and 28 orbitals which arises from spin polarization. Only small amounts of spin density in the 18 and 28 orbitals give large isotropic couplings since a pure 18 fluorine coupling is 321,000 gauss and a 20,10u The radicals with pure Zs coupling is l6,u00 gauss. fluorine substituents show near cylindrical symmetry for the fluorine couplings and the unique direction is taken parallel to the 2p".orbital of fluorine. The anisotropic hyperfine tensor for carbon-l3 nuclei depends mainly on the unpaired electron density in the 2p atomic orbital of the carbon atom and the isotropic part is a measure of the 013 and 023 character of the odd electron. If the unpaired electron was entirely localized in a 2pz orbital on carbon, the dipolar tensor would be axially symmetric. Since the electron is located in the positive region of the expression (3 cos2 9 - 1), the sign of the coupling is positive. Usually, there are small deviations from axial symmetry due to some small unpaired electron density being delocalized onto neighboring atoms. CHAPTER III EXPERIMENTAL ASPECTS A. Sample Preparation Most of the single crystals in this study were grown from saturated ethanol-water solutions. Anhydrous sodium acetate was the notable exception. Commercial anhydrous sodium acetate (Matheson, Coleman and Bell) was placed in an oven at 120° C for at least eight hours.113 The anhydrous salt was then transferred in a desiccator to a dry box where it was dissolved in absolute ethanol and returned to a desiccator for crystal growing. Thin platelets of the salt were obtained. Care had to be taken in handling the crystals since they were deliquescent. The anhydrous salt was also used to grow crystals of the deuterated trihydrate (CH3COONa-3D20) by dissolving it in a solution of D20 (K 8 K Laboratories, Inc.) and deuteroethanol, CH3CH20D (Diaprep, Inc.). The d3-sodium acetate (CD3COONa'3D20) was prepared by neutralizing du-acetic acid (Diaprep, Inc.) with NaOD (made by dissolving NaOH in D20) to a phenolphthalein end- point and growing the crystals from D20 solution. The sodium and ammonium salts of the monofluoro, di- fluoro and chlorodifluoro acetates were prepared by neutraliz- ing CFHZCOOH and CF2HCOOH (Columbia Organic Chemical Co.) 32 1' ..I . {H1 war-I _ 33 and CFZClCOOH (PCR, Inc.) with ammonium hydroxide or sodium hydroxide, respectively. Crystals of the monofluoroacetate, CFH2COONDu, and the trifluoroacetate, CF3000NDu, were pre- pared by neutralization of the acid with NDuOD (26% in D20, Diaprep, Inc.) followed by slow evaporation. B. Crystal Morphology The single crystals were observed with a polarizing microscope to determine extinctions.11u Using the crystal morphology and extinction axes, preliminary crystal align- ments were made for observation in the ESR Spectrometer. In some cases the radical could be aligned using the maxi- mum or minimum splitting features of the spectra as dis- played on the oscilloscope mode of the spectrometer. As far as is known, only the crystal morphology of sodium acetate trihydrate has been studied. Groth115 lists the crystal as monoclinic with eight molecules per unit cell and B = 112.1° and C2/m space group. C. Sample Handling One of the difficult experimental techniques in ESR is the accurate alignment of single crystals in the mag- netic field. Ideally, one would like to mount a sample and collect data for rotations about three mutually orthogonal axes. Space requirements in the cavity and irradiation techniques combine to make this rather dif- ficult. Research workers in this laboratory have 3H experimented with and modified procedures to arrive at a fairly routine and relatively good technique for obtain- ing the desired data. Various irradiation sources were used. These included a y-ray source which delivered a dose rate of 2 X 106 rad/hr., a 1 MeV G. E. Resonant Transformer which produced accelerated electrons, a General Electric XRD-l Model X-ray unit operating at 20/50 kV and 12.5 ma, and a G. E. mercury arc BH-6 capillary uv lamp operating at 900 watts power. In addition, several samples were ir- radiated using the University of Michigan Ford Reactor neutron source in the hope of producing different radicals. In all cases, however, these irradiation sources yielded essentially the same radicals. Since the y-ray source proved to be the most convenient, the technique will be described more fully. Samples were either placed in a one-dram vial or mounted with glue on a flattened copper wire attached to a glass rod. They were then either left at room tempera- ture or placed in a liquid nitrogen filled Dewar and centered in the y-cell. Irradiations were carried out for 2—6 hours. Several methods for mounting crystals in the spec- trometer were used. For room-temperature systems, the crystals were mounted on a Teflon sample holder as pre- viously described.llu In the case of low-temperature 35 studies two methods were generally used. In one, the sample was placed between the prongs of a brass clip which had been glued to a quartz rod. Mounting and aligning was done under liquid nitrogen conditions as previously 116 This method afforded the use thoroughly described. of the same crystal for more than one rotation study; however, alignment accuracy was slightly reduced. In the second method, the crystal was aligned at room tem- perature on a flattened copper wire attached to a glass tube. The crystals were then glued (PlioBond Cement, Goodyear Tire and Rubber Company) to the copper wire and irradiated. Signals from the glue proved not to be a significant problem. This method usually did not allow use of a crystal for more than one alignment. However accurate crystal alignments could be made at room tem- perature with the aid of the polarizing microscope. These rods were then mounted in a specially designed )116 and rotated Dewar (previously described by Watson in the magnetic field using a pointer and protractor. Alternatively, a Varian variable-temperature Dewar was used. In most instances, the crystals were rotated in the cavity. However, provisions have been made for rotating the magnet about the crystal and this method was used in the case of the Q-band experiments. Irradiated sodium acetate crystals were allowed to warm to 198° K for five minutes by placing the samples 36 in a test tube immersed in a dry ice-ethanol bath; they were then returned to liquid nitrogen temperatures in order to maintain radical concentration. When these experiments were completed, the crystals were warmed to room temperature for five minutes and returned to liquid nitrogen for further studies. D. Instrumentation Various Varian components comprised the ESR Spec- trometer systems. Preliminary spectra were recorded on an E-H Varian X-band system using a Fieldial calibra- tion. The bulk of the experimental work was then car- ried out on a Varian v-u502 X-band system with a 100 kHz field modulation control unit, Mark II Fieldial regulation and a l2-inch magnet. Both first- and second- derivative displays of the absorption were used. Spectra were monitored on an oscilloscope display and recorded on a Moseley 7000A or a Hewlett Packard 7005B X-Y recorder. The field was calibrated with the NMR signal from water using a marginal oscillator which,with a Monsanto counter, gave the field value in frequency units. The microwave frequency was determined by a calibrated TS-lHB/UP U.S. Navy Spectrum Analyzer. Frequent checks of the field and frequency measurements were made using the normal ESR standards of pitch, DPPH (diphenyl picrylhydrazine) and Fremy's Salt (peroxylamine disulphonate). A Varian 37 v-u533 rotating cavity or a Varian V-u53l multipurpose cavity was ‘used along with the Varian V-HSHO variable temperature accessory. Spectra were also recorded on a Varian v-usos spec- trometer system operating at Q-band frequencies (12,500 gauss and 35 GHz). There was no calibration system avail- able for this range, so the field and frequency as in- dicated on the spectrometer were used, with a significant loss in accuracy. Information was also gleaned from a Varian E-700 ENDOR accessory operating in conjunction with the X—band spectrometer in the range 3-H8 MHz and using the Varian special large-access cylindrical cavity modified to deliver 1 kilowatt of peak rf power at the sample. E. Studies Below 77° K A considerable effort was made setting-up, modify- ing and devising new experimental techniques for studies at liquid helium temperatures. The first system to be used was an Air Products and Chemicals, Inc. AC2—110 Cryo-Tip refrigerator with a Ventron Magnion cylindrical sample cavity. After a considerable time adapting this Cryo-Tip to the ESR system, including modification of the cavity, the feas- ibility of using hydrogen gas from the point of view of safety precluded the use of this equipment. The second system,which proved to be the easiest 38 to operate, involved what will be called the "flow-method". A liquid helium transfer tube was designed and modified from a version of that used in Dr. Richard Sands' labora- tory at the University of Michigan. It consists of a double-walled stainless steel tube wrapped with Mylar film and provided with a port for evacuating the outer chamber. The tube was designed for insertion in a thirty- 1iter liquid helium Dewar. The boil-off gas, produced by heating a resistor immersed in the liquid, was allowed to pass through the transfer tube and into a specially designed quartz Dewar as shown in Figure 1. Evacuation of the Dewar was necessary before each run since the warmed helium gas was shown to be permeable to the quartz. The sample was then inserted from above and the temperature was regulated by the rate of helium boil-off. With an efficient transfer, it was possible to force liquid to the tip of the transfer tube and hence approach u.2° K at the sample. Temperatures were recorded with a 0.07% iron-doped gold-copper thermocouple. This method resulted in large expenditures of liquid helium but proved to be very stable. The third system to be used was an Andonian Associates variable-temperature Throttle-System Dewar (Model MHD/O- l7/7M). This consisted of (Figure 2) a three-liter modular liquid helium Dewar with liquid nitrogen shield, a tail section with Suprasil quartz inner and outer tubing, a 39 ' fl Suprasil ‘ II A Quartz 6mm id.—- — — -— J. '/2 mm wall 3 %Il xsamplo support 6 1/200 0H y’a I I I +—-—2lmm ad. . 12mm ad. —- ‘-- -- 19mm and. 318/9 m!°.b°" I / , I pom! / I ’9mm id. 3" b/ ({’{______,/”\7 _T— “/2 u I.- 1 ll __VOCUU t x mm m p" k— ] 7/8” __fi Figure 1. Liquid helium quartz Dewar for "flow—method" studies below 77°K. insert in Varian V- cavity. Dewar is designed to “531 variable temperature N0 Thermocouple Food Through - — Ho Fill _ ’,'_ ,j'. Ho Throttle ON GO! Port I: 7’. a Volvo i 59—13% i ‘ g3" N2 Vent "’57 ‘ I ' N2 Fill a I I a $\\\\‘2 l \ \\\\"-I ‘7 Vacuum\ 24 “36” K— 6 5,3" oo. 1 ix: I '2 .. 