Ab initio calculations for the anharmonic vibrational resonance dynamics in the overtone spectra of the coupled OH and CH chromophores in CD2H-OH

We report high level ab initio calculations (treating correlation by second order Moller-Plesset perturbation theory, MP2) of a five-dimensional norma l coordinate subspace of the potential and electric dipole hypersurfaces of the C-s conformer of dideuteromethanol, CD2HOH. Accurate vibrational varia tional calculations are carried out using a discrete variable representatio n (DVR) for the five anharmonically coupled modes (three coupled CH stretch ing and bending modes and the OH stretching and high frequency OH bending m ode). The overtone spectra of the OH chromophore are calculated and analyze d in detail with respect to their anharmonic resonance dynamics leading to short time intramolecular vibrational redistribution (IVR) via the close re sonance coupling of 5 nu(OH) (5 nu(1)) with 4 nu(OH)+nu(CH)(4 nu(1)+nu(2)), as previously observed and assigned experimentally. While the assignment o f the resonance is confirmed by the ab initio calculation, a sequence of ca lculations including various subspaces (two-dimensional to five-dimensional ) lead to the conclusion that the resonance contains important contribution s from coupling to the various bending modes, not just involving the CH- an d OH stretching modes. Furthermore, even in the two-dimensional subspace th e effective coupling constants k(1112) and k(1222) characterizing the reson ance are not identical with the anharmonic potential constants C-1112 and C -1222 in the Taylor expansion of the potential, but rather an expansion to sixth order is needed to describe the resonance quantitatively. A similar c onclusion holds true with other low order perturbation expansions of the re sonance coupling, involving sequences of cubic couplings to other modes. We furthermore predict important resonances between OH stretching and OH bend ing also involving CH bending modes, which contribute to IVR at higher leve ls of excitation. (C) 1999 American Institute of Physics.