ESR Study of Quantum Tunneling of a Methyl Group: A Simple Tunneling Model

The ESR absorption of the intermediate radical CH₃⋅CHR from l‐alanine was studied at 4.2°K. It was found that the methyl group spectrum for the $a$ axis was different than that for the $c$ axis. While Freed's theory J. Chem. Phys. 43, 1710 (1965)] failed to explain this fact, a simple quantum‐tunneling model was found to account for the observations satisfactorily, attributing the observed difference to an anisotropic dipole–dipole interaction. It was shown that an hfs splitting having any functional dependence on the angle of twist Θ results in the characteristic splitting of the $E$ states. A methyl proton which has the cos²Θ dependence assumed by Freed was found to be a special case exhibiting this $E$ state splitting.