QM/MM dynamics of CH3COO(-)-water hydrogen bonds in aqueous solution.

Two combined QM/MM molecular dynamics (MD) simulations, namely, HF/MM and B3LYP/MM, in which the central CH(3)COO(-) and its surrounding water molecules were treated at HF and B3LYP levels of accuracy, respectively, using the DZV+ basis set, have been performed to investigate the characteristics of CH(3)COO(-)-water hydrogen bonds in dilute aqueous solution. Both HF/MM and B3LYP/MM simulations clearly indicate relatively strong hydrogen bonds between CH(3)COO(-) oxygens and their nearest-neighbor waters compared with those of water-water hydrogen bonds in the bulk. In addition, it is observed that first-shell waters are either "loosely" or "tightly" bound to their respective CH(3)COO(-) oxygen atoms, leading to large fluctuations in the coordination number, ranging from 2 to 5, with the prevalent value of 3. Among the HF and B3LYP methods for the description of the QM-treated region, the latter predicts slightly higher hydrogen-bond strength in the CH(3)COO(-)-water complex.