COMMUNICATIONS TO THE EDITOR
3707
Electron Spin Resonance Study of the Neopentyl Radical from the Radiolysis of Solid Neopentane in the Presence
of Nitrous Oxide'
Sir: It is generally considered that many of the freeradical intermediates produced in the radiolysis of hydrocarbons originate from the recombination of ions and electrons, and it has been shown2 that the yields and distribution of stable radiolysis products derived from hydrocarbons are modified in the presence of an electron scavenger. Accordingly, it occurred to us that corresponding cliff erences in the free-radical distribution might be revealed directly by esr studies following the y radiolysis of solid hydrocarbons. I n this communication we wish to report an example of this effect, and this work also serves to clarify and interpret the hyperfine structure of the neopentyl radical. As previously r e p ~ r t e d , the ~ , ~ esr spectrum of yirradiated pure neopentane at 77°K is rather complex because the indjvidual components overlap as a result of anisotropic broadening. However, several lines can be clearly recognized as belonging to the t-butyl r a d i ~ a l ,and ~ ) ~we have observed similar results. Fessenden and Schuler6were able t o obtain a well-resolved spectrum of the transient radicals present during electron irradiation of solid neopentane at 213"K, and the lines from the t-butyl radical predominated under these conditions. By contrast, we find that after y irradiation of neopentane containing 4 mol % of nitrous oxide at 77°K the esr spectrum A shown in Figure 1 consists largely of a broad 1: 2: 1 triplet with a hyperfine splitting constant of 21.5 G. This spectrum is characteristic of coupling to two a-protons and is assigned t o the neopentyl radical. There is a strong indication of additional substructure in the spectrum taken at 77"K, and this is well resolved in spectrum B of the same sample at 128°K. Each line of the triplet is split into four components with a separation of 4.0 G and approximate intensity ratios of 1:3:3:1. Some of the additional lines in the spectrum are attributable to t-butyl, but it is clear that neopentyl is the main free radical formed under the conditions of this experiment. Thus it appears that the use of nitrous oxide as an electron scavenger leads to selective free-radical formation in neopentane radiolysis. The second purpose of this communication is to comment on the structure of the neopentyl radical. The quartet splitting can be attributed t o the interaction of the unpaired electron with three equivalent y-protons, but the coupling constant of 4.0 G is much larger than the range of values usually observed for y coupling in hydrocarbon free radical^.^ It is proposed that in the temperature range of our measurements, the radical adopts a preferred conformation with
G
Figure 1. Esr first-derivative spect,ra of ?-irradiated neopentane containing 4 mol % nitrous oxide (dose, 1.1 X lofgeV g-1; microwave power, 0.01 mW). Spectrum A was recorded with the sample at 77"K, and spectrum B was recorded with the sample at 128°K. Spectrometer gain settings were 630 and 400, respectively.
a small dihedral angle between one C,-C, bond and the axis of the p orbital cotanining the unpaired electron, as illustrated in the structure below. Regard-
less of the mechanism of hyperfine interaction, the main y-proton interaction will then be confined t o the three protons in the one methyl group corresponding to the position of maximum orbital overlap. This will be true provided that the rate of interconversion between the three equivalent conformations obtained by rotation about the C,-C, bond is less than the frequency separation of the lines due t o the methyl coupling. At higher temperature, the jump rate between these conformations may increase and so equalize the interaction of all nine y-protons; this would explain the smaller y-coupling constant inferred from the previous studies.K The preceding analysis assumes free rotation about the C,-C, bonds so that the three y-protons in each methyl group are rendered magnetically equivalent. (1) This research was supported by the U. S. Atomic Energy Commission under AEC Contract No. AT-(40-1)-2968. This is AEC Document No. 0R0-2968-39. (2) P. R. Geissler and J. E.Willard, J . A m e r . Chem. Soc., 8 4 , 4627 (1962). (3) B. Smaller and M.S. Matheson, J. Chem. P h y s . , 28, 1169 (1958). (4) J. Roncin, Mol. Cryst., 3, 117 (1967). (5) R. W. Fessenden and R. H. Schuler, J . Chem. Phys., 39, 2147 (1963). V o l u m e 72, N u m b e r 10 October 1968
3708
Figure 2. Esr first-derivative spectrum of 7-irradiated isobutane containing 4 mol % nitrous oxide (dose, 1.5 x 1019 eV g-1; microwave power, 0.01 mW). The spectrum was recorded with the sample at 77°K and there was no significant change at higher temperature. Spectrometer gain setting was 800.
An esr spectrum similar to that shown in Figure 1B was obtained by Roncin4 on warming a y-irradiated xenon matrix containing neopentane to 130°K. He also attributed the spectrum to the neopentyl radical but interpreted the y coupling as the result of an overall interaction with all nine y-protons. It was argued4 that the large splitting of 4.5 G (the value observed in ref 4) could not be characteristic of coupling to a single y-proton but instead is the sum of the coupling constants for the interaction of three y-protons in each of the three methyl groups which are themselves taken to be equivalent. This argument presupposes that rotation about the C,-Co bond is sufficiently rapid to average the interaction between the unpaired electron and the three methyl groups. I n order to explain the quartet structure as the result of a coupling to three equivalent methyl groups, it then had to be assumed that in the interaction with the three y-protons in each
The Journal of Physical Chemistry
COMMUNICATIONS TO THE EDITOR methyl group only the two outer lines contribute to the observed splitting, the inner lines being broadened beyond detection by positional interconversion at a rate of 107 to io* cps. There are two facts which argue against Roncin’s ingenious e~planation.~First, the quartet splitting is unaffected over a relatively large temperature range (Figure l),and this is an unlikely result if the spectrum were critically determined by the state of rotation within each methyl group since this motion generally involves only a small energy barrier.5 Secondly, it was anticipated4 that the magnitude of the y-coupling to the protons of the two methyl groups in the isobutyl radical would be similar to that observed for neopentyl. However, we have been unable to observe any corresponding triplet hyperfine structure in this case, and the spectrum (Figure 2 ) is interpretable in terms of coupling with two a-protons and one @-proton with hyperfine splitting constants of 20 It 1 and 36 G, respectively. On the other hand, our interpretation of the spectrum of the neopentyl radical suggests only a small coupling with the y-protons in the isobutyl radical because the large coupling to the @-proton in this case means that the position close to maximum orbital overlap is now largely occupied by the C,-H bond. Thus it is not surprising that the y-proton splitting in isobutyl is small and consequently unresolved under these experimental conditions. DEPARTMENT OF CHEMISTRY JACOB LIN FFRANCON WILLIAMS UNIVERSITY OF TENNESSEE KNOXVILLE, TENNESSEE 37916 RECEIVED AUQUST9, 1968
