Parameterization of reactive force field: Dynamics of the [Nb ₆O₁₉H_x]^(8-x)- lindqvist polyoxoanion in bulk water

We present results on parameterization of reactive force field [van Duin, A. C. T.; Dasgupta, S.; Lorant, F.; Goddard, W. A. ReaxFF: A Reactive Force Field for Hydrocarbons. J. Phys. Chem. A 2001, 105, 9396-9409] for investigating the properties of the [Nb₆O₁₉H_x] ^(8-x)- Lindqvist polyoxoanion, x = 0-8, in water. Force-field parameters were fitted to an extensive data set consisting of structures and energetics obtained at the Perdew-Burke-Ernzerhof density functional level of theory. These parameters can reasonably describe pure water structure as well as water with an excess of H⁺ and OH^- ions. Molecular dynamics simulations were performed on [Nb₆O₁₉H _x]^(8-x)-, x = 0-8, submerged in bulk water at 298 K. Analysis of the MD trajectories showed facile H atom transfer between the protonated polyoxoanion core and bulk water. The number of oxygen sites labeled with an H atom was found to vary depending on the pH of the solution. Detailed analysis shows that the total number of protons at bridging (terminal), η-O (μ₂-O), sites ranges from 3(1) at pH 7, to 2(0) at pH 11, to 1(0) at pH 15. These findings closely reflect available experimental measurements.