Exploration of the Mechanism of the Activation of ClONO₂by HCl in Small Water Clusters Using Electronic Structure Methods

Abstract

High-level electronic structure calculations were used to study the mechanism of the reaction of ClONO 2 with HCl in neutral water clusters containing one to five solvating water molecules. For the reaction between molecular HCl and ClONO 2 , the barrier decreases from 42 kcal mol -1 (uncatalyzed) to essentially zero when catalyzed by only two water molecules, where the reaction products involve Cl 2 and HONO 2 . The calculations thus predict that the gas-phase reaction may be important in the stratospheric reactivation of ClONO 2 . The reaction between ClONO 2 and solvated H 3 O + Cl -, as on the polar stratospheric cloud (PSC) surface, was investigated with clusters involving up to seven water molecules. The ice-catalyzed reaction involves an ionic mechanism whereby charge transfer to ClONO 2 from the attacking nucleophile leads to significant ionization along the Cl-ONO 2 bond. The effect of the size of the first solvation shell of Cl -is addressed by our calculations. In a cluster containing three waters and a five-water cluster structurally related to hexagonal ice, ClONO 2 reacts spontaneously with HCl to yield Cl 2 /HONO 2 in the three-water reaction and Cl 2 /H 3 O + -NO 3 -in the five-water-catalyzed reaction. The calculations thus predict that the reaction of ClONO 2 with HCl on PSC ice aerosols can proceed spontaneously via an ionic pathway.

Theoretical Investigation of the Reaction Mechanism of ClONO2 + HCl → HNO3 + Cl2 on (H2O)n (n = 0–3) Cluster

“…In the following discussions, for all hydration numbers of clusters n = 1, 2, and 3, the same letters are used for molecular labels for products (A, B), for optimized structures (C, D, E, ...) in increasing order of the relative energy, and for TSs the asterisk is added to these letters for the sake of convenience. Two isomeric structures are found as reactants in n = 0 cluster (Figure 1), where isomer 0D was previously found by McNamara et al 7 Although 0D was reported to be the reactant with the hydrogen bond between two atoms O5 and H7, the new reactant isomer 0C is found to be more stable than 0D with a hydrogen bond between O2 and H7. In 0C, HCl molecule coordinates to ClONO 2 in the direction perpendicular to the molecular plane of ClONO 2 .…”

“…McNamara et al 7 elucidated the reaction mechanisms for n ≤ 3 clusters and few structures with n = 4 and 5 using ab initio molecular orbital (MO) calculations by the second-order Møller−Plesset (MP2) perturbation theory for optimized structures obtained by Kohn−Sham density functional theory with a combination of Becke's three-parameter exchange and Lee−Yang−Parr correlation functional (B3LYP) with the 6-311++G(d,p) basis set. They showed that the reaction barriers were lowered systematically by adding water molecules to the complex.…”

The nature of the interface and the diffusion coefficient of HCl/ice and HBr/ice in the temperature range 190–205 K

“…greatly improved our detailed understanding of heterogeneous reactions involving HCl on ice surfaces. 35,36 The studies of HBr on ice are less numerous than those in the case of HCl. In fact, a phase diagram of the HBr/ice system has not yet been constructed for low temperatures even though an ionic trihydrate of HBr reportedly has been found.…”

Exploration of the atmospheric reactivity of N2O5 and HCl in small water clusters using electronic structure methods

“…Thus the ice surface reaction would involve where H 3 O`Clã n initial step may involve the desolvation of Cl~and thus contribute to increasing the barrier somewhat. 39,43,44 The increased availability of water on the ice surface (compared to the gas phase) may also lead to the Ðnal reaction products involving ionised nitric acid However, Sodeau O`Cl~) whereby developing charged species is stabilised by additional waters. This has been shown to be the case for reactions (2), (4) and (5).…”