Harmonic and anharmonic calculations and experiment

The vibrational spectroscopy of SO4 2− · H2O n is studied by theoretical calculations for n=1–5, and the results are compared with experiments for n=3–5. The calculations use both ab initio MP2 and DFT/B3LYP potential energy surfaces. Both harmonic and anharmonic calculations are reported, the latter with the CC-VSCF method. The main findings are the following: 1 With one exception H2O bending mode , the anharmonicity of the observed transitions, all in the experimental window of 540–1850 cm−1, is negligible. The computed anharmonic coupling suggests that intramolecular vibrational redistribution does not play any role for the observed linewidths. 2 Comparison with experiment at the harmonic level of computed fundamental frequencies indicates that MP2 is significantly more accurate than DFT/B3LYP for these systems. 3 Strong anharmonic effects are, however, calculated for numerous transitions of these systems, which are outside the present observation window. These include fundamentals as well as combination modes. 4 Combination modes for the n=1 and n=2 clusters are computed. Several relatively strong combination transitions are predicted. These show strong anharmonic effects. 5 An interesting effect of the zero point energy ZPE on structure is found for SO4 2− · H2O 5: The global minimum of the potential energy corresponds to a Cs structure, but with incorporation of ZPE the lowest energy structure is C2v, in accordance with experiment. 6 No stable structures were found for OH− · HSO4 − · H2O n, for n 5. © 2007 American Institute of Physics. DOI: 10.1063/1.2764074