THE APPARENT TORSIONAL BARRIER REDUCTION EFFECT IN METHANOL COMPLEXES

Abstract

Rotational analyses of several methanol complexes have indicated that the methyl group torsional barrier heights in these complexes are substantially lower than in bare methanol. It was proposed recently that this barrier reduction effect is only apparent. It results from neglect of the large amplitude "librational" motion of the hydroxyl hydrogen in methanol. $Fraser et al.^{1}$ proposed a simple Hamiltonian to analize this effect, $H = {Fp_{a}}^{2} + F_{1}p_{e}^{2} - 2F_{1}P_{a} P_{e} + V_{3}(1-\cos 3\alpha)/2+v_{1}(1-\cos \theta)/2 \quad (1)$ where $v_{1}$ is the potential barrier that hinders the hydroxyl group internal rotation (or wagging). Data taken for $CH_{3}OH \bullet Ar, CH_{3}OH \bullet SO_{2}$, and $CH_{3}OH\bullet HCl$ will be presented to illustrate the barrier reduction effect. Isotopic variation of the apparent barrier height will be used to check the validity of Eq. (1). The physical significance of $v_{1}$ in these complexes will be discussed.