Orbital Following in the Methyl Radical

Abstract

Incomplete orbital following of the molecular vibrations in the methyl radical is examined through its relationship to the hyperfine splittings observed in the ESR spectrum. A semiempirical nonionic valence-bond theory with different degrees of carbon sigma-orbital following is used to estimate the proton and C13 splitting constants as a function of the nuclear positions that correspond to the out-of-plane bending mode. It is found that a considerable ``lag'' in the orbital motion with respect to that of the nuclei is required to achieve a consistent interpretation of the hyperfine spectrum in the methyl radical and its isotopic analogs. A variety of additional factors (i.e., integral parameter variation, breakdown of Born—Oppenheimer approximation, ionic contributions, other vibrations) are also investigated to determine their effects on the hyperfine splittings. Although some of the factors are found to be significant (particularly the choice of integral parameters), it appears that they cannot serve to explain the hyperfine data in the presence of complete following. Comparisons of the present conclusions with other studies of hyperfine splittings and related investigations of orbital following are presented.