Ab Initio Molecular Dynamics Study of the Solvated OHCl⁻ Complex: Implications for the Atmospheric Oxidation of Chloride Anion to Molecular Chlorine

Abstract

We have studied the OHCl− complex in a six-water cluster and in bulk liquid water by means of Born−Oppenheimer molecular dynamics based on generalized gradient-corrected BLYP density functional theory. Self-interaction-corrected results, which predict a hydrogen-bonded OH···Cl− complex, are compared to the uncorrected results, which predict a hemibonded (HO—Cl)−. A second-order Møller−Plesset potential energy landscape of the gas-phase complex in its ground-state was computed to determine which of the two configurations represents the true nature of the complex. Because no evidence of a local minimum was found in the vicinity of the geometry corresponding to (HO—Cl)−, we conclude that the self-interaction-corrected results are more accurate and, therefore, that the complex is held together by a hydrogen-bond-like interaction in both an asymmetric solvation environment, as represented by the cluster, and a symmetric solvation environment, as represented by the bulk system. We postulate that the mechanism that governs the atmospheric oxidation of Cl−(aq) to Cl2(g) on the surface of marine aerosols is initiated by the formation of a H-bonded OH···Cl− complex. Furthermore, because no evidence of charge transfer from Cl− to OH was found, in either the liquid or the cluster environment, we propose that the second step of the oxidation of Cl− is the reaction of the complex with a second Cl−, resulting in the formation of the species Cl2− and OH−. Cl2(g) could then be formed via an electron-transfer reaction with an impinging OH molecule.