Ab initio study of the chlorine nitrate protonation reaction: Implications for loss of ClONO₂ in the stratosphere

Abstract

The energetics of the protonation reaction of chlorine nitrate, ClONO2, have been investigated using state-of-the art ab initio quantum mechanical methods. Specifically, equilibrium structures, vibrational spectra, and thermochemical data have been obtained using second-order Møller-Plesset perturbation theory (MP2), singles and doubles coupled-cluster theory (CCSD), and the CCSD(T) method, which includes a preturbational estimate of the effects of connected triple excitations. It is shown that the lowest energy form of protonated ClONO2 corresponds to a complex between HOCl and NO2+ - similar to the situation for HONO2 and CH3ONO2. The proton affinity of ClONO2 is determined to be 176.8 ± 3 kcal/mol (0 K) while the second most stable isomer of protonated ClONO2 is 20 kcal/mol higher in energy. The HOCl⋯NO2+ complex is bound by 12.9 ± 2 kcal/mol. The heat of formation of ClONO2 is computed to be 7.4 kcal/mol at 298 K, in good agreement with the experimental value of 6.3 kcal/mol. The present study supports a recent hypothesis that the reaction of ClONO2 on the surface of polar stratospheric clouds (PSCs) is proton catalyzed, although the specific mechanism is different. The results are also consistent with other recent findings concerning the reactions of ClONO2 and HOCl with HCl on the surface of PSCs. © 1993 American Chemical Society.