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.