15,, Cavity Adaptor Sample Support 7mm od. J Figure 2. Schematic diagram of the Andonian Associates variable-temperature throttle-system Dewar (Model MHD/O-l7/7M). ”1 liquid helium throttling system and a removable sample. positioner and support tube with heat reflecting baffles. With this system, after the vacuum jacket is cryo-pumped, three liters of liquid helium are introduced into the body of the liquid-nitrogen cooled Dewar. A throttling valve then allows passage of liquid helium through a stainless steel capillary to the sample port and an intimate flow of evaporating helium around the sample allows for cooling. Control of the valve is used for varying the flow of helium and, under full throttling conditions, liquid can actually be introduced at the sample. Since evaporat— ing helium passes up through the sample port and out the top, the sample support may be removed for changing samples without seriously disturbing the system. This Dewar had to be modified with a beryllium window to allow for irradiation from the bottom and a mounting collar was designed to allow for rotation of the sample in the cavity. Temperatures of u.5° K were recorded with the iron- doped gold-copper thermocouple. This thermocouple had the advantage of greater sensitivity in the temperature range M-60° K (greater than 10 uV/°K). The thermocouple was calibrated at several temperatures and the accuracy of temperature measurements was estimated as about iO.25° K. F. Analysis of Data Generally, a single crystal was rotated about three 42 mutually perpendicular axes in the magnetic field of the spectrometer. A judicious choice of axes can make analysis of data easier. The rotations were carried out over 180° in steps of 5 or 10 angular degrees. In some instances smaller steps were taken depending upon the complexity of the spectra. A plot of rotations in degrees versus absolute line positions in gauss gives an isofrequency plot from which the nuclear coupling scheme can usually be deduced. The couplings are then determined and a tensor is set up and diagonalized to find the principal values and direction cosines. Several diagonalization routines are available for computer analysis of the data. These were used inter- changeably to assure consistency of results. One such method,developed from programs written by W. G. Waller117 of this laboratory,will be described. The coupling values were determined as a function of orientation for the three rotations in the magnetic field. vThese values were then 117 submitted in a FITCURV routine to produce a least- squares fit of the data to the equation: 118 a + B cosZ 8 + Y sin 2 8. (22) A plot of this curve and the aBy parameters were the out- put of the program. These nine parameters were then sub- mitted in a diagonalization program the output of which gave the principal values and direction cosines of the us tensor. In order to confirm these results, a third pro- gram, PLOTll, was used to give a calculated curve for comparison with the experimental values. Certain constants and conversion factors have been used in this work. They are given here for units of gauss, which will be designated by G throughout this thesis. (0.71QHB9) (ve MHz) g: Ho (gauss) H (gauss) = (2.3u8682 x 102) (up MHz) where ve is the klystron frequency and v is the proton P oscillator frequency. In addition, the standard field markers used as a check against the spectrum analyzer and.NMR probe are given: DPPH: g 2.0036 2.0028 Pitch: g 2.00550 Fremy's salt: g a N 13.0 10.1 G CHAPTER IV IRRADIATED SODIUM ACETATES A. §_I-_I_,COONa-3D2_O_ In order to proceed with the study of motional ef- fects in the spectra of methyl radical, it was necessary to become familiar with the spectra of the radicals produced on irradiation of sodium acetate. Rogers and Kispert98 had previously reported irradiation studies at 77° K of the trihydrate salt. It was decided to study this system in the form of the deuterated trihydrate salt. 1. Analysis of Spectra at 77° K. The ESR spectra of CH3COONa-3D20 irradiated and observed at 77° K gave an intense quartet with approximate intensity ratios 1:3:3zl. In addition, small satellite lines on either side of each line of the quartet appeared. These satel— lites have been observed before119 and were associated with "spin-flip" transitions occurring at the usual NMR spacing, HNMR = hw/gNBN gauss, symmetrically placed on both sides of each of the main lines of the quartet.120 They arise from simultaneous changes in the spin state of a neighboring nucleus and that of the electron induced by weak dipole-dipole coupling between the magnetic moments of the electron and the nucleus. Even deuteration did not completely remove these lines, indicating either IILI H5 long—range coupling with protons associated with the diamagnetic molecules or exchange of D and H induced by radiation. Further, increased microwave power had the effect of saturating the main quartet while not affect- ing these spin-flip transitions (Figure 3a). Spectra from rotation of the crystal about three orthogonal axes indicated a slight anisotropy in the 0* direction, with the smallest coupling of 21.5 G occur- ring when the magnetic field is along this axis. Site splitting could be observed when the magnetic field ap- proached this unique c* direction. Rotation in one plane produced essentially no change in the coupling, indicat- ing a cylindrically symmetric system. The radical has been shown to be the -CH3 radical 93 The by the carbon-13 studies of Rogers and Kispert. radical has a unique axis and shows two slightly skewed orientations (Figure 3b). It was therefore concluded that the methyl radical produced in CH3COONa'3D20 is a planar n—electron radical reorienting rapidly about its threefold axis with a rate greater than the anisotropic hyperfine interaction between the electron and proton which is about 108 sec-l. Principal values and direction cosines for this radical are given in Table M. The methyl radical has been of considerable interest both experimentally and theoretically. Table 5 provides a convenient summary of the experimental ESR results a) b) Figure 3. 146 Wis | 20(3l U HI/c* . 20 G I Second-derivative ESR spectra of °CH radical in y-irradiated single crystals of CH3COONa-3D20 at 77° K. Spectrum (a) shows a relat1ve in- crease in the "spin flip" transitions as a result of the saturation of the methyl lines at higher microwave power. Spectrum (b) shows site splitting when the magnetic field is parallel to the c* direction. #7 Table u. ESR parameters for radicals produced in y-irrad- iated single crystals of CH3COONa-3D20. Principal Direction CosinesI Values * Radical Temp. (Gauss) a - b c ~CH3 77° K A(H)=-22.u 0.383 -0.92H 0.00% -22.M 0.922 -0.382 -0.061 -21.5 0.058 0.019 0.998 a. :-22.1 130 -CH2COO' 198° KJKHl):-10.8 0.679 -0.679 0.278 -31.0 0.5u3 0.720 o.u32 -21.1 -o.u9u -0.1u3 0.858 a. =-21.0 iso A(H2):-12.H 0.827 0.333 0.H53 -33.6 -0.237 0.937 -0.256 -20.1 -0.510 -0.10H 0.85u a. =—22.0 1so °CO2- 300° K g=2.0034 o.u12 -0.283 0.866 2.0012 -0.678 0.5u0 0.M99 1.097” -0.609 -0.792 0.030 giso:2'0007 1. Directign cosines with respect to the crystallographic a, b, c axes. us OOs O.NNIa.NN o OOOIO seesaw sosw ONs O.O a.NN .s.m pm On sOmO O.OO N.ON x ass as >2 sOmO ONs .OOI.OO O.NNIO.ON OOOOIoaaO Eosm mmmaw :0 H mo aNs mss some w O.N :.ON x ass at OmesmO soo>> so H mo ONs aN x OON we OOO ONs susus s aO.NN ex Ossmsesss shamed: O.ON x ON.O we smo :Ns OOa.O sa.NN o ONOI smwOOO zosw sN.ON Nm ONs O0.0N x 0N.O s< as sOmO s N.N N.N s aO.NN redness NNs Ow .s: NO .ON x ass or sOmo st 0.00.0.0N OO.O aO.ON x OOO passe x OOO we mo .mom mpcoaaoo “Umave save «Amve mosdom .Hmowons ahepoa pom oousoaos muaomos Mmm mo hemaadm .m manna 49 .mmdmw mo muse: cw oopsomos mosam> % mcssmooooz was mimm 0.00 O.sN s.NN a.NN NOs sermsso Osmssm a.NO O.sN ONmO. sparrow assOoO 0.0s O.NN a.aO O.ms O.NN O.NN OO sermsso Oswesm a.NO O.sN ONmO. swarmed asssom am smumsso esmssm as.NN mm Hmvmhso mammam aw mm opossum ocON sOs O.NN o OON um edeOOO 30am GO 3000 no .Mmm mFCGEEOU AUmHvd Amvd _ fiamvd mOQSOm ooscwpcoo m manna 50 reported on methyl radical. When available, deuteromethyl and carbon-l3 data are also reported. In addition, Sch- 101 and Davidson et al.133 have reported rader and Karplus theoretical studies on methyl radical. ‘In most cases, the radical appears to be undergoing a free tumbling motion in the matrices. This may be due to the relatively small size of °CH3, to the inertness of the matrix used, or to the method employed to generate the radical. Con- sequently the proton and carbon-l3 couplingssflunrisotropic behavior. In some instances the methyl radical showed changes in coupling with temperature (Table 5). Studies of methyl radical formed by UV photolysis of CH31 on porous Vycor glass at 77° K indicated an interaction of 2.6 gauss believed to arise from interaction of °CH3 with boron-ll nuclei found in the glass.127 132 have presented evi- Janecka, Vyas and Fujimoto dence for a superhyperfine interaction of sodium with the methyl radical in carbon-13 substituted CH3000Na°3D20. In our studies we have not been able to detect this inter- action. 2. Analysis of Spectra at 198° K. The crystal was allowed to warm to dry ice-ethanol temperature for ap- proximately five minutes, then returned to liquid nitrogen temperature to prevent rapid decay of the radical. For most orientations, the ESR spectra showed a doublet of 51 doublets of approximately equal intensities. This quartet was complicated by the appearance of another set of quar- tets with a slightly different 9 value, indicating a second site of the radical. When the two couplings of the doublet of doublets became coincidentally equal, a triplet resulted with relative intensities approximately 1:2:1 (Figure u). The coupling scheme was typical of that of two a-protons and, hence, the radical was assigned to the -CH2COO‘ species. Since the complete analysis of the spectra of this radical in sodium acetate had not been done, the necessary data were collected for rotations about three orthogonal axes. The coupling values were submitted in a computer least-squares fitting routine and the a, B,“yparameters which resulted (Equation 22) were diagonalized according to the method previously described. Typical a-proton principal values were obtained and are given in Table u. During the course of this investigation a report by Fujimoto and Janecka60 appeared on the study of radicals produced in this same matrix along with data for the carbon- 13 substituted racial. In addition to the °CHZCOO' radical produced at 198° K and observed at 77° K, they noted motional averaging effects on warming the crystal. It is curious that their information on the a-proton coup- lings and g values show that the HCH angle is 107°. This is unusually small for an spz-hybrid (expected 120°) and, 52 y I Hllb Wm I Figure u. Second-derivative ESR spectra for two orientations of the magnetic field with respect to the -CH2COO' radical produced in irradiated CH3COONa:3D20 single crystals which have been warmed to ~198° K for five minutes. 53 further, their intermediate proton principal values, along with the smallest 9 value, do not correspond to a planar radical. However, the present study of this same radical shows a planar structure with an HCH angle of 116°. These values were determined on the basis of the direction cosines for the minimum proton principal values which are known to be directed along the C-H bonds.”0 A study of the literature shows a number of matrices which give the °CH2COO- radical and the data are presented in Table 6. Comparison of the data shows the HCH angle to be on the order of 115-120° for all the radicals, in- cluding many which undergo motional averaging. It will be further noted that the 9 value data are inconsistent. This is not surprising since no consistent standard of measurement has been utilized in ESR spectroscopy as, say, the use of TMS in NMR. In malonic acid, it was shown“0 that the smallest 9 value should lie perpendicular to the plane of the radical with the other two being in the plane of the radical and nearly equal. Most of the results are consistent with this interpretation. 