Unimolecular and hydrolysis channels for the detachment of water from microsolvated alkaline earth dication (Mg2+, Ca2+, Sr2+, Ba2+) clusters

Abstract

We examine theoretically the three channels that are associated with the detachment of a single water molecule from the aqueous clusters of the alkaline earth dications, [M(H2O) n ]2+, M = Mg, Ca, Sr, Ba, n ≤ 6. These are the unimolecular water loss (M2+(H2O) n-1 + H2O) and the two hydrolysis channels resulting the loss of hydronium ([MOH(H2O) n-2]+ + H3O+) and Zundel ([MOH(H2O) n-3]+ + H3O+(H2O)) cations. Minimum energy paths (MEPs) corresponding to those three channels were constructed at the Møller–Plesset second order perturbation (MP2) level of theory with basis sets of doubleand triple-ζ quality. We furthermore investigated the water and hydronium loss channels from the mono-hydroxide water clusters with up to four water molecules, [MOH (H2O) n ]+, 1 ≤ n ≤ 4. Our results indicate the preference of the hydronium loss and possibly the Zundel-cation loss channels for the smallest size clusters, whereas the unimolecular water loss channel is preferred for the larger ones as well as the mono-hydroxide clusters. Although the charge separation (hydronium and Zundel-cation loss) channels produce more stable products when compared to the ones for the unimolecular water loss, they also require the surmounting of high-energy barriers, a fact that makes the experimental observation of fragments related to these hydrolysis channels difficult.KeywordsAlkaline earth dication aqueous clustersUnimolecular dissociationPotential energy curveElectronic structureHydrolysis channel