KAZUMITORIYAMA AND MACHIO IWASAKI
1824 The essential point is that the two protons contributing to the esr spectra were originally a protons of adjacent oxime molecules. Thus, both the heat-stable and uv-stable radicals are bimolecular reaction products formed between two oxime molecules. The reacting oximes are undoubtedly oriented in head-to-head fashion in the urea lattice. The geometrical restraints
imposed by the tubular cavities of urea inclusion crystals facilitate reactions that would not be easily observed in studies of the pure oximes.
Acknowledgments. We are grateful to Professors John F. W. Keana and Charles E. Klopfenstein for useful discussions.
Electron Spin Resonance Evidence for the C-F Bond Rupture by Dissociative Electron Attachment by Kazumi Toriyama and Machio Iwasaki* Government Industrial Research Institute, Nagoya, Hirate, Kita, Nagoya, Japan
(Received M a y 6 , 19Yl)
Publication costs assisted by the Government Industrial Research Institute
To examine the possibility of the C-F bond rupture by dissociative electron attachment to fluorine, esr study has been carried out using irradiated glasses of 2-methyltetrahydrofuran containing small amounts of fluorinated compounds such as CFHtCONHz, CHF&OOEt, CFsCONH2, CFaCOOEt, CF&OOH, c-CgF12, and CF&lCONHZ. Among these, the first three compounds gave clear evidence that radicals are formed from the rupture of the C-F bond by electron attachment followed by the dissociative process. The results obtained from a series of compounds indicate that the partially fluorinated compounds are favorable to the dissociative electron attachment and that the electron-withdrawing tendency of the functional group is another important factor for the dissociative process. Our results may suggest that the dissociative electron attachment plays an important role in the formation of radicals in some irradiated fluorine-containing compounds, although the cross section is not so large as compared with that in chlorine-containing compounds.
Dissociative electron attachment to organic halides [RX e- 4 . R X-] in irradiated organic glasses has been thoroughly studied by esr and optical measurements.’ It has been pointed out that the dissociative process occurs when the electron affinity of X exceeds the bond dissociation energy of R-X.It2 Since the C-F bond strength exceeds the electron affinity of the fluorine atom, it is believed that the electron attachment to some fluorinated compounds is nondissociat i ~ e . l J - ~I n this connection, the mechanism of the C-F bond rupture in the y radiolysis of fluorocarbons and hydrocarbon-fluorocarbon mixtures has been the subject of many recent ~ t u d i e s . ~ - ~ On the other hand, we have found in our previous work’O that cH2CONH2is produced in a crystal of monofluoroacetamide irradiated at 77°K and that upon warming cHzCONH2abstracts an hydrogen atom from the neighboring molecule forming cHFCONHe. This means that the primary radical in partially fluorinated compounds is the one formed by the selective breakage of the C-F bond and that the secondary radical is produced by the selective abstraction of the hy-
+
+
The Journal of Physical Chemistry, Vol. 76, N o . 13, 1972
drogen atom. It was also found that the radical pair between cH2CONH2 and cHFCONHz is formed at 77°K as a minor product. Although a mechanism involving hot fluorine atoms liberated from the direct breakage of the C-F bond was assumed in our previous paper,I0 we became aware of the importance of ionic processes shortly after submitting our manuscript.” (1) W. H. Hamill, “Radical Ions,” E. T. Kaiser and L. Kevan, Ed., Interscience, New York, N. Y . ,p 321. (2) D. W. Skelly, R. G. Hayes, and W. H. Hamill, J . Chem. Phys., 43, 2795 (1965). (3) L. A. Rajbenbach, J . Amer. Chem. SOC.,88, 4275 (1966). (4) N. H. Sagert and A. S. Blair, Can. J . Chem., 46, 3284 (1968). (5) L. A. Rajbenbach, J . Phgs. Chem., 73, 356 (1969). (6) N. H. Sagert, J. A, Reid, and R. W. Robinson, Can. J . Chem., 47, 2655 (1969). (7) N. H. Sagert, ibid., 46, 95 (1968). (8) N. H. Sagertand J. A. Reid, ibid., 48, 2429 (1970). (9) M. B. Faligatter and R. J. Hanrahan, J . Phys. Chem., 74, 2806 (1970). (10) M. Iwasaki and K. Toriyama, J . Chem. Phys., 46, 4693 (1967). (11) M. Iwasaki and K. Toriyama, Tenth Symposium on Radiation Chemistry, Hiroshima, Japan, Oct 1967.