3. Analysis of Room-Temperature Spectra. Finally, the crystal was allowed to warm to room temperature for approximately five minutes and again returned to the liquid nitrogen bath for observation of the spectra of the radical. Radical concentrations decayed if the crystal was kept at room temperature for more than 5n O.O s.sN s.OO x xOOOs am NONOO.N OOOOO.N NOOO.N . s I 0.0 O.sN O NO x O..O.Oss spreads oesN a.O O.sN O.NO N: .s.m Na ONOO.N OOOO.N OOOO.N s I a.Os O.sN O.sO x O. st so: Oesmossmm OOOO.s OOOO.s OOOO.N a.os O.Os s.ON Nmusm smeseeuomc .s.m OO O.O s.sN O.NO Nm xosa O.Os O.sN O.NO sm Os Oss remassm resosswsss 0.0 O.sN O.NO Nm xOOss mmH 5.0H o.H~ ~.mm a: mad Hom.mCOo>Hu OO.OO O.ae O.O O.Oa sOOsOe s.ss O.ON O.ON.Nmusm xOOaN OOs ONOO.N OOOO.N OOOO.N O.Os N.Os O.sO xOOOs eesossm :ms Ac.s0mv mmmoo.~ N.smue s.som N.Os s.sN O.NO Nm s.s.mo O: ONOO.N OOOO.N NOOO.N a.Os O.ON O.NO sm Os Oss Osom assess: .wmm some smseo reuse a neon OIO sOOamOO snzsu oeaoaeoo mosam> m wmmwmwmmm .Hmumzpo -Osmesm es eases sheaves -OOONmo. how oophomoh upszmop mmm mo >noEEDm .m manna 55 .o>fiumwo: ooESmmm who mwcwaaooo one no mcmwms AH.hmV .B.m a.O N.OO a.OasOOseq O.sN O.Os 0.0N s.ON Nausm .s.m sN.OOO xoaa s.sN O.O 0.0N O.NO Nm xoaa OO O.Os O.N a.OOsoOses OsOO.N sNOO.N sOOO.N s N O.sN O.ss O.ON O.OO x oaOs o OO. assures easeom N.ss N.ON a.NO Nm s 0.0Ns 0.0s a.ON O.sO m xOOON N.ss a.Os a.NO N: shseosemm NO s 0.0ss OO 0.0s 0.0N O.NO m spheres sasssossm s.O O.sN O.OO Nm OO OsOO.N ONOO.N sOOO.N s O.Oss O.ss 0.0N O.OO m xoaa humerus oasN .mom sumo Loewe sense A coon oIo Awmsmwv mom» ocsoasou mozsm> moosm> O wmwvuwsam poscspcoo m manna 56 I 20023 Figure 5a. Second-derivative ESR spectrum of 'C02- radical produced in irradiated single crystals of CH3COONa-3D20 at room temperature. The two lines indicate two magnetically distinguishable sites of the radical. Figure 5b. Second-derivative ESR spectrum of -CD radical produced in irradiated single crystals of CD3COONa'3D20 at 77° K. (:1 \3 several hours. Spectra observed at liquid nitrogen tem- perature for this room-temperature radical showed a singlet for certain orientations, which split into two singlets for most of the orientations in the three orthogonal planes (Figure 5a). A determination of the g values gave principal elements of 1.9974, 2.0012 and 2.003%. These values are similar to those reported for a °C02' sigma-type radical which has been studied by several workers in irradiated sodium formate.l37’138 137 to be a sigma The -C02- radical has been shown radical with the unpaired electron localized in an op hybrid-type orbital directed along the 02v symmetry axis. The radical retains the bent configuration with an angle of 128°. Fig. 6. Structure of the-002' radical. The unusual feature of this particular radical is the low isotropic 9 value, with the smallest value determined 58 107 137 both theoretically and from carbon-l3 data to be along the y axis (Figure 6). Also,from carbon-13 data it has been shown that the large isotropic part of the coupling is due to unpaired spin density in the carbon 23 orbital. The intermediate 9 value is directed along the z axis in the plane of the radical. Further, the anisotropic part of the coupling which is nearly cylindrical about the z axis derives mainly from the carbon sz electron density. It is interesting to note that several other studies 139 of -COZ' in irradiated sodium formates, in sodium 150 lul,1u2, in sodium deposits 1% hydrogen oxalate, in calcite, 1N3 in dry ice and in potassium bicarbonate are in good agreement with the above analysis. On the other hand, spectra of irradiated carboxylic acids (e.g., suc- 1M5 cinic ) thought to produce :CO - radicals were shown 2 ul,1u6 to actually result from a car- rather conclusively bonyl-type radical RC=0. This radical, with g values similar to carboxyl radical, is postulated to be produced by C-O bond scission of negative primary ions: 0 0 O i’ _ / /' - R-C\ + e "_>R-C° —->R-C, 4' OH OH OH The g andJN13C)tensors reported by Fujimoto and Janecka,60 13 13 in - C02- produced from CH3 COONa-BDZO did not 59 have the same principal axes as would be expected. In addition, the near cylindrical symmetry of A(13C) found in other studies is not present. Nevertheless, they do 13 not report any coupling from C in radicals produced from irradiation of 13CH3000Na as would be expected if the carbonyl radical were produced,so their radical must be .13 - co2 . H. Analysis of reaction scheme. Formation of -CH3 on irradiation of sodium acetate must result from a rup- ture of the C-C bond. This bond breakage at low tempera- tures is not unusual. Upon warming, the ~CH3 radical apparently abstracts a proton from a neighboring molecule to give ~CH2000-. Again, this is not surprising since most studies of methyl radicals indicate that it is an unstable species at higher temperatures. Deuterium labelling studies of similar radicals produced in irradiated zinc acetate57 support this mechanism. However, it is dif- ficult to postulate a mechanism for formation of “€02- from the n-electron radical 'CH2C00-. Although some carboxyl radicals may be present at lower temperatures they are more easily saturated and so may not be observed. Nevertheless the simultaneous increase in :COZ' and de- crease in 'CH2COO' concentration on warming must certainly indicate a radical reaction of some kind. 60 B. _C_D.,COONa° 3D 9_ 2 In addition to the 'CH3 radical it was desirable to produce °CD3 radicals for study at low temperatures so single crystals of the perdeuterated acetate were irradiated and the spectra studied at 77° K. The seven-line spectrum observed was typical of a rotating trideuteromethyl radical species with the predicted intensity ratios of approximately l:3:6:7:6:3:l (Figure 5b). The splittings ranged in value from 3.2a to 3.60 gauss which are about 15% of the cor- responding proton values. Since the ratio of magnetic moments is (0.85738/2.79270) = 0.3070 and the spins are I = 1 and I = 1/2, respectively, the ratio of splittings should be 0.1535. Rotation of the crystal in the magnetic field produced small anisotropy of the order of 0.” gauss. In addition, lines from another radical were observed on either side of the septet. This radical could not be identified but the ESR spectrum showed considerable an- isotropy. Upon gradual warming, the central septet col- lapsed and the outer lines from this unidentified species increased. The splitting, which is #0 gauss between peaks and 13 gauss between nearest lines, appears too large to arise from the expected °CD2COO- radical. The possibility of a proton impurity cannot be ruled out since radical reactions which favor one isotopic species have appeared in several other instances.lu7 61 C. ENDOR Studies After the data for the ESR studies of irradiation damage in CHacOONa°3D20 single crystals had been collected, two papers appeared in the literature on similar studies.60’132 132 In one, Janecka, Vyas and Fujimoto indicated that there was a coupling of the methyl radical to a neighboring sodium atom at 77° K,as observed for the -C02 radical of sodium formate.137 In the present investigation such a sodium coupling was not observed. One might expect to see such a sodium splitting in the 'CD3 spectrum since the deuterium coupling is of the order of that reported for sodium (3 gauss). No such coupling was observed. In order to resolve the problem of whether sodium coupling was present in the ESR spectrum of 'CH3, it was decided to do an ENDOR experiment for evidence of sodium hyperfine interaction. Double resonance has the inherent advantage of being capable of detecting very small coup- lings which may be unobservable in the ESR spectrum be- cause they are of the order of linewidths. Sodium has a nuclear spin of 3/2, hence four elec- tron resonance lines are expected. Since sodium may also have a quadrupole coupling, there are six possible ENDOR transitions, three above and three below VNa = 3.66 MHz (the frequency of free sodium at 9.2 GHz and 3200 gauss). The range of the Varian ENDOR system is approximately 3 to 48 MHz, therefore only the high—field lines would be 62 observable. In sodium ENDOR measurements done on the lu8 these °CO2' radical found in irradiated sodium formate, high-field lines were reported to be more intense. How- ever, only a transition in the ESR which connects one of the energy levels in the ENDOR transition will be observed. Thus, if the M = 13/2 lines are saturated only one high- field line would be expected. The sodium ESR hyperfine splitting for 'CH3 was not resolved therefore it was not possible to determine which of the sodium transitions was saturated in the ENDOR experiment. A signal was detected in the ENDOR at 5.80 MHz corresponding to a sodium splitting of n.28 MHz, or 1.53 gauss. However, vD for deuterium would be expected at 2.13 MHz, and consequently a deuterium coupling of approximately 2.6 gauss would also account for the ENDOR line observed. Therefore deuterium cannot be ruled out as an alternative. No additional hyperfine coupling which might be attributable to sodium was observed in the case of the room temperature °C02- radical by Janecka et al. or in this work. There remains, then, some doubt whether there is a sodium coupling. Since the compound studied is a deutero-substituted species, care must be taken in inter- preting the spectra due to possible deuterium exchange reactions.lug D. Anhydrous Sodium Acetate It was of interest to observe the results of 63 irradiated crystals of the anhydrous salt. Rogers and Kispert,+7 have shown that production of methyl radicals depends on water of hydration in a number of acetate salts. That is, the greater the number of waters of crystallization in the salt, the greater the probability of methyl radical production and the higher the concentra- tion of methyl radicals formed at 77° K. 113 observed back in 191% a remark- Vorlander and Nolte able difference in the anhydrous salts formed from de- hydration at 120° C and above 200° C, the former being probably rhombic and the latter probably monoclinic. They noted, "it may be assumed that within the molecule there exist differences in the intensity of the energy between individual parts of the molecule or, especially, as a result of these intramolecular differences, there may likewise be variable extramolecular intensity dif- ferences between like molecules." 1. The Doublet ESR Spectrum. The crystal struc- ture of anhydrous sodium acetate has not been determined. However, the crystals grow in very thin plates in the shape of a parallelogram with extinctions occurring along the diagonal of the parallelogram. These two extinction axes, designated.b and c, were used for rotations in the magnetic field along with the axis perpendicular to the plates. Samples irradiated and observed at 77° K gave 64 unusually symmetric ESR patterns. For rotations about b and c, essentially identical coupling curves were ob- served. Most orientations gave a doublet of doublets with typical a-proton couplings; these became triplets for R parallel to 0° and 90°. From these spectra the radical was assigned the structure -CHZCOO-. The third orienta- tion, a, gave a triplet of nearly isotropic coupling. Since the measured coupling was approximately 20 gauss, which is a typical intermediate principal value for an a-proton, it was concluded that the be plane was the plane containing the C-C bond and the bisector of the HCH angle. Simulated spectra were obtained using a computer program118 and assuming that there are two a protons with -10, -20, and -30 gauss principal splitting values. These confirmed that there would be equivalent coupling from the two protons for all orientations in this be plane, with values ranging from 17.5 gauss to 20 gauss. The experi- mental tensor was then diagonalized and gave the principal values and direction cosines listed in Table 7. It will be noted here again that the HCH angle is 115.5° from direction cosine data (Table 6). In contrast to the radical found in the CH3COONa°3D20, this system shows considerably more symmetry, with only one site being observed for all orientations. A crystal structure analysis at this point would be required for further com- parisons. 