Transition states for hydride-water (H-)(H2O)n clusters, n = 2-6, 20

Hydride anion (H-) reacts quickly in aqueous solution to form molecular hydrogen (H2) and hydroxide ion (OH-), but (H-)(H2O)n clusters have many PES local minima in which the hydride is solvated by two or more water molecules making dihydrogen bonds. Using ab initio methods applied to (H-)(H2O)n clusters for n ≤ 6 and for n = 20, we explored these clusters' reaction pathways and transition states. At the B3LYP/6−311++G∗∗ level of theory, all of the small (2 ≤ n ≤ 6) clusters are unstable or barely stable at 0 K: all activation barriers for reaction to form H2 are under 0.5 kcal/mol, all but one are under 0.2 kcal/mol, and most are negative. In some instances the lowest-barrier reaction pathway is multi-step, requiring H-bond rearrangements first, and those rearrangements are consistent with improving the “presolvation” of the emerging (OH-) in accordance with a “solvation hierarchy.” The arrangements explored for n = 20 consist of (H-) encapsulated by a dodecahedral cage with coordination of 3, 4, 5, or 6 at the hydride anion. The lowest-energy (H-)(H2O)20 cluster found has 4 H-bonds to the anion. Of several reaction pathways explored, the lowest barrier height is 3.68 kcal/mol at 0 K, suggesting that a low-temperature stable solvated hydride can exist. For the (H-)(H2O)20 clusters at their transition states to H2 formation, the H--H distances are at or below the van der Waals cutoff for covalent H-H bonding (92 pm). In all, 21 transition states to (H2)(OH-)(H2O)n-1 and eight H-bond rearranging transition states are presented. During a 25 ps AIMD simulation of H- in a box of 32 H2O's at 300 K, reaction to form H2 did not occur, the hydride ion spent 95% of the time being 4-coordinated, and two first-shell exchanges were observed. This and other calculations suggest the mean survival time of (H-) in bulk water at 300 K is likely between 20 and 100 ps.