Isoprene Reactivity on Water Surfaces from ab initio QM/MM Molecular Dynamics Simulations

Isoprene is the most abundant volatile organic compound in the atmosphere after methane. While gas-phase processes have been widely studied, the chemistry of isoprene in aqueous environments is less well known. Nevertheless, some experiments have reported unexpected reactivity at the air-water interface. In this work, we have carried out combined quantum-classical molecular dynamics simulations of isoprene at the air-water interface, as well as ab initio and density functional theory calculations on isoprene-water complexes. We report the first calculation of the thermodynamics of adsorption of isoprene at the water surface, examine how hydration influences its electronic properties and reactivity indices, and estimate the OH-initiated oxidation rate. Our study indicates that isoprene interacts with the water surface mainly through H-π bonding. This primary interaction mode produces strong fluctuations of the π and π^* bond stabilities, and therefore of isoprene's chemical potential, nucleophilicity and ionization potential, anticipating significant dynamical effects on its reactivity at the air-water interface. Using data from the literature and free energies reported in our work, we have estimated the rate of the OH-initiated oxidation process at the air-water interface (5.0×10¹²  molecule cm^-3  s^-1 ) to be about 7 orders of magnitude larger than the corresponding rate in the gas phase (8.2×10⁵  molecule cm^-3  s^-1 ). Atmospheric implications of this result are discussed.