Anharmonic Vibrational Spectroscopy of Hydrogen-Bonded Systems Directly Computed from ab Initio Potential Surfaces: (H2O)n, n = 2, 3; Cl-(H2O)n, n = 1, 2; H+(H2O)n, n = 1, 2; H2O−CH3OH

Abstract

Vibrational energy levels and infrared absorption intensities of several neutral and ionic hydrogen-bonded clusters are computed directly from ab initio potential energy surfaces, and the results are compared with experiment. The electronic structure method used to compute the potential surfaces is MP2, with Dunning's triple-ζ + polarization basis set. The calculation of the vibrational states from the potential surface points is carried out using the correlation corrected vibrational self-consistent field (CC-VSCF) method. This method includes anharmonicity and the coupling between different vibrational modes. The combined electronic structure/vibrational algorithm thus provides first-principles calculations of vibrational spectroscopy at a fairly accurate anharmonic level and can be useful for testing the accuracy of electronic structure methods by comparing with experimental vibrational spectroscopy. Systems treated here are (H2O)n, n = 2, 3; Cl-(H2O)n, n = 1, 2; H+(H2O)n, n = 1, 2; and H2O−CH3OH. In the cases of (H2O)3 and H2O−CH3OH, over 13 000 potential surface points are computed. For each system treated, all the fundamental transitions are computed, but the experimental data for comparison is mostly available for the OH stretches or other stiff modes only. The results show very good agreement between the calculated and experimental frequencies for all systems. The typical deviation for OH stretching modes is on the order of 50 cm-1, indicating that the ab initio potential surfaces are of good accuracy. This is very encouraging for further pursuing MP2 calculations of potential energy surfaces of hydrogen-bonded systems.