Vibrational spectroscopy of (SO^2-₄ ) · (H ₂O)n clusters, n=1-5: Harmonic and anharmonic calculations and experiment

The vibrational spectroscopy of (S O4 2-) (H2 O)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 (H2 O 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 (S O4 2-) (H2 O)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 (O H-) (HS O4-) (H2 O)n, for n≤5.