ESREVIDENCE FOR THE C-F BOND RUPTURE We therefore examined the possibility that radicals are formed from the rupture of the C-F bond by the dissociative electron attachment in irradiated 2-methyltetrahydrofuran (R'ITHF) glasses of monofluoroacetamide and its related compounds. We have then found evidence for the dissociative electron attachment to such fluorinated compounds as monofluoroacetamide, ethyl difluoroacetate, and trifluoroacetamide in M T H F glasses. No such evidence has been obtained for other fluorinated compounds such as trifluoroacetic acid, ethyl trifluoroacetate, and perfluorocyclohexane.
Experimental Section M T H F used. was purified by passing through a column of silica gels. CHzFCONH2, CF&ONHZ, CHFZCOOEt, CF3COOEt, CF3COOH, and CCIFzCONHz, which were used as solutes in this experiment, were obtained from Pierce Chemical Co. Perfluorocyclohexane was supplied from the fluorine chemistry laboratory of our Institute. Esr samples were prepared in a vacuum line by distilling M T H F into the Spectrosi1 tube containing each of these fluorinated compounds in an amount of 1-2 mol %. Irradiations were made by 6oCoy rays to a. dose of 5 X lo4rads a t 77°K in the dark. Esr spectra were measured at 77°K by a JEOLCO 3BS-X spectrometer operated a t X band.
1825
+ e-+
CHFzCOOEt
aCHFCOOEt
+ F-
(1)
A similar experiment was also carried out using CHzFCONHz as a solute. I n this case, the radical expected from dissociative electron attachment is ~ H z CONHz having no a fluorine, so that the superposition of the spectrum due to the M T H F radical makes it difficult to obtain clear evidence for the formation of this radical. However, the signal due to the trapped electron was greatly reduced by adding 2 mol % of CH2FCONHz, and the spectrum obtained after photobleaching of the remaining trapped electron was found t o be a superposition of the signals due to CHzCONHz and M T H F radical. This was confirmed by comparing the spectrum with that obtained from the M T H F glass containing CHzCICONHz which is expected to give cH2CONHz. The spectra were reasonably similar except that the addition of 1-2 mol % of CHzCICONHzresulted in the complete disappearance of the signal of the trapped electron. This may mean that the cross section for electron attachment is smaller in CHzFCONHz than in CHZClCONHz. To compare the cross section for dissociative electron attachment to fluorine with that for chlorine, a similar experiment was also carried out using CC1F2CONH2. In this case the only radical formed was cFZCONHz,
Results and Discussion Figure l a shows the esr spectrum of the trapped electron in the MTHF glass containing no additives. When 2 mol of CHFZCOOEt is added, the single line due to the trapped electron is replaced by a pair of doublets as shown in Figure lb. The central seven lines are mainly from the RITHF radical. The pair of doublets can be interpreted by the hyperfine line shape as due to anisotropic a-fluorine coupling. As previously r e p ~ r t e d l ~the * ' ~large anisotropy of the a-fluorine coupling is known to give the wing peaks corresponding to the large value of A 1 1 , which ranges from 180 to 200 G.14 It has also been demonstrated that the wing peaks further split into the hyperfine lines due to the small couplings with other nuclei. It is evident that the wing peak. separation of about 200 G in Figure l b arises from the coupling with one a-fluorine nucleus. The small splitting, 22 G, of each wing peak indicates the existence of the a-proton coupling. The hyperfine lines corresponding to the Al component of the a-fluorine coupling tensor are superposed on the central lines due to the M T H F radical, resulting in considerable changes in the intensity ratio of the seven-line spectrum of the M T H F radical. Thus the radical responsible for this spectrum can be assigned to CHFCOOEt produced by the C-F bond rupture. Since this radical is formed in the M T H F glass, we conclude that the C-F bond scission occurs by dissociative electron attachment
II I I I I I I ~
AH H t
II II\I 11'
AF(mnx1
H I
Figure 1. Esr spectra of (a) MTHF glass and (b) CHFaCOOEt (2 mol %) in MTHF glass, ?-irradiated at 77°K to a dose of about 5 x lo4 rads. Spectrum b was measured with a gain five times higher than spectrum a. (12) M. Iwasaki, K. Toriyama, and B. Eda, J . Chem. Phys., 42, 63 (1965). (13) M.Iwasaki, ibid., 45, 990 (1966). (14) M.Iwasaki, Fluorine Chem. Rev., 5, 1 (1971).
