The mobility and solvation structure of a hydroxyl radical in a water nanodroplet: a Born–Oppenheimer molecular dynamics study

Abstract

Hydroxyl radicals (OH*) play a crucial role in atmospheric chemistry and biological processes. In this study, Born–Oppenheimer molecular dynamics simulations are performed under ambient conditions for a hydroxyl radical in a water nanodroplet containing 191 water molecules. Density functional theory calculations are performed at the BLYP-D3 level with some test calculations at the B3LYP-D3 level. In two 150 ps trajectories, either with OH* initially located in the interior region or at the surface of the water nanodroplet, the OH* radical ends up in the subsurface layer of the nanodroplet, which is different from the “surface preference” predicted from previous empirical force field simulations. The solvation structure of OH* contains fluctuating hydrogen bonds, as well as a two-center three-electron hemibond in some cases. The mobility of OH* is enhanced by hydrogen transfer, which has a free energy barrier of ∼4.6 kcal mol⁻¹. The results presented in this study deepen our understanding of the structure and dynamics of OH* in aqueous solutions, especially around the air–water interface.