An ESR and ENDOR study of tunneling rotation of a hindered CH3 group in C2H5 radicals trapped in xenon matrices at 4.2 K: Environment effect on internal rotation

Two kinds of ethyl radicals have been observed in Xe matrices depending upon the reactions of radical formation. The CH3 group in C2H5(I) formed from H abstraction by H atoms at temperatures below 50 K exhibits an ESR hyperfine pattern characteristic of tunneling rotation in a threefold hindering potential, whereas that in C2H5(II) formed from homolytic scission of the C–H bond exhibits a conventional spectrum typical of free or random hopping rotation with a small barrier. The g and hyperfine coupling tensors indicate that both the ethyl radicals possess a conventional planar or nearly planar structure. It is concluded that the difference in the internal motion arises from an environmental effect which lowers the rotational symmetry from a six- to threefold potential. The tunneling splitting has been determined to be 450 MHz for C2H5(I). From this the barrier to internal rotation is estimated to be about 1 kcal/mol. In estimating the barrier height the influence of the interaction of the CH2 group with the surroundings is taken into consideration. The contribution of a zero-point torsional amplitude in the averaging process of the β-proton coupling constant is also discussed.