J. Phys. Chem. 1982, 86, 325-326
325
Electron Spin Resonance Study of the 2-Adamantyl Radical Robert J. Wattman,+ A. Campbell Ling,' Department of Chemistry, Sen Jose Stete University, San Jose, California 95192
and J. Bargon IBM Research Labor8tory, San Jose, California 95 193 (Received: November 18, 198 1)
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A liquid-phasenarrow-linewell-resolved ESR spectrum attributed to the 2-adamantyl radical has been observed. The a-proton hyperfine splitting constant (hfsc) is found to be 22.72 G , and tentative values for other hfsc values have been postulated.
Introduction The use of adamantane as an isolation matrix for the study of solute organic radicals generated by radiolysis methods is now well established, but considerable controversy in the literature has existed as to the nature of the radicals produced by y radiolysis of adamantane it~e1f.l-l~These data have been summarized by Wood and ~o-workers.'~More recently, Migita and Iwaizumi15investigated the temperature dependence of ESR and ENDOR spectra of y-irradiated adamantane, and suggested the presence of some ring-opened radicals along with the 1-and 2-adamantyl radicals. Hyperfine splitting constant (hfsc) values based on ENDOR spectra were also reported for the 1-and 2-adamantyl radicals. Indirect evidence that H atom loss in adamantane may preferentially occur from the bridgehead rather than the secondary position was provided by Waltman and Ling.16 These conclusions were based on vapor-phase ionization studies of both adamantane and its homologue diamantane by conventional mass spectrometry. Finally, reported solid-state wide-line ESR spectra of both 1-and 2-adamantyl radicals have been obtained by simultaneous deposition of sodium with 1- or 2-bromoadamantane in carefully purified adamantane.17 The hfsc values obtained for the 1-adamantyl radical matched well with the high-resolution data for 1-adamantyl radicals obtained by Krusic et a1.16 but the spectrum of the 2adamantyl radical consisted of only a broad doublet which yielded an CY hfsc value of 20.5 G. Resolution of the controversy concerning the predominant radical produced by radiolysis of adamantane has been severely hampered by the lack of an authentic well-resolved 2-adamantyl ESR spectrum, and further aggravated by the difficulties associated with obtaining such a detailed spectrum. We report herein observation of a liquid-phase narrow-line well-resolved ESR spectrum attributed to the 2-adamantyl radical, obtained via photolysis of a solution of 2-bromoadamantane in the presence of a bromine-abstracting agent according to a method described by Hudson et a1.18 Experimental Section 2-Bromoadamantane, cyclopentane, cyclohexane, ditert-butyl peroxide, triethylsilane, hexamethylditin, hexamethyldigermane, hexamethyldisilane, and tributyltin hydride were obtained from commercial sources. 2Bromoadamantane was purified via recrystallization from freshly distilled cyclohexane and vacuum sublimed prior 'Taken from work presented in partial fulfilment of the requirementa for a MS degree, San Jose State University, 1981. OO22-3654/82/2O86-O325$0 1.2510
to use. ESR spectra (Varian Associates E-104A EPR spectrometer) were obtained via photolysis of a mixture of a 10-2-lo-' M solution of 2-bromoadamantane in either cyclopentane or cyclohexane with triethylsilane and ditert-butyl peroxide, with a 1OOO-W Hg-Xe arc lamp (Oriel Corp.) focussed into the ESR cavity via a quartz lens. Typical spectrometer settings were 0.3-1.0-mW microwave power, 0.20-0.80-G modulation amplitude at a frequency of 100 kHz, with a field setting at 3240 f 50 G at frequency 9.1 GHz, with scan times from 30 to 60 min. Optimum signals were observed at experimental temperatures below -80 "C with either cyclopentane or cyclohexane as solvents. The sample mixtures were placed in a quartz ESR tube and degassed by the "freeze-pump-thaw" method, then sealed under vacuum and used directly for photolysis. When hexamethylditin, hexamethyldigermane, hexamethyldisilane, and tributyltin hydride were used in place of triethylsilane, the results were unsatisfactory. Hexamethylditin did produce the 2-adamantyl radical, but of poor intensity, while hexamethyldigermane gave rise to a self-derived radical species. No recognizable signals were obtained with either hexamethyldisilane or tributyltin hydride. The g value of the 2-adamantyl radical was obtained by injecting a dilute solution of DPPH in cyclopentane after photolysis while maintaining all other conditions as before. The g value of DPPH (g = 2.0036; see ref 19) was cali(1)K. W. Bowers, G. J. Nolfi, and F. D. Greene, J. Am. Chem. SOC., 86,3707 (1963). (2)M. T. Jones, J. Am. Chem. SOC.,88,174 (1966). (3)D. R. Gee, L. Fabes, and J. K. S. Wan, Chem. Phys. Lett., 7,311 (1970). (4)L. Bonnazola and R. Marx, Chem. Phys. Lett., 8,413 (1971). (5)J. R.Ferrell, G. R. Holdren, R. V. Lloyd, and D. E. Wood, Chem. Phys. Lett., 9,343 (1971). (6)P. J. Krusic, T. A. Rettig, and P. von R. Schleyer, J.Am. Chem. SOC.,94,995 (1972). (7)W. G. Filby and K. Gunther, Chem. Phys. Lett., 14,440 (1972). (8)W. G. Filby and K. Gunther, Chem. Phys. Lett., 17, 150 (1972). (9)R. Marx, Chem. Phys. Lett., 17,152 (1972). (IO) R. V. Lloyd and M. T. Rodgers, Chem. Phys. Lett., 17,428(1972). (11)W. G. Filby and K. Gunther, 2.Naturforsch., 27, 1289 (1972). (12)G. J. Hyfantis and A. C. Ling, Chem. Phys. Lett., 24,335(1974). (13)G. J. Hyfantis and A. C. Ling, Can. J. Chem., 62,1206 (1974). (14)G. C.Dismukea and J. E. Willard, J. Phys. Chem., 80,1435(1976). (15)C.T.Migita and M. Iwaizumi, Chem. Phys. Lett., 71,322(1980). (16)R.J. Waltman and A. C. Ling, Can. J. Chem., 58,2189 (1980). (17)R.V. Lloyd, S. DiGregorio, L. DiMauro, and D. E. Wood, J.Phys. Chem., 84,2891 (1980). (18)A. Hudson and R. A. Jackson, Chem. Commun., 1323 (1969). (19)J. E.Wertz, C. F. Koelsch, and J. L. Vivo, J. Chem. Phys., 23, 2194 (1955).
0 1982 American Chemical Society
Letters
326 The Journal of Physical Chernktry, Vol. 86, No. 3, 7982 a
1*'S3
Figure 2. Assigned hfsc values for the 2-adamantyi radical from experimental data. The assignments were made based on INDO calculations of Migita and Iwaizumi (see ref 15): -18.04,1.14,2.34, -1.73,and 5.03 0 for the a (l),6 (2), y-1 (4), y-2 (4),and A (2) protons, respectively.
t crrter
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b
2-RDAMANTY L C
A
the 2-adamantyl radical obtained by photolysis of a solution of 2-bromoadamantane in cyclohexane with triethylsilane and di-tert-butyl peroxide. Although the spectrum is well resolved, only the a-proton hfsc value of 22.72 G can be presently assigned with certainty. The four lines labeled A, with line widths larger than those of the main spectrum, are attributed to another radical species and appeared in every experiment. Occasionally, the triplet labeled B in Figure ICappeared, partially masking one or more lines belonging to the 2-adamantyl radical. This makes unequivocal assignment of the outer lines difficult at present. Power saturation studies did not reveal any new data, and no enhancement of one radical species at the expense of another could be observed. If the lines starting from arrow 1 in Figure l a are assigned to the 2-adamantyl radical, albeit with possible overlap from another species, then a set of hfsc values shown in Figure 2 can be obtained. The only unambiguous hfsc value from these experiments is that attributed to the a proton at 22.72 G; the others must at this time be considered speculative, and are assigned via INDO calculations from Migita and 1wai~umi.l~ Figure l b is the first-derivative computer simulation of the 2-adamantyl radical based on the parameters discussed above. Although the basic line patterns and envelope match well, they are still not a perfect fit with the experimental spectrum. Further work to clarify and assign hfsc values involving both selective deuteration and additional computer simulations are currently in progress. The g value of the 2-adamantyl radical was found to be 2.00248 f O.oooO8, similar to the value of 2.0025 obtained by Lloyd et al."
Center
Figure 1. (a) Firstderivative X-band liquld-phase ESR spectrum for the radical derived from 2-bromoadamantane via bromine abstraction initiated by trlethytsilyl radicals in cyclohexane at -96 'C. The four Unes indicated by arrow A are attributed to a second radical specles. (b) A computer simulation of the P-adamantyl radical based on the hfsc values obtained from (a). (c) The presence of triplet B at -94 "C. The broad and intense line to the high-field sue of the center of the spectrum in (a) and (c) is due to color centers caused by photolysis of the quartz insert used for variable temperature control in the ESR cavity.
brated from Fremy's salt in a dual-cavity setup and found to be 2.00353 f 0.00008. Results and Discussion Figure l a shows the first-derivative ESR spectrum of
Conclusions A liquid-phase narrow-line well-resolved ESR spectrum of the 2-adamantyl radical has been obtained for the first time. An a-proton hfsc value of 22.72 G was assigned, and the g value was determined to be 2.00248 f O.oooO8. Furthermore, hfsc values of 0.98,2.90,1.83, and 4.73 G for the 6 (2), 7-1 (4), 7-2 (4), and 6 (2) protons, respectively, are tentatively postulated from experimental data and computer simulation studies. Acknowledgment. The authors thank Melanie Capuno for assistance with sample purification and preparation.
