CHAPTER 3: Rate Constants and the Kinetic Isotope Effects in Multi-Proton Transfer Reactions: A Case Study of ClONO₂+HCl→HNO₃+Cl₂ Reactions with Water Clusters with Canonical Variational Transition State Theory using a Direct Ab Initio Dynamics Approach

We performed variational transition state theory calculations for multiple proton transfers in ClONO₂ + HCl → HNO₃ + Cl₂ with water clusters containing one to two water molecules based on high-level quantum mechanical potential energy surfaces (PSCs), which can be used as a model of the reactions occurring on ice surface in stratospheric clouds. Two and three protons in these reactions with one and two water molecules, respectively, were transferred concertedly and asynchronously; the energy barriers depend substantially on the number of water molecules involved. The potential energy barrier at the MP2/6–311++(3df,3pd)//MP2/6–31G(d,p) level was 4.8 kcal/mol for the triple proton transfer involving two water molecules, and the rate constant was 1.6×10³ s^–1 at 197 K, which agrees very well with experimental values. The agreement between experimental and theoretical rate constants suggests that the reaction between ClONO₂ and HCl in the PSCs occurs concertedly by asynchronous multiple proton transfers through the hydrogen-bonded water molecules between them. The potential energy curve near the saddle point is very flat, suggesting that the tunneling effect on the multi-proton transfer in the PSCs is negligible. This study revealed that the rule of the geometric mean, which has been used for many years as an experimental criterion for the concerted mechanism, is not valid for highly asynchronous multiple proton transfers.