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 ΔH ⁰_{n?1, n}. Using the ab initio closed-shell self-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 CH₃ becomes more positive, and these values of CH₃O^?(H₂O)_n approach the corresponding values of CH₃OH 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 Å 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. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1138–1144, 1999