Ab initio molecular orbital study on structures and energetics of CH3O-(H2O)n and CH3S-(H2O)n in gas phase

The purpose of this article was to calculate the structures and energetics of CH3O-(H2O)(n) and CH3S-(H2O)(n) in the gas phase: the maximum number of water molecules that can directly interact with the O of CH3O-; and when n is larger, we asked how the CH3O- and CH3S- moiety of CH3O-(H2O)(n) and CH3S-(H2O)(n) changes and how we can reproduce experimental Delta H-n-1,n(0). Using the ab initio closed-shell sell-consistent field method with the energy gradient technique, we carried out full geometry optimizations with the MP2/aug-cc-pVDZ for CH3O-(H2O)(n) (n = 0, 1, 2, 3) and the MP2/6-31 + G(d,p) (for n = 5, 6). The structures of CH3S-(H2O)(n) (n = 0, 1, 2, 3) were fully optimized using MP2/6-31 + + G(2d,2p). It is predicted that the CH3O-(H2O)(6) does not exist. We also performed vibrational analysis for all clusters [except CH3O-(H2O)(6)] at the optimized structures to confirm that all vibrational frequencies are real. Those clusters have all real vibrational frequencies and correspond to equilibrium structures. The results show that the above maximum number of water molecules for CH3O- is five in the gas phase. For CH3O-(H2O)(n), when n becomes larger, the C-O bond length becomes longer, the C-H bond lengths become smaller, the HCO bond angles become smaller, the charge on the hydrogen of CH, becomes more positive, and these values of CH3O-(H2O)(n) approach the corresponding values of CH,OI-I: with the n increment. The C-O bond length of CH3O-(H2O)(3) is longer than the C-O bond length of CH3O- in the gas phase by 0.044 Angstrom at the MP2/aug-cc-pVDZ level of theory. The structure of the CH3S- moiety in CH3S-(H2O)(n) does not change with the n increment. (C) 1999 John Wiley & Sons, Inc.