Molecular Spectroscopy and Chemical Reactions of Molecular Clusters : Computational Studies

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1) The noble structures of water cluster anions (H2O)n- were found in a series of ab initio molecular orbital calculations. The excess electron is trapped under the electrostatic field of HO bonds of water molecules. The similar structure was found in the water cluster complexes with the group 1 metal atom, M(HィイD22ィエD2O)ィイD2nィエD2 (n≧4). In the complexes, the metal atom is ionized, and the ejected electron is trapped within HO bonds. The ionization threshold of the complexes are size-independent as well as metal-independent.2) The coupling of the OH stretching with the intermolecular O-X stretching and O-X-H bending was examined for XィイD1-ィエD1(HィイD22ィエD2O) complex anions (X=F, Cl, Br and I). The observed vibrational bands for X=Cl are assigned if the internal energy of the anions is high.3) By combining the multicanonical sampling for Monte-Carlo simulation, the reweighting technique and the ab initio MO calculations, the temperature dependence of averaged structures of water dimers was examined. A few changes of the dominant structures were found. At high temperature, the structures similar to the transition states, which are entropy-favored, are dominant.The technique was applied for successfully assigning the photoelectron spectrum of SiィイD22ィエD2CィイD22ィエD2ィイD1-ィエD1, which was recently experimentally reported.