65 Table 7. acetate. Principal hyperfine splitting values and direction cosines for the ~CH2COO' radical produced at 77° K in irradiated single crystals of anhydrous sodium A(H) (gauss) . O . * D1rect1on Cos1nes A(Hl) = -l2.9 .56” -33.0 .H03 -21.0 -.721 aiso' -22 3 A(H2) = -13.6 .567 -33.2 -.365 -20.8 —.7H0 aiso: —22.5 (HCH = 115.5° -.5H5 .620 .837 .370 .OHl .692 .520 .639 .85” -.371 -.022 .67” * . With respect to the laboratory axis system specified in the text. 66 . Ln Q I V‘J "I" . 2. The Triplet ESR Spectrum. In addition to the lines from the above radical, an inequivalent set of lines appeared on either side of the main quartet for certain orientations, with a maximum splitting of 150 G along the axes of rotation. The couplings for the lines on the low-field side are one-half the coupling of the main spectrum, while the lines on the high-field side show couplings of approximately one-fourth those of the main spectrum (Figure 7a). For most orientations, the lines were incompletely resolved and the intensities very quickly were reduced to the level of base-line noise. Such sig- nals are typical of a radical pair. However, the unusual result is that the components of the pair are apparently different species. To verify these results, a powder spectrum was taken (Figure 7b). The two signals at either side of the main spectrum correspond to a splitting of approximately 162.8 gauss. In addition, the half-field transition at g = H was observed (Figure 7c) as a very weak signal. When two radicals are produced about 5-10 A apart, a triplet state interaction between the two electrons results. Characteristics of the triplet state spectrum are the occurrence of the AMS = 1 transition and of a weak half—field signal at g = H due to the AMS = 2 "for- bidden transition". This transition becomes "allowed" by mixing of the MS = 0 and M8 = :1 levels in a second- 67 ch b) c) Figure 7. ESR spectra of radicals produced in irradiated anhydrous sodium acetate at 77° K. a) Single crystal second-derivative spectrum showing ~CH2COO' main radical with dissimilar coupling from the radical pair. b) First derivative powder spectrum. c) Low-field (m1650 G) transi- tion for radical pair, g=u, with lOOO-fold in- crease in gain. 68 order perturbation treatment. The electronic spin moment in the triplet state is that of two coupled electrons (S = l) and only one-half of this moment, on the average, effectively interacts with the nuclei; hence the hyper- fine separations are approximately one-half those of the 150 main singlet. The spin Hamiltonian for such a pair is given by: H: = BEE-(791 + $2) + £132 + 31-13-32 (23) where D is the traceless dipole-dipole coupling tensor and J is the exchange coupling. Since the distance between the two unpaired electrons is so large, a point-dipole ap- proximation is fairly good, resulting in zero-field split- ting constants with IEI m0 and D, therefore,axially sym- metric. Hence, the vector R between the electrons is parallel to D and 22 D = £3; (1- 3c0328) (2“) 2B or, rearranging, R = 3.06/0, (25) where D is in gauss and R is in angstroms. From these equations, the distance of 5.77A was deter- mined to be the maximum distance between the pairs in anhydrous sodium acetate. This is in good agreement with values found in pairs previously studied. Iwasaki151 69 has presented a table of data for radical pairs in terms of two modes of formation. His table (Table 8) is pre- sented here and expanded to include more recent work for comparison. The radical obtained in this study cannot be identified from the spectra obtained, but appears to be a dissimilar pair (that is, a pair in which there are two different kinds of radicals interacting). Only one other study has shown this kind of behavior.158 Box has shown161 that no pairs exist in dimethyl- glyoxime at u.2° K and only form on warming to 77° K. This strongly supports the idea that pairs are not ran- dom formations of two closely-spaced molecules but must result from some radical reaction. A mechanism has been suggested161 whereby a cation loses a proton to a neigh- boring molecule to form a radical. Electrons then migrate and combine with the protonated species with the evolution of H2 and production of a second radical in the vicinity of the first to form the pair. Upon further warming above 77° K, the pairs diffuse apart by intermolecular hydrogen transfer to give the final discrete radicals. Anhydrous sodium acetate would be an interesting system to test this reaction sequence. It would be neces- sary, however, to study the irradiation damage process at lower temperatures. One further observation should be notedOthat in most of the pairs previously studied, forma— tion occurred where a protonated species such as -OH or same Nmzooamo. OOs OO.a N soafiaflmmwo Nmzoo mo. Axossv oUOEMpoomososamocoz mm.m .o>ncm aOs OO.O .Ossss .OOOIOOOm osmoasv Osor osssxo opossumao OOs s.O .OIOmOOIOm Axoaav OssswseosromImsoesaOossOm OOs N.a .OstmOOOOOO- sxosae Oraaosaosaoesesxofiam O.O Issss OOs s.O IOOO .OzmouamOOIsO Axoaav OssxoesmaersosossOIa 0 m.~.v 7 OOs N.Ov .OstmOOOImozom Axoaav masxossmssssmz O.Ov OOs O.sv .OszImOzOm Axoaav rasxosso OOs O.O «ma :.m .ozsmm0voIoAmmovzom AXossv mexo>HmH>£PoEHQ .wrm smv assesses senses sees essoaaoo moan wan .aEmB moasooHoE pcoommom 039 Eosm cospom mnwmm .< .manpmhpo onCOm Umpmwomssfl CH ocsom msflom Hmofions mo moOoSHm Mmm .m manna l 7 mo ON.a O mma N.m Axouhv HOCHOQOmmm NOs m: AMossv whoszaom can sOs Oa.O oeossoo sOss< weoooso< axoaav Oeossooososmue Os. OO.O I.+smooomo"mooooxc sxoaav assesses sameness assmoosoe sI.+V O .\ / Ouw who 13 mm .m \o N o/ Axes: ooflsomnco Osman: m m Hma ovocoame OOs 0.0 mooomooooux+ sxON.:O somososs aasmoosoa NO aO.a ssoosoosssv mzszOmzmz so mzmzoomszz axoaav oossoosoo 0.0N OOs O0.0 .Omzoonz Axoaa.0OO.OOO mosssxososm .wom smv «soosoom Odessa sees ossoaaoo won pose .QEoB concepcoo m manna 2 7 .Uo>powno mos sang oco cozy whoa mommo oEOO CH um O.O Ososux .OOOOmOO ooswsamso ssossooso as OOH m.m msm>aouonm .mmmo Axom.:v ooflxosoa HmoNconHQ aOs O.Os IOOOOO. s.O.mv opossomsoo sasmmosoa 0.0s msonpo can OOH m.m mmmoIo. AXossv opocoonmoazcoeafla a-O OOs O.O NsOmOOOz. Axoasc ossNOsOOesOeosamswos OOs O zoNsOmOOO. Axoasv osssssaoossoooOsIosooa< OOs Oa.O Omo. .m sOON.sO sooner: .Oom sme esoosoom oossoa sxov osaoaaoo mo: pmwn .mEmH oasooaoe mamcwm m mo cowpflmomeoooo one >p omanom mason .m Umsnflvcoo m mHAMH 73 -NH was available. In anhydrous sodium acetate, no protons of this type exist. One further experiment with anhydrous sodium acetate indicated that under controlled warming conditions the °C02- radical did not appear to be produced. A detailed crystal structure analysis is needed, but certainly it can be concluded that the crystal packing is strongly in- fluencing radical formation in these acetate systems. E. Methyl Radical Below 77° K. 125,128,130 Several authors have reported unusual ESR line intensities for methyl radical at low temperatures. Normally, a l:3:3:1 quartet would be expected from equi- valent coupling of three protons with the unpaired elec- 125 observed an unusual l:l:l:l tron. Jackel and Gordy quartet for the radical found in inert matrices such as xenon at u.2° K. They ascribed this phenomenon to separa- tions in the low-level rotational energies as predicted 168 The over-all wave function for the three by McConnell. equivalent hydrogens in the lowest rotational state is required by symmetry to be a symmetric function. Each function must correspond to a separate hyperfine component and each would have equal weight. Hence, the predicted intensities should be l:l:1:l. As the temperature is raised, higher rotational states are populated and the intensities approach the expected l:3:3:l quartet. 168 McConnell points out that if there is a barrier to 7H the rotation in some matrices, then the normal quartet will be observed; this appears to be the case for most matrices other than xenon which have been used (Table 5). It was of interest to study the ESR spectra of the methyl radical found in sodium acetate trihydrate down to temperatures below 77° K. If the radical did exper- ience restricted motion, perhaps the type of motion could be determined and the barrier to rotation calculated. There were a number of experimental difficulties en- countered. Since the radical was unstable in the acetate matrix above liquid nitrogen temperatures, it was dif- ficult to transfer the crystals after irradiation at liquid nitrogen temperature without introducing slight warming and resultant interference from the spectrum of the -CH2COO- radical so formed. In experiments with the Andonian Dewar system it was found that the crystals irradiated at 77° K could not be inserted into the helium cold finger without some warming difficulty. Hence the helium "flow method" was used to collect data. The °CH2COO- radical may even exist at 77° K but may not be observable due to differences in saturation properties, the methyl radical being less susceptible to power satura- tion. Evidence from these liquid helium ESR experiments indicates that -CH2COO- is indeed present at 77° K and appears to undergo further motional changes at lower temperatures. 75 The second problem encountered was power saturation from the incident microwaves at the cavity. At tempera- tures approaching u.2° K the crystal relaxation mechanisms prevent the rapid decay of electrons to the lower energy state and, hence, a situation is created whereby the electron levels become equally populated with resultant loss of signal. Spectra were recorded with some degree of success for both the ~CH3 and ~CD3 radicals using the low-power arm of the microwave bridge. The spectra are shown in Figures 8a, b, and c. It will be noted that the spectra above 30° K show some interference from a second radical, presumably 'CHZCOO'. Nevertheless, it was possible to record spectra of methyl radicals at temperatures down to H.5° K. Sev- eral features may be observed. In all studies, the separa- tion between the two outer lines of the quartet did not change with change in temperature. At approximately 30° K significant changes in peak intensities occurred such that the inner lines were reduced to a ratio of less than one with respect to the outer lines. The -CD3 spectra were also observed to undergo a similar change in intensity ratios. The temperature at which the intensity ratios for ~CH3 becomes approximately l:l:l:l (30° K) is con- 125 on the siderably higher than that calculated by Gordy basis of rotational level energies. At lower temperatures, these inner lines appear to be split into additional 76 AO'K (In W IGPK I206' Figure 8a. Second-derivative ESR spectra for -CH3 radical at temperatures below 77° K. 28’K t:— Figure 8b. 77 l l l l i laSflt I l l 1 l figsa S'K ”MI I Second-derivative ESR spectra for 'CH radicals (arrows) at temperatures below 77° K. Extra lines at 33° K indicate the presence of a second species, probably ~CHZCOO‘. 