THEORETICAL-STUDY OF VIBRATIONAL OVERTONE SPECTROSCOPY AND DYNAMICS OF METHANOL

Experimentally observed coupling between OH and CH stretching modes in the high overtone region has been modeled successfully in terms of a curvilinear internal coordinate Hamiltonian including harmonic couplin g between anharmonic OH and CH stretching oscillators and cubic Fermi resonance kinetic and potential energy couplings between CH stretches and HCH bends. The Hamiltonian matrices have been set up in block diag onal forms including only resonant states, The potential-energy parame ters have been optimized by the least-squares method using experimenta l vibrational term values as data. The OH/CH stretch interaction param eter obtained agrees well with the one calculated by perturbation theo ry from a published ab initio harmonic force field. The model has repr oduced well experimental band origins in the OH stretching overtone re gion, and it has provided assignments for the bending overtones in the CH stretching fundamental region. Finally, a unitary transformation i s found from the internal coordinate representation to the correspondi ng normal coordinate representation providing a set of normal coordina te parameters like diagonal anharmonicity parameters, Darling-Dennison resonance constants and cubic Fermi resonance force constants. The re sults confirm the experimental finding of energy redistribution betwee n the OH and CH stretching modes on subnanosecond time scale at 5 nu(O H). (C) 1997 American Institute of Physics.