Proton Transfer Rates in Ionized Hydrogen Chloride-Water Clusters: A Direct Ab Initio Molecular Dynamics Study

Reactions of the microhydrated hydrogen chloride radical cation, [HCl-(H₂O)_n]⁺ (n = 1-5), following the ionization of the parent neutral cluster were investigated by the direct ab initio molecular dynamics (AIMD) method to elucidate the cluster size dependence of the proton transfer (PT) rate in the ionized state. The ionization occurred from the HCl moiety of the clusters. The proton of HCl⁺ was transferred to the neighboring water molecule in the cluster. The time of PT was strongly dependent on the cluster size (n); the time of PT decreased with increasing n and reached a limiting value at n = 4-5 (the time of PT was ca. 7 fs). The acceleration of the PT rate was mainly caused by the shortness of the hydrogen bond between HCl⁺ and H₂O in larger clusters, that is, a short hydrogen bond causes fast PT. The electrostatic effects of the water cluster further accelerated the rate of PT. After the first PT from HCl⁺ to H₂O, the second PT (H₃O⁺ + H₂O →H₂O + H₃O⁺) was detected for n = 3-5. The times of the first and second PTs were calculated as 7-15 and 30-40 fs, respectively. The reaction mechanism was discussed based on the theoretical results.