78 30"( )0 CB 1 U or IS'K Figure 8c. Second-derivative ESR spectra for 'CD3 radical at temperatures below 77° K. 79 components. However, there is considerable interference from lines due to the other radical present. It can be concluded that no classical Brownian-type motion is taking place since there are no line-broadening regions as ob- served for other methyl-type rotations.70 Results obtained in this work appear to be comparable to those attributed to quantum mechanical tunnelling through a barrier as observed (Table l) in several other systems.5”’7l’73’7” In particular, the spectra observed by Clough 33 21.76 for methyl malonic acid show some similarities to the spectra of Figure 8a. No further analysis of the spectra can be reported. It can be concluded that the methyl radical in sodium acetate trihydrate exhibits unusual motional behavior be— low liquid nitrogen temperatures. Further work must be done, such as irradiating the samples in the cavity to eliminate warming problems and improving the spectrometer to reduce power saturation problems, before detailed analysis of the motional behavior can be made. CHAPTER V IRRADIATED FLUOROACETATES An attempt was made to prepare and study the ESR spectra of the radicals -CFH2 and -CF2H. By analogy with the results reported above for methyl radical in irradiated CH3COONa-3H20 the salts CFH2COONa and CF2HCOONa were irradiated. Unfortunately, any low tem- perature handling of the crystals resulted in shatter- ing, making analysis of the ESR spectra impossible. Consequently, the ammonium salts of the corresponding fluoro-substituted compounds were irradiated and the results of these studies are reported here. Only the crystal structure of ammonium trifluoro- acetate has been determined. Cruikshank, Jones and 169 have found this salt to be monoclinic with Walker space group P21/a, four molecules per unit cell and B = 100°. The structures of the monofluoro- and difluoro- salts are not known. Extinction axes for both salts were parallel to the diagonals of the flat face. Two of the axes of rotation were chosen parallel to these diagonals with the third axis being the mutually orthogonal direc- tion. In all systems studied, the c direction will cor- respond to maximum fluorine splitting and the a direction 80 81 to the minimum fluorine splitting. A. Ammonium Monofluoroacetate 1. Analysis of Room Temperature Spectra. Irradia- tion at room temperature of ammonium monofluoroacetate produced an ESR spectrum consisting of a doublet of doub- lets. Typical second—derivative ESR spectra of the single crystal are shown in Figure 9. The splitting for the smaller doublet was approximately 20 gauss with an an- isotropy of :10 gauss. The larger doublet showed typical a-fluorine anisotropy with a maximum splitting of about 185 gauss and a minimum splitting close to zero. The radical was therefore assigned the structure F O‘ \/ -C-C /\ H 0 Although a similar radicaL 'CHFCONH2,had been previously studied in the amide,CF2HCONH2,by Cook, Rowlands and Whiffen85 it was decided to carry out the present study for a number of reasons. The number of single-crystal investigations of fluoro-organic radicals has been rela- tively small. Most of the radicals found have had more than one fluorine atom and carbon-l3 hyperfine splitting data have not been reported (Table 2). In addition, the 85 study of °CHFCONH2 in monofluoroacetamide had added dif- ficulties; more than one magnetically equivalent site HI/a L———‘" G H/Ib Figure 9. H/lc Second-derivative ESR spectra for -CFHCOO' radical produced in irradiated single crystals of ammonium monofluoroacetate at room tempera- ture. 83 was seen and long-range coupling from the nitrogen, typical of the amides, was observed. Also they did not report g values for this radical. The radical °CFHCOO- was well suited for study. As observed from the isofrequency plots in the following Figure10(a-c), two planes of the rotation produced only one radical site while the other plane had two magnetically distinguishable sites. With the presence of an.a proton a careful evaluation of the radical structure and spin density distribution can be made. The system was analyzed and diagonalized tensors were obtained for the 9 value and for the fluorine and proton hyperfine splittings. These principal components, along with their corresponding direction cosines, are listed in Table 9. Values of the hyperfine interactions A(H) and A(F) for rotation of the magnetic field in the three orthogonal planes are shown in Figures 11 and 12 and the corresponding plots of the g values are shown in Figure 13. Since the radical concentration was quite high for the room temperature radical, it was possible to increase the signal gain without much loss in sensitivity, hence the 1.1% natural abundance carbon-l3 lines were detectable. In view of the fact that the directions of the maximum components of the carbon-13 and fluorine-19 hyperfine splitting tensors coincide within experimental error, the value for Azz (13C) is obtained directly. It was not an bIIH cIIH- bII 180° 1 - 90° Figure 10a. Variation with magnetic field orientation of the ESR hyperfine lines in the be plane for the °CFHCOO' radical in irradiated single crystals of ammonium monofluoroacetate at room temperature. 85 CIIH” a 180° aIIH-- CH - .00 Figure 10b. Variation with magnetic field orientation of the ESR hyperfine lines in the ca plane for the -CFHC00' radical in irradiated single crystals of ammonium monofluoro- acetate at room temperature. 86 aIIH - bIIH-- a/IH-t- 'v A '7 -r 90° vv'V'Uv' AAA-I- - - v AA v A v- A AAA‘ v.' AA AA‘ ‘ ‘ v O .. Aso Figure 10c. Variation with magnetic field orientation of the ESR hyperfine lines in the ab plane for the -CFHCOO‘ radical in irradiated single crystals of ammonium monofluoro— acetate at room temperature. 87 Table 9. Principal values and direction cosines for.A(F), A(H), g and 4(130) for the -CFHCOO' radical produced by room temperature irradiation of ammonium monofluoroacetate. Principal * Elements Spherical Direction Cosines (Gauss) 8 ¢ a b c A(F) -12.2 90 1165.7 —0.969 10.2H6 0.000 -17.8 90 i 75.8 0.296 i0.969 0.000 181.3 0 90.0 0.000 0.000 1.000 a. 50.9 180 A(H) -l3.0 90.1 + 3. +0.998 10.052 -0.002 -21.5 1.0 100 0.003 $0.019 0.9998u aiso -2208 9 2.0055 90.0 i 0.999 20.093 0.000 2.0051 90.0 : -0.0u3 10.999 0.000 2.0023 0 0.000 0.000 1.000 giso 2.00u3 A(13C) 20.0 90.0 i H. 0.997 10.080 0.000 18.6 89.1 19H.6 -0.080 10.997 0.015 85.3 0.0 190.0 0.000 10.015 1.000 aiso ”1.3 * With respect to the text. the laboratory axis system specified in 88 I“... III- ( III- an- ”‘i" no 6' f 01)“ i rim. ‘9‘ ”2‘ '9‘ Q... JL ( w-WWO I i a} i ‘ Inn! Figure 11. Plot of hyperfine splitting with respect to magnetic field direction for fluorine coupling in the ~CFHCOO' radical in irradiated single crystals of ammonium monofluoroacetate at room temperature. 89 up :95 IIII-I 15” 6° ' 9'0" ' I IngloJ CH1 Ifli (#H Sir ' . 225* n ( 15:L .L J «I» 1)- mole! Figure 12. Plot of hyperfine splitting with respect to magnetic field direction for proton coupling in the -CFHCOO' radical in irradiated single crystals of ammonium monofluoroacetate at room temperature. J ‘fiu‘ t. 90 Ht! olH bl}! ”0'5? ‘ 90" L 100' amino end on! cfli 2.0055" '° "- 9 210035" :- ¢ - : : 10°" 0° ' 90" 190' unwed out 601 aflH I zooas-I- “t I: I. : : z°° ’ 90' so: who Figure 13. Plot of g values with respect to magnetic field direction for the °CFHCOO' radical in irradiated single crystals of ammonium mono- fluoroacetate at room temperature. 91 possible to observe carbon-l3 coupling in the plane of the radical since the lines are not resolved as a result of the very small coupling values. However, since two rotations showed near cylindrical symmetry (Figure 1”) as expected for an electron in the 2p1r orbital of carbon, the use of 118 as described the program COPLANR from Waller and Rogers, in the experimental section, gave principal carbon-l3 values which agree quite well with data available for similar radicals. The A(13C) principal values and direction cosines are also given in Table 9. Relatively large "spin-flip" transitions were also observed for this radical. These transitions have been 9” The observed in other fluorine-containing radicals. satellites at igNBNH, on either side of the normal transi- tions, appeared and were independent of power saturation, whereas spectra for the radical itself had to be attenuated by 20db since they tended to saturate quite readily. Only the absolute values of the principal components of the hyperfine splitting tensors can be determined from the first-order spectra at X-band frequencies. It is generally agreed that the large fluorine coupling has a positive value. This leaves several choices for the signs of the couplings of the other two principal values. Here again, since near cylindrical symmetry would be expected, the choice becomes either both positive or both negative. It would be extremely desirable to determine the 92 _J l l l I 1 I I I U’ 90’ “NP omflmo Figure 14. Plot of hyperfine splitting with respect to magnetic field direction in the be and ca planes for carbon-l3 coupling in the -CFHCOO' radical in irradiated single crystals of am- monium monofluoroacetate at room temperature. 93 relative signs of the principal couplings to permit analysis of the electronic structure of the radical. As can be seen from Table 2 not many signs have been determined for these radicals. One method to resolve the problem of signs is analysis of the "forbidden" or second-order couplings. Since these become important only when the nuclear Zeeman interaction is of the same order as the nuclear hyperfine interactions they are observable only near the minima of A(F)(about l2 gauss). When spectra are taken at Q-band frequencies, the intensities of these second-order transitions become important at larger values of A(F) (up to about 35 gauss). Spectra were collected for rotation of the crystal in the ca plane on the Q-band spectrometer. Since no field calibration was available, other than direct readings of the "Fieldial", only the relative line positions could be accurately analyzed. A computer analysis of the spectra of the °CFHCOO' radical was made using a program called MAGNSPEC (QCPE Program 150, by Marcel Kopp) modified to run on the CDC 6500 computer. With this program, the theoretical line positions, including second-order transi- tions were calculated for the choices (+++) and (+--) of the principal values of A(F). The best agreement between calculated and experimental angular variation of the line positions with the magnetic field was determined. The best choice of signs is that 9” with the two small principal values negative. This is in good agreement with the results of a similar analysis made 85 92 for radicals in fluoroacetamide and difluoromalonamide. 