The Journal of Physical Chemistry, Vol. 76,No. 13,1972
G. POLLARD, N. SMYRL, AND J. P. DEVLIN
1826 and no evidence for &XFCONH, was obtained. This means that the cross section for dissociative electron attachment to fluorine is very much smaller than that to chlorine. On the other hand, no spectral evidence was obtained for the formation of CF2COOEt in M T H F glass containing CF3COOEt. This is in marked contrast to the case of CFzHCOOEt and may result from the lower C-F bond strengths of the partially fluorinated compounds compared to those of the trifluoro corn pound^.'^ It was, however, found that in a series of trifluoro compounds, CF3COOH, CF3COOEt, and CF3CONH2, having different functional groups, only the last compound having the least electron-withdrawing group gave the radical CF2CONH2 formed from C-F bond rupture. This suggests that the electron-withdrawing tendency of the functional group is another important factor for the dissociative process. Since the y radiolysis of cyclohexane-perfluorocyclohexane mixture has been the subject of many recent studies in view of the mechanism of the C-F bond rupt~re,~ a similar -~ experiment was also carried out using perfluorocyclohexane. However, no spectral evidence was obtained for the formation of c - C O F ~in . the M T H F glass containing c-C6FI2,although its formation is reported in pure c-CsF12 irradiated at 770K.16
I n our previous worklo on an irradiated single crystal of CHZFCONHZ, we have assumed the direct breakage of the C-F bond by ionizing radiation producing the CH2CONHz and a hot fluorine atom which reacts with the surrounding molecules to form the radical pair. However, the results obtained in the present study suggest that the ionic process plays an important role in the formation of the isolated CHzCONH, radical as well as the radical pair as described in our paper on the pairwise trapping of radicals in normal hydrocarbons.’7 Acknowledgment. The authors wish to thank Dr. Z. Kuri of Nagoya University for his suggestion of the possibility of the ionic process in irradiated CH2FCONH2. They also wish to thank Mr. H. Muto of our laboratory for his cooperation in the experiments carried out for the M T H F glasses containing C-COFIZ and CFsCOOH. They are grateful to Dr. S. Nagase and Mr. H. Baba of our Institute for providing the sample of C-CBFlz. (15) L. Pauling, “The Nature of the Chemical Bond,” Cornel1 University Press, Ithaca, N . Y., 1960, p 314. (16) C. Chachaty, A. Forchioni, and M. Shiotani, Can. J . Chem., 48, 435 (1970). (17) M . Iwasaki, T . Ichikawa, and T. Ohmori, J . Chem. Phys., 50, 1991 (1969).
Infrared and Raman Spectra of Group I Nitrate Aggregates in Carbon Dioxide Matrices and Glassy Thin Films by Gary Pollard, Norman Smyrl, and J. Paul Devlin” Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74074 (Received January 10, 1978) Publication costs assisted by the National Science Foundation
Aggregates of the group I nitrates in COz matrices have been prepared by reducing the matrix gas-to-sample ratio to well below the value required for complete monomer isolation. The infrared and Raman spectra of the resulting LiN03 aggregates, through comparison with published melt spectra, clearly indicate a glassy rather than crystalline structure. It has further been discovered that deposition of the nitrate vapors a t -180°, in the absence of any matrix gas, permits the formation of pure glass thin films and presents the possibility of preparation of bulk quantities of the group I nitrate glasses.
Introduction It has recently been shown that the alkali metal nitrates and TWO3 can be volatilized smoothly with very little decomposition at temperatures near their respective melting Consequently, it was possible to characterize spectroscopically the monomers (&lN03), and, in some cases, the dimers (XINO&, The Journal of Physical Chemistry, Vol. 76, No. IS,19%
of a number of the group I nitrates4 using the familiar methods of matrix isolation of high temperature (1) A . Blichler and J. L. Stauffer, J . Phys. Chem., 70, 4092 (1966). (2) C. J. Hardy and B. 0. Field, J . Chem. Soc., 5130 (1963). (3) J. P. Nalta, N. W. Sohubring, and R . A. Dork, from “Thin Film Dielectrics Symposium,” The Electrochemical Society, New York, N. Y., 1969, p 236.
