ESR studies of internal rotation in radicals

The β hyperfine splittings in the ESR spectra of a number of alkyl radicals were previously shown to be temperature dependent. The results of detailed measurements over a range of temperatures are presented here for propyl, isobutyl, 3-butenyl and CHD2CD2• radicals. The dependence, for radicals of this type, of the coupling constant upon cos2 of the azimuthal angle, θ, between the particular β proton and the axis of the p orbital containing the unpaired electron suggests that the departure of the splittings from that found for ethyl radical arises from an anisotropic averaging over the rotation about the bond between the α and β carbons and that the temperature dependence reflects the expected changes in this averaging with temperature. A treatment is applied which averages cos2 θ first over the rotational eigenfunctions for the case of a two-fold barrier to internal rotation and then over a Boltzmann distribution among the various rotational levels. The observed splittings are obtained if barriers of the order of 300 cal/mole are taken for the first three radicals and if a 90 cal/mole barrier is used for CHD2CD2•. A discussion is presented which attempts to show the relationship of these barriers to those found in other molecules. The similarity is noted of the isotope effect in CHD2CD2• and that in 7-deutero-cycloheptatriene which causes a slight preference for one of the two ring configurations