Ab initio calculations of mode selective tunneling dynamics in (CH3OH)-C-12 and (CH3OH)-C-13

A simplified formulation of the harmonic reaction path Hamiltonian (RPH) approach is used to calculate mode specific tunneling splittings and stereomutation times in (CH3OH)-C-12 and (CH3OH)-C-13. 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 nu(2) and nu(9) and of the CH-rocking mode nu(11) 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 Delta(ν) over tilde (2)=-3.6 cm(-1), Delta(ν) over tilde (9)=-3.2 cm(-1), and Delta(ν) over tilde (11)=-8.2 cm(-1) that are in satisfactory agreement with experimental data. Negative tunneling splittings are also predicted for the asymmetric CH-bending modes nu(4) and nu(10) and the CH3-rocking mode nu(7). A smooth decrease of the tunneling splitting is calculated for increasing OH stretching excitation [Delta(ν) over tilde(nu(1))=6.2 cm(-1),...,Delta(ν) over tilde (6nu(1))=1.5 cm(-1)] in quantitative agreement with experiment [Delta(ν) over tilde(nu(1))=6.3 cm(-1),...,Delta(ν) over tilde (6nu(1))=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. (C) 2003 American Institute of Physics.