A comparative ab initio study of torsion-inversion coupling in CH3NH2, CH3OH2+ and CH3CH2

Two-dimensional, partially optimized torsion-inversion potential energy surfaces for methylamine, protonated methanol and ethyl radical were obtained by ab initio methods up to CCSD(T)/6-311++G(3df,2p). At this level, the zero-point corrected torsional barriers for CH3NH2 and CH3OH2+ were found to be 703.8 and 399.6 cm−1, respectively, and corresponding inversion barriers are 1946.9 and 877.2 cm−1. The torsional barrier for CH3CH2 was found to be 21.2 cm−1. Although the barrier heights are very different, the dominant torsion-inversion coupling term in each of the three cases has the same form, V1,3τ cos 3α, where τ is the inversion angle and α is the torsional angle, and the magnitudes of the coupling, 280 ⩽ V1,3 ⩽ 450 cm−1, are similar.