2. Analysis of Spectra at 77° K. Irradiation of ammonium fluoroacetate at 77° K did not produce the 'CFH2 radical as expected. The spectrum contained a central por- tion consisting of a doublet of doublets for most orienta- tions. This collapsed to a triplet with intensity ratios approximately 1:2:1 when the magnetic field was directed along the c direction. The outer set of lines, of consider- ably less intensity, was similar to that observed in the spectrum of the room temperature radical 'CFHCOO'. The central set was assigned to the -CH2COO' radical; it was similarly found in the irradiation studies in fluoroacetamide that -CH2CONH2 was produced at 77° K.158 The ratio of the area under one of the outer lines of the °CFHCOO' radical to the area under one of the outer lines of the 'CH2COO' species gives the approximate ratio of the radical concen- trations as 0.25. The areas were estimated by multiplying the half-width at half-height by the height of the peak. Upon controlled warming, the lines from the 'CH2COO- radical decayed while those from 'CFHCOO' increased. There seems to be no complete correlation of the decay of the °CH2COO" species with concomitant increase in the 'CFHCOO' species. Hence, the decomposition of the 'CH2COO- radical 95 j can either lead to a diamagnetic species or to a -CFHCOO' radical by abstracting a fluorine atom of a neighboring molecule, with no simple rate being observable. Finally, at room temperature the only radical present in substantial amounts is the fluoro species. Trace amounts of the other species always remain and even when samples were irradiated at room temperature slight traces of the °CH2COO- species were noted. As with monofluoroacetamide158 where only the pro- tonated radical was present at low temperatures, a stereo- selective irradiation process is taking place. One would expect to find twice as much of the fluoro radical if the loss of hydrogen and fluorine atoms were equally probable. However, the spectra observed established that there must be a large preference for C-F bond breaking over C-H bond breaking at lower temperatures. 3. Variable Temperature Studies. Another feature of the irradiation of ammonium monofluoroacetate was an im- portant change in the ESR spectra on warming from 77° K to 300° K. Various changes have been noted in other fluor- 99 ine-containing radicals, but were not, apparently, ob- servable for the radical in irradiated monofluoroacetamide.83 The room-temperature radical 'CFHCOO’ was therefore selec- tively cooled and warmed to study this change in coupling with temperature. 96 Two recent publications indicate that there is a restrict- ed rotation or torsional oscillation about the C-C bond in 58 'CHZCOO' radicals found in zinc acetate single crystals and in 'CFZCOO’ radicals found in trifluoroacetamide83 single crystals. With these studies in mind, the ESR spectra of the monofluoro radical were observed on selective warming to 100°C (where decomposition of the crystal oc- curred). Crystals were also cooled to -1H0° C which is the most convenient stable low temperature available with the Varian variable-temperature unit; further cooling did not significantly change the spectra. These changes are shown for selected orientations of the crystal in the magnetic field in Figure lSa-i. Several features should be noted in the spectra (Figure 16). Changes in coupling did not produce any change from the four lines of equal intensity to a triplet with 1:2:1 intensity ratios as was observed in the two previously cited cases.58’83 The only significant change is in the fluorine coupling, while the proton splitting changes only about one gauss for any of the orientations studied. Further, no changes were observed for either the g values or the maximum carbon-13 coupling such as had been noted for the °CF2CONH2 radical studied by Bogan and Kispert.83 In order to determine whether the radical was under- going any motional changes with change in temperature, the room-temperature radical was studied also at -100° C. o . a) if”, ‘b ‘< 1r v F 18" it if 0 .60 «In 7' in lumpfc 34" h t H > qzar"’fio a 'U 13) g / 20“ .. 3: ‘( 1b 4b ”1 J» i F i .. -120 ‘ Jo + 4b 1bmpfc Figure 15(a-i). Variation with temperature of the fluorine (F) and proton (H) hyperfine splittings for the -CFHCOO“ radical. a) The magnetic field is parallel to the b direction. b) The magnetic field is directed 95° from b in the be plane. 98 .s c) d) 4230 : ~45 I 5 Fumxfl: Figure 15 continued - c) The magnetic field is directed 11° from a in the ca plane. d) The magnetic field is parallel to the a direction. 99 1. J. o 4» 4+ e) f) Figure 15 continued e) The magnetic field is parallel to the c direction. Subscripts I and II represent the two different radicals. f) The magnetic field is directed 25° from c in the ca plane. 4 “C’- 1. l 100 1» db g) h) m” H 4b 00 0 : f A : A ‘0 50 u) u” fimpvc Figure 15 continued g) The magnetic field is directed 25° from a in the ab plane. h) The magnetic field is directed 20° from b in the be plane. These fluorine and proton splittings are shown at higher temperatures. 101 “2+ 13% AJflNfl i) #:(lc Figure 15 continued 1) The magnetic field is directed 95° from b in the be plane. Subscripts I and 11 represent the two different radicals; sub— scripts I' and 11' represent the two magnetically inequivalent sites of Radicals I and II respec- tively. ‘ 102 N N H/la c2194 R H/lb Mm I'll/c. N” W‘ Wiwwwwwm I.- Figure 16. Second-derivative ESR spectra for 'CFHCOO’ radical produced in single crystals of ammonium monofluoroacetate irradiated at room temperature and observed at -1H0° C. 103 Rotations were made about the three orthogonal axes pre- viously selected. Alignment of the crystal was done at room temperature; to confirm the rotation angles the crystals were periodically warmed to room temperature and spectra taken for comparison with those previously obtained. Room temperature spectra taken with the magnetic field in the be plane show only a single radical in two magnetically inequivalent sites, while at -1u0° C two radicals each with two magnetically distinct sites appear to be present. Also, the magnitudes of the fluorine couplings, for all orientations, were significantly larger than those at room temperature. Isofrequency plots (Figure 17a,b,c) show that for rotation in the ca plane, both the maximum and minimum couplings have increased and are shifted approximately 10° from the corresponding magnetic field directions found for the room temperature radical. A comparison of the behavior of the minimum fluorine splitting on warming is shown in Figure 15c and d. When the magnetic field direc- tion is 11° from a in the ca plane (Figure 150), it is seen that the fluorine splitting increases with increasing tem- perature. On the other hand, it is seen that the fluorine splitting decreases with increasing temperature so that at room temperature, the minimum fluorine splitting now occurs for the magnetic field direction parallel to the a axis (Figure 15d). 10” blH-I— bill-Id- s 180° Figure 17a. Variation with magnetic field orientation of the ESR hyperfine lines in the be plane for the :CFHCOO' radical in irradiated ammonium monofluoroacetate at -lu0° C. Radicals I and II have two magnetically inequivalent sites and lines for the second sites are labeled 1', II'. 105 7 P1 80° clH— aIIH-- IZSG, C'H "' I "" 0° Figure 17b. Variation with magnetic field orientation of the ESR hyperfine lines in the ca plane for the -CFHCOO‘ radical in irradiated ammonium monofluoroacetate at -1u0°c. Radicals I and II have two magnetically inequivalent sites. 1.; III,"- i‘l‘l I 106 I 180° 1 all-la blHi-v - (r’ Figure 17c. Variation with magnetic field orientation of the ESR hyperfine lines in the ab plane for the -CFHCOO’ radical in irradiated ammonium monofluoroacetate at -190° C. 107 Further, observation of the isofrequency plot for the ca plane (Figure 17b) indicates two sets of spectra which appear to arise from two radicals unrelated by any crystal symmetry operation. This would indicate that there are two radicals present (Radical I and Radical II) with different crystal environments. The isofrequency plot for the be plane (Figure 17a) shows this same behavior but each radical is now in two magnetically inequivalent sites related by a mirror plane of symmetry; this indicates a site splitting for each radical based on the crystal sym- metry. The diagonalized tensors at -190° C gave the prin- cipal values and direction cosines listed in Table 10. Two sets of principal values resulted from a diagonalization of the data based on measurements from each of the two sets of lines. A computer calculation of the angular variation of the coupling with respect to the magnetic field, as discussed previously, showed that each of the two sets of principal values and direction cosines were related to another set only by a change in the signs of the direction cosines in column b of Table 10. A plot of the fluorine hyperfine coupling with respect to orientation in the mag- netic field for the three mutually orthogonal planes (Figure 18) clearly shows the presence of two radicals (I and II) and the corresponding set of magnetically inequivalent sites (I' and II'). A plot of the proton hyperfine coupling with respect to orientation in the magnetic field is shown I .IIIIItlItII. ill. I] 1!! NI. .. III-III j)b\\| 108 Table 10. Principal values and direction cosines for A(F) andA(H) for the two ~CFHCOO‘ radicals produced at room temperature and observed at -190° C by irradiation of ammonium monofluoroacetate. Principal . * Elements Spherical Direction Cosines (Gauss) 8 ¢ a b c Radical I A(F) -10.5 97.0 i 1.9 0.992 10.020 -0.l22 -28.0 98.8 192.2 -0.038 10.987 -0.153 193.9 11.2 152.9 0.117 10.157 0.981 “150 51.8 A(H) -11.7 106.3 $11.0 0.992 $0.183 -0.281 -39.9 88.6 178.6 0.198 10.980 0.029 -21.9 16.5 $17.0 0.271 $0.078 0.959 “180 -22.8 Radical II A(F) ~13.2 99.1 i 3.9 0.985 10.060 -0.159 -2l.2 98.7 199.9 -0.089 10.985 -0.152 188.1 12.6 197.2 0.198 10.163 0.976 “180 51.2 A(H) -12.9 100.9$ 8.0 0.979 $0.136 -0.180 —35.2 81.0 180.9 0.165 10.979 0.157 -2l.9 13.8 $99.9 0.159 $0.182 0.971 * With respect to the laboratory axis system specified in the text. Figure 18. 0’ 90" w Plot of hyperfine splitting with respect to magnetic field direction for fluorine coupling in the ~CFHCOO‘ radicals in irradiated single crystals of ammonium monofluoroacetate at -l90° C. h!‘)a\\- 110 in Figure 19. Because the range of couplings is small in the ab plane, the "in-plane" splittings are not as accurately determined as they were for the room temperature radical (Figure 17c). Nevertheless, the "in-plane" splittings observed for rotations in the ca and be planes (Figure 18) agree well, for example, with the principal A values ob- tained by diagonalization of the A(F) tensor utilizing all the data. That is, for the rotation in the ca plane, a maximum value of 191.6 G and a minimum value of 10.5 G are measured for Radical I. These are close to the values predicted from the A(F) tensor (Table 10) for rotation in a plane containing the maximum and minimum principal direc- tions. The relative signs of the principal A(F) values could not be determined from the spectra at -190°C but are presumably the same as those found at room temperature. Several observations can be made concerning the two sets of principal values at -190°C and the set at room temperature (Table 9). If the signs of the two smaller couplings at -190° C are assumed to be negative, as in the room temperaturecase, then only very small changes in the isotropic coupling are observed. Consequently, no important structural changes in the radical appear to occur. However, the observed decrease in anisotropic coupling with increase in temperature suggests that the radical is undergoing some motion (oscillation) such that the principal A(F) values 111 T) h 6- * ' ° 4 ii f 3i Figure 19. Plot of hyperfine splitting with respect to magnetic field direction for proton coupling in the :CFHCOO' radicals in irradiated single crystals of ammonium monofluoroacetate at -l90° C. “Uh! /\..