Direct dynamics calculations of multiple proton transfer through hydrogen-bonded wire and the role of micro-solvation in ClONO2 + H2O → HNO3 + HOCl reactions

Abstract

The hydrolysis of ClONO2 on polar stratospheric ice has been considered as a major factor causing stratospheric ozone depletion. We have theoretically investigated the reaction dynamics of hydrolysis on ice surface and the role of micro-solvation. No theoretical studies have been performed for the micro-solvent effect of multiple proton transfer in the hydrolysis of ClONO2. Rate constants and tunneling coefficients were calculated using variational transition state theory including multidimensional tunneling. The dispersion corrected, spin-component scaled, double hybrid PBE functional with the P86 correlation that can reproduce the MP2/CBS + ∆CCSD(Q) result was used to generate potential energy surfaces. No more than three water molecules could form a cyclic hydrogen (H)-bonded chain to catalyze the reaction and the other is bound to the chain to act as a micro-solvent. The catalytic water reduced not only the barrier but also the tunneling effect significantly. The micro-solvent effect of lowering the barrier is smaller and depends on the position. The multiple proton transfer path through H-bonded chain, in some cases, varied with the position of micro-solvent, and consequently the H-bonded structure of HOCl–HNO3 cluster became completely different from the reactant and TS structures. The predicted rate constant was 0.671 at 193 K, and the Arrhenius activation energy was 8 kcal/mol. This rate constant was smaller by three orders of magnitude than that of ClONO2 + HCl on ice, which is consistent with the experimental observations that at low HCl concentration conditions ClONO2 hydrolysis competes with ClONO2 + HCl reaction.