Ab initio calculations of mode selective tunneling dynamics in CH312OH and CH313OH

Abstract

A simplified formulation of the harmonic reaction path Hamiltonian (RPH) approach is used to calculate mode specific tunneling splittings and stereomutation times in CH312OH and CH313OH. The experimental torsional spectrum is very well reproduced, as well as the few known isotope shifts. The mode specific changes in tunneling splitting are investigated for the excitation of fundamentals and OH stretching overtones. Good agreement between experiment and the RPH model is obtained, except for excitations of modes which are perturbed by anharmonic resonances. The inverted tunneling splittings (E level below A) experimentally observed for the fundamental transitions of the CH-stretching modes ν2 and ν9 and of the CH-rocking mode ν11 are shown to result from a pure symmetry effect and not from a breakdown of vibrational adiabaticity. Introducing a proper geometrical phase factor but retaining the adiabatic separation of the torsional dynamics yields calculated values of Δν̃2=−3.6 cm−1, Δν̃9=−3.2 cm−1, and Δν̃11=−8.2 cm−1 that are in satisfactory agreement with experimental data. Negative tunneling splittings are also predicted for the asymmetric CH-bending modes ν4 and ν10 and the CH3-rocking mode ν7. A smooth decrease of the tunneling splitting is calculated for increasing OH stretching excitation [Δν̃(ν1)=6.2 cm−1,…,Δν̃(6ν1)=1.5 cm−1] in quantitative agreement with experiment [Δν̃(ν1)=6.3 cm−1,…,Δν̃(6ν1)=1.6 cm−1]. The effect is shown to result in about equal parts from the increase of the effective torsional barrier and the effective lengthening of the OH bond.