Anharmonic calculations and 3D PES study of the O-H group vibrations in the methanol dimer

Abstract

Geometry, harmonic and anharmonic vibrations of O-H group of the donor molecule of methanol dimer are studied using ab initio methods employing DFT in conjunction with extended basis set. To describe more accurately proton large amplitude motions, its Cartesian coordinates were used as the vibtrational in order to build the 3D potential energy surface (PES). Using this 3D PES, a flexible Hamiltonian is built numerically and then employed to find the frequencies of the stretching and bending O-H group vibrations. Calculated fundamental frequencies are in reasonable agreement with the corresponding values obtained from the diagonalization of the Hamiltonian matrix. Some discrepancies in the results of the two approaches is largely offset by analyzing the structure of the normal modes and additional accounting of anharmonic interaction of the OH groups vibrations with the rest of the normal modes when 3D PES is used. Comparison of the calculated results with the experimental data presented in the literature was done.