— 112 at -190° C are partially averaged at room temperature. The motion cannot be a rotation since that would result in a cylindrically symmetric hyperfine tensor. An oscillation about the C-C bond direction would decrease Azz(F) and Ayy(F) would move toward less negative values as observed for both radicals. The Axx(F) value should also become less negative,which is found in the case of Radical II,but in Radical I it becomes more negative by 1.7 G. Also Axx(H) and Ayy(H) should move toward one another with increase in temperature and Azz(H) remain more or less un- changed; this is the observed behavior for both radicals. In view of the fact that the largest probable errors are those for the "in-plane" fluorine splitting values, the agreement with the behavior expected for an oscillation about the C-C bond is reasonably good. The changes in ESR spectra resulting from motions of a radical may be computed using a modified Bloch theory.170 Hayes and coworkers58 showed that the Bloch theory was not adequate to account for the spectral changes with tempera- ture for °CHZCOO' in a single crystal of zinc acetate, since it does not take into account effects of spin ex- change. That is, in a single crystal, hindered internal motion may lead to an exchange in spins between sites having hyperfine fields which must be averaged both in magnitude and in direction. Consequently, they found it necessary to resort to the more complete density matrix /\ 113 theory. The density matrix formalism has been well-developed 8,171 for many years. This method provides a nearly exact treatment for calculating spectral changes with motion. Recently, computational methods have been developed172 which make the calculations more feasible. This approach to magnetic resonance problems has been expanded by Binsch173 and subsequently was modified by Hayes and coworkers58 to treat the problem of motional averaging in °CH2COO‘ in zinc acetate. Bogan and Kispert83 followed these same methods but chose to define their tensors along the effec- tive magnetic field rather than along the crystal direc- tions. The results, however, were analogous. It was of interest to test this method on the °CFHCOO' radical. The density matrix program of Hayes et al.58 was modified to run on the CDC 6500 computer. Calculations for °CFHCOO' showed that the resultant computed spectra did not fit the experimental spectra satisfactorily. The motion does not seem to be well described by the torsional oscillation model. This is not too surprising since the density matrix calculation predicts an averaging of the inner lines of the spectrum along with a change in the coupling of the outer lines. Experimentally, no averaging of the inner lines is observed. It appears that more complex changes are occurring on cooling than it is possible at present to include in .1 II'II ll I’I‘ I IlllI. I )s\iI.I 119 the density matrix formalism. In particular, the single radical observed at room temperature is replaced between 25°C and -30° C (Figures 15e and 151) by two radicals with different orientations and different A(F) and A(H) tensors. This change could result from a change in crystal structure of the matrix to a lower symmetry space group. On warming the principal components of A(F) and A(H) for each radical change in the manner predicted for a small oscillation about the C-C bond. However, A(F) and A(H) for Radicals I and II do not average to the room temperature tensors. The density matrix method as applied here could not include all these factors and so could not be expected to reproduce the observed spectra. Further progress would require crystallographic data over the temperature range studied. B. Ammonium Difluoroacetate 1. Room Temperature Radicals. Irradiation of am- monium difluoroacetate was accomplished at room tempera- ture. Selected orientations indicated that the spectrum was considerably more complicated than that of the mono- fluoroacetate. No crystal structure has been reported, so alignment was made along the extinction axes using the polarizing microscope. For orientation of the magnetic field along one axis, a much simplified spectrum was ob- tained. There were two sets of lines with slightly dif- ferent g values indicating that there were two distinct 115 radical species. One set, a doublet of doublets, was similar to the spectrum of the monofluoro radical and was assigned to the °CFHCOO' radical. The triplet pattern with intensity ratios approximately 1:2:1 showed a maximum separation of about 360 gauss in- dicating coupling from two equivalent fluorines; this was assigned to the 'CF2COO' radical. An isofrequency plot for rotation of the magnetic field in the ca plane (Figure 20) shows the complexity of the spectra resulting from the pres- ence of magnetically distinguishable sites. The spectra show that there are as many as four sites for the difluoro radical and two sites for the monofluoro radical. Hence, either the crystal structure is different from that of the monofluoroacetate and/or the radicals are oriented dif- ferently in this crystal. As the coupling of the fluorine approaches a minimum, it becomes nearly impossible to distinguish between the sites or between the radicals (Figure 21). Spectra obtained on rotating the magnetic field in the planes of the radicals were not interpretable. Never- theless, a maximum fluorine coupling could be assigned, along with carbon-13 hyperfine splitting values, for the specific orientation which is perpendicular to the plane of both radicals and parallel to the p1r orbital containing the unpaired spin (Figure 22a). Again, the directions of the maximum components of the carbon-l3 and fluorine-19 hyperfine splitting tensors coincide within the experimental 116 CM 1 T. I alH1 clIH _ 00 Figure 20. Variation with magnetic field orientation in the ac plane of the ESR hyperfine lines for the -CFHCOO’ and -CF2COO‘ radicals produced in irradiated single crystals of ammonium di- fluoroacetate at room temperature. [506; b) MD Figure 21. 117 H/lc’ H/Ia Second-derivative ESR spectra of the room tem- perature radicals, ~CFHCOO‘ and °CF2COO’, pro- duced in irradiated single crystals of ammonium difluoroacetate. Spectrum (a) was obtained with the magnetic field parallel to the maximum fluorine coupling (designated the c direction). Spectrum (b) was obtained with the magnetic field 90° from c (designated a). 118 Figure 22a. Second-derivative ESR spectrum of the radicals produced :hl single crystals of ammonium di- fluoroacetate irradiated at room temperature. The direction of the magnetic field is parallel to c. The gain has been increased by 1000 to permit observation of the 13C satellites since the natural abundance of 13C is only 1.1%. .A A O // {I B .50<3. C: Figure 22b. First-derivative ESR powder spectrum of ammonium difluoroacetate irradiated at 77° K. t f 119 3(19 C) may be ob- error and the values of A2 F) and A33(13 tained directly. In this case, A22: 180.2 G (E of Figure 22a) for the two fluorine atoms and A23 = 130.5 G (e of Figure 22a) for carbon-13 in the ‘CFZCOO’ radical. For the -CFHCOO’ radical the values are: 223(19r) = 185.2 0, Azch) = 21.9 G (r of Figure 22a), and Azz(13C) = 86.2 G (f of Figure 22a). 2. Analysis of 'CFHCOO- and 'CF2COO- Spectra. With the available data, it is possible to discuss the radical geometry and electronic structure. Since the 'CFHCOO' radical is nearly axially symmetric, the primary contribu- tion to the anisotropy must arise from electron spin density delocalized into the F2?" orbital: Contributions of the second structure are of the order of 12%. This estimate is based on the values (1080, -590, -590) calculated for an electron in a pure 2p orbital. For the ammonium fluoroacetate radical, the anisotropic components (Table 9) are (130.9, -67.8, -62.8). On the other hand, the departure from complete cylin- drical symmetry indicates a spin polarization of the 2pO orbital of the C-F bond; it has been proposed85 that ‘Im‘ LI- \v" [\I 120 this is negative and of the order of -0.003. This value is determined by a further separation of the anisotropic tensor to give: 130.5 132.2 ' -1.7 -67.8 = -66.1 + -l.7 -62.8 -66.1 +3.9 The Fermi contact interaction resulting from the odd electron density in the F23 orbital leads to the isotropic coupling of 50.9 G. An upper limit to the spin density in the fluorine 23 orbital may then be determined from the value calculated for an electron in a pure 23 orbital, (16,900 G), to be 0.0031. There is some contribution to this value from configuration interaction with the Is orbital. The analogous values for carbon-l3 are given by (99.0, —21.3, -22.7) for the anisotropic part and 91.3 gauss for the isotropic part of the coupling. The 02p1T spin density calculated from these values is approximately 0.70. Certainly one expects this value to be somewhat larger (0.83) based on the spin densities on hydrogen (018(H) = 0.095) and on fluorine (02p(F) = 0.12). Using McConnell's 110 relation for a-proton coupling, aCH = Qp , one obtains Cg" 02p(C) 5 1.0 using Q = -23 G. This disagreement is perhaps not unexpected since the effective charge for the wave function used to determine the B parameter undoubtedly 4/1] 121 differs from the effective charge on carbon in the fluorine- substituted radical. The significant difference observed between Azz(13C), the coupling value along the pH orbital direction, for ~CFHCOO' and -CF2COO' would indicate that there is a dif- ference between the isotropic values a(130) for these radicals. The three components of the A(lBC) tensor for the ~CF2COO' radical in ammonium difluoroacetate could not be deter- mined with certainty; however, an estimate of a(13C) could be made. Several tensors for these fl-type radicals have been obtained and the anisotropic part of the coupling is observed to be nearly symmetrical with the form (2B, -B, -B) and a value of 2B 5 95 G. Hence, the a(l3C) value becomes approximately 85 gauss for °CF2COO'. Following 97 it is reasonable to attribute the Fessenden and Schuler, increase in a(13C) with increase in fluorine substitution to increasing deviation of the radicals from planarity and consequently, an increase in the 3 character of the orbital. Values for 8, the angle of bending from the plane of the radical, are computed on the basis of the formula 13 a( G) = a0(130) + 1190(2 tange), (26) with a0(130) taken as 38.5 gauss assuming 'CH3 is planar.101 This formula leads to a nearly planar structure for -CFHCOO' radical while the -CFZCOO‘ system bends slightly,to ap- proximately 8° from the plane. The odd-electron orbital 122 would then have 9-5% 8 character in the difluoro radical. Similar values were determined by Rogers and Kispert89 for other fluorine containing radicals. 