Ab initio molecular orbital study on the structures and energetics of CH3OH(H2O)n and CH3SH(H2O)n in the gas phase

Abstract

The purpose of this study was to calculate the structures and energetics of CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n in the gas phase: we asked how the CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} moieties of CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n change with an increase in n and how can we reproduce the experimental values ΔH°n−1,n. For this purpose, we carried out full geometry optimizations with MP2/6-31+G(d,p) for CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n (n=0,1,2,3,4,5) and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n (n=0,1,2,3,4). We also performed a vibrational analysis for all clusters in the optimized structures to confirm that all vibrational frequencies are real. All of the vibrational frequencies of these clusters are real, and they correspond to equilibrium structures. For CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n, when n increases, (1) the CO bond length decreases, (2) the CH bond lengths do not change, (3) the OH bond lengths increase, (4) the OCH bond angles increase, (5) the COH bond angles decrease, (6) the charge on CH3 becomes less positive, and (7) these predicted values, except for the OH bond lengths of CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n, approach the corresponding values in CH3OH. The CO bond length in CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)5 is shorter than that in CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} in the gas phase by 0.061 Å at the MP2/6-31+G(d,p) level. Except for the SH bond lengths in CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n, however, the structure of the CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} moiety does not change with an increase in n. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 125–131, 2001