The electron-methyl group spin-spin interaction

The nuclear spin symmetry conversion transition, whereby a methyl group changes tunnelling state (A↔E) and total nuclear spin (ΔI = ± 1), is made resonant by the flip of an unpaired electron spin (Δms = ± 1). The coupling between an unpaired electron in a free radical and a methyl group in a nearby molecule is via the inter-molecular spin-spin interaction. The matrix elements and transition probabilities for this transition are calculated explicitly. The motivation behind the calculation is to aid in the interpretation of electron spin relaxation experiments in γ-irradiated 4-methyl-2,6-t-butylphenol where these resonant transitions have a profound effect. The results presented have also been useful in the interpretation of nuclear magnetic resonance experiments in the same substance