High temperature aqueous solvent effect on translational and hydrogen bond dynamics of the hydroxyl radical - MD simulation study

MD simulations using flexible models were performed to show how velocity autocorrelation functions, diffusion (D-OH) and self-diffusion (D-W) coefficients, continuous and intermittent lifetimes of H-donor and H-acceptor bonds depend on: i) transformations in the hydrogen-bond network (HBN) of water below the critical point, and ii) decreasing density at the supercritical isotherm (similar to 670 K). Contrary to simulations neglecting short-range hydrogen-oxygen interactions, we show that complexation of (OHaq)-O-center dot with H2O is enhanced by breakage of the continuous HBN, resulting in the increase of (OHaq)-O-center dot hydrodynamic radius and the D-OH values smaller than D-W by 30%. Difference between D-OH and D-W increases to 65% at 670 K below the critical density. It is explained by enhanced aggregation of water molecules around (OHaq)-O-center dot, earlier recognized as the self-trapping mechanism. Except near-ambient conditions, the intermittent lifetime of H-bonds is longer than the translational time, indicating that (OHaq)-O-center dot is complexed with water and moves as a molecular aggregate.