Ab initio MO study on the structures of OH−(H2O)n in the gas phase

Abstract

The purpose of this article is to show that (i) the OH− ion binds strongly to up to five water molecules in the gas phase, and (ii) the OH− (H2O)n has a Cn symmetry in the gas phase. For this purpose, the structures of OH− (H2O)n (n=0–6) were fully optimized with MP2/6-31++G(d), and those of OH− (H2O)n−1(H2O) (n=4,5) were partially optimized with MP2/6-31++G(d), and the energy differences between OH− (H2O)5 and OH− (H2O)4(H2O) were calculated by MP3/6-31++G(d), MP4/6-31++G(d), MP2/6-31+G(2d, 2p), MP2/aug-cc-pVDZ and MP2/6-311+G(3df, 2p) at the MP2/6-31++G(d) optimized structures. We also performed a vibrational analysis for OH− (H2O)n (n=0–5) using MP2/6-31++G(d) at the optimized structures to confirm that all vibrational frequencies are real. These calculations show that the OH− ion binds strongly to up to five water molecules in the gas phase, and that OH− (H2O)n has a Cn symmetry in the gas phase.