3. Spectra at 77° K. Identification of the radicals formed by irradiation of ammonium difluoroacetate at liquid nitrogen temperature has not been successful. Apparently the two radicals found at room temperature are present at 77° K, but at different orientations so that analysis of the spectra is difficult. In addition, though, a very weak coupling pattern was noted with a large maximum hyperfine splitting value. This radical would have to be charac- terized as arising from two fluorine atoms, because the coupling is greater than 360 gauss, plus a smaller doublet from an a proton. Unfortunately, due to the weak intensity of the lines and to overlapping from those of the other more abundant species present no further results may be reported from the single crystal. It is interesting to note the powder spectrum for this sample irradiated at 77° K (Figure 22b). Normally, powder 17” However, in this spectra appear too complex to analyze. case several observations may be made. At increased gain, a pair of doublets (A, Figure 22b) separated by about 900 G is observed. This splitting must arise from two fluorine nuclei and hence, one of the peaks in the central portion of the spectrum must form the central line (C) of a triplet 123 with a fluorine coupling of 211 G. The splitting for the doublet substructure present for each of the outer pairs (A) is estimated to be 22 G and so probably arises from an a proton. The intermediate pair of doublets (B) with considerably greater intensity, cannot belong to the same multiplet as the triplet (A,C,A) since the splitting is different. The separation of 190 G between these lines (B, Figure 22b) suggests that they arise from an a fluorine and the doublet substructure of 22.6 G is indicative of an a proton. The outer-line couplings in a powder correspond 179 to 922’ the 9 value in a direction perpendicular to the plane of the radical. Consequently, a tentative assignment of the inner and more intense set (B) to the 'CFHCOO" radical is made and the outer set (A,C,A) is assigned to the 'CFZH radical. Since the isotropic hyperfine fluorine interaction in °CF2H (89.2 G)97 is considerably larger than that in °CFHCOO- (50.9 G), it is not unreasonable to assign the values 211 G and 190 G for A33(F) to -CF2H and -CFHCOO', respectively. C. Ammonium Trifluoroacetate O O O O O 0 88 Prev1ous studies on 1rrad1ated ammonium tr1f1uoroacetate gave the 'CFZCOO- radical at room temperature. However, low- temperature (77° K) irradiation yielded an unusual spectrum with three anisotropic fluorine couplings and one coupling which was assigned to a proton. The radical was reported 129 to be [CF3COOH-NH31- on the basis of the doublet splitting since its magnitude (19.7 G) was about that expected for a long-range proton coupling. However, the isotropic fluorine coupling data indicate that the radical is probably 'CF3 since the splitting is comparable to that found for 'CF3 in trifluoroacetamide.87 If it is 'CF3 two points may be noted,(l) the fluorine couplings are all inequivalent, indicating restricted motion, and (2) there would be no explanation for the extra doublet coupling. With this ambiguity in mind crystals of CFBCOOND1+ grown from a solution of D20 were irradiated at 77° K. The spectra for rotations in the ac and be planes chosen by Srygley and Gordy88 were exactly comparable to their spectra. Further, on slight warming, no changes in the spec— tra which would indicate motional averaging were observed in the very small temperature range of stability of this radical; about 200° K,-CF2COO- is formed. The unusual doublet structure persists even in this deuterated sample. Consequently, one can assign the doublet coupling to a long- range interaction such as that with a fluorine in the lattice. This unusual type of coupling has been reported for one of the 'CF2COO' radicals found in irradiated sodium chlorodi- fluoroacetate.89 Here the coupling was explained as probably arising from two fluorine atoms in the lattice to give a triplet. The magnitudes of the couplings are rather similar. The assignment of °CF3 as the structure of the radical is .NIIIN nN|.||.tl 125 thus reasonable. D. Ammonium Chlorodifluoroacetate The usefulness of these ammonium salts for electron spin resonance studies was further shown with the irradia- tion of ammonium chlorodifluoroacetate. Irradiation at 77° K indicates,from the spectrum presented in Figure 23,that the °CF2C1 radical is present. Here again, the hyperfine structure is very complex and interference from another radical species makes only tentative assignment possible. However, observations of the spectrum taken at an orientation showing maximum splitting (360 gauss) of the outer lines indicates that at least two fluorine atoms must be present. The fine structure then, on the outer lines can only be explained on the basis of superhyperfine coupling from a chlorine atom leading to the choice of °CF2C1 radical as the probable species. On warming to room temperature, the spectrum clearly indicates that the °CF2COO- radical is the only species remaining and there is no longer any chlorine fine struc- ture associated with the spectrum. E. Radical Formation The large amount of information now availablel’2 on the formation of radicals in single crystals of fluorine- containing organic compounds enables some generalizations to be made. For the sequence CFHZCOONHu, CF HCOONHu, 2 IX 1‘2. 126 .mvoamflpase sopdo one mo wcfippwamm EBEOme mm>sm nosnz pony mw soap Imvcofiso 039 .x one we cowpmfiomspw zn ounpoomososamaoosoano ESOCOEEM mo Hmvmmpo oawcwm m ow ooosoopm mamowoms mo Edspomam mmm o>wpm>wsoolocooom .mm osowsm soon. 127 CF3COONHu and CF2C1COONHu it appears that two processes are occurring on irradiation at 77° K; carbon-carbon bond breaking and carbon—halogen bond breaking. There seems to be a preference for CF bond breaking over CH bond breaking, for example, in CFHZCOONHu. One would expect to find twice as much °CFHCOO- if the loss of hydrogen and fluorine atoms were equally probable. However, approximately four times as much 'CHZCOO' is found. And, in the difluoro- acetate there is a large preference for °CFHCOO' at 77° K. On the other hand, the loss of chlorine appears to be favored over loss of fluorine in the ammonium chlorodifluoroacetate system. This is supported by the evidence reported for radicals produced in irradiated sodium chlorodifluoroacetate?9 Carbon-carbon bond breaking also appears to be one result of low-temperature irradiation. However, this seems to depend on the number and type of halogen substituents. That is, CC bond breaking takes place in the monofluoro system. However, successively larger amounts of radical are produced with successive fluorine substitution, with relatively large concentrations in the trifluoro system. It is interesting to note that the ~CH3 radical is the predominant radical formed on irradiation of CH3COONHu single crystals at 77° K.“7 On warming, abstraction reactions appear to be taking place. For CFHZCOONHu, the 'CH COO- abstracts a proton 2 from a neighboring molecule to give 'CFHCOO- at room 128 temperature. In CFZCOONHQ, both 'CF2H and some CFHCOO- must abstract protons from the neighboring molecules to account for the relative abundance of -CF2COO' at room temperature. Also, the -CF3 radical abstracts a fluorine from the lattice to give -CFZCOO- in ammonium trifluoro- acetate while 'CFZCl must abstract a chlorine from a neighbor to produce 'CFZCOO' at room temperature in am- monium chlorodifluoroacetate. CHAPTER VI SUMMARY Electron spin resonance studies of radicals in several irradiated acetates and halogen-substituted acetates have been carried out. (1) Single crystals of CH3COONa'3D2O have been ir- radiated at 77° K and 300° K and the ESR spectra studied over a range of temperatures. Three radicals were found; 'CH3 predominates at 77° K, -CH2COO' at 198° K and 'C02- at room temperature. The 9 tensor and proton hyperfine splitting tensors were determined by analysis of the spec- tra. In addition, -CD3 was studied in irradiated single crystals of CD3COONa°3D O at 77° K. 2 A study of the ESR spectrum of methyl radical in the range 9.2° K - 77° K was made. At 77° K the ESR parameters show that it is a n-electron radical with three equivalent hydrogens and that it is undergoing rotation about the threefold axis. At 30° K relative intensities of the four lines were found to change from l:3:3:l to l:l:1:1 indicating some restriction of rotation at low temperature. The ESR parameters for 'CHZCOO- and 'C02- are similar to those observed for the same radicals in other matrices. 129 130 The mechanism by which 'CHZCOO- is produced from -CH3 appears to be hydrogen abstraction from a CH3COO- ion of the matrix. (II) A study of the ESR spectra of y-irradiated single crystals and powders of anhydrous sodium acetate was made at 77° K. The °CH2COO- radical was obtained and the A(H) tensors evaluated. In addition, a triplet spectrum was observed which was attributed to a radical pair. Although the components of the pair appeared to be different,the spectra did not permit their positive identification. (III) The ESR spectra of y-irradiated single crystals of ammonium monofluoroacetate were studied over the tempera- ture range 77° K a 373° K. It was found that the radicals -CH2COO- and 'CFHCOO- were produced in the approximate ratio 9:1 on irradiation at 77° K. On warming to room tem- perature only the 'CFHCOO- radical was present. The °CFHCOO' radical at room temperature was found to occupy two magnetically inequivalent sites. On cooling, the spectra were observed to change such that two °CFHCOO-, each with different A(F), A(H) tensors, were obtained and each occupied two magnetically distinct sites. The g and the fluorine and hydrogen hyperfine splitting tensors were evaluated for both radicals at -l90° C and for the room— temperature radical; for the room-temperature radical the 13 C hyperfine interaction tensor was also obtained. An oscillation of the CHF group of the radical about 131 the C-C bond was postulated to account for the dependence of the ESR spectra on temperature. However changes in crystal structure may also occur. (IV) A study of the ESR spectra of y-irradiated single crystals and powders of ammonium difluoroacetate was made at 77° K and at room temperature. Material irradiated and observed at 77° K gave complex spectra which could not be analyzed in detail but the powder spectra indicated that both -CF2H and °CFHCOO- were produced, the former in relatively low concentrations, and possibly a third radi- cal. On warming the -CF2H radical disappeared and at room temperature the spectra showed the presence of °CF2COO_ and -CFHCOO- in approximately equal proportions. Measure- ments of the maximum components of the 13C hyperfine »splitting tensors were made for both radicals and from these the isotropic components were estimated. These values show that 'CFHCOO' is a nearly planar n-electron radical while °CF 000' is bent about 8° out of the plane. 2 (V) An investigation of the radical species present in irradiated ammonium tr1f1uoroacetate-du (CF3COONDH) was made. ESR spectra of single crystals irradiated and observed at 77° K show strong evidence that the °CF3 radical is produced, along with 'CFZCOO-. A long-range coupling from a neighboring atom was not eliminated in the deuterated salt as would have been expected from a study reported in the 132 literature in which the radical was assigned the structure [CF3COOH..NH3]- instead of -cr3. (VI) Single crystals of ammonium chlorodifluoroacetate were irradiated and the ESR spectra observed at 77° K. These show the presence of at least two radicals, one show- ing chlorine and fluorine hyperfine splitting and the second fluorine hyperfine splitting only. The radicals were assigned the structures 'CF2C1 and -CF2COO-. 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