Stepwise Activation of Water by Open-Shell Interactions, Cl(H2O)n=4-8,17.

Chemical activation of water by a single chlorine atom was examined computationally for clusters of chlorine radicals and water in a size regime just prior to internal hydration of water/ions, Cl·(H2O)n=4-8,17. This investigation follows a recent analysis of this radical-molecule interaction [Christensen et al. J. Phys. Chem. A 2019, 123, 8657] for n = 1-4, which demonstrated that n = 4 marked a transition in which an oxidized-water structural motif became viable, albeit high in energy. Thousands of unique isomers were computed in the present analysis, which resulted in three structural classes of isomers, including intact hydrated chlorine, hydrogen-transferred (HCl)(OH·)(H2O)n-1, and charge-transferred (Cl-)(H3O+)(OH·)(H2O)n-2 configurations. The electronic structures of these classes were investigated, along with harmonic vibrational signatures that probed the degree of water-network perturbations and generated experimentally verifiable computational predictions. The main outcome of this analysis is that the charge-transferred isomers were stabilized considerably upon increased hydration-leading to an energetic crossover with the hydrogen-transferred forms-but the degree of hydration was surprisingly still not sufficient to achieve crossover between the intact chlorine-water complexes and these charge-separated configurations. Internal hydration of the ions appears to be necessary in order to achieve this separation, which will likely occur at larger cluster sizes.