Characteristics of the “Hypercoordination” of hydroxide (OH−) in water: A comparative study of HF/MM and B3LYP/MM MD simulations

Abstract The solvation structure and dynamics of hydroxide (OH − ) in water have been studied by means of two combined QM/MM MD simulations, namely HF/MM and B3LYP/MM, in which the central OH − and its surrounding water molecules were treated at HF and B3LYP levels of accuracy, respectively, using a 6-311++G(d,p) basis set. On the basis of both the HF/MM and B3LYP/MM MD simulations, it is observed that the hydrogen bonds (HBs) between OH − and its first-shell waters are quite strong, i.e. , compared to that of bulk water. Nevertheless, the solvation shell of OH − is flexible, in which the first-shell waters can be either “loosely” or “tightly” bound to OH − (with either a short-lived or long-lived exchange period). In this respect, the oxygen site in OH − prefers to be dynamically hypercoordinated by the surrounding waters, i.e. , by accepting four or five HBs, while the OH − hydrogen is found to donate a HB, but only transiently. This observed phenomenon strongly supports the concept that the mechanism of OH − transport in aqueous solution relies on the dynamical hypercoordination scenario. Comparing the HF/MM and B3LYP/MM results, the observed differences are discussed with respect to the quality of the HF and B3LYP methods in describing such condensed phase system.