Molecular dynamics of water hexamer anions at cryogenic temperatures

Abstract

We studied the dynamics of water hexamer anions [(H2O)6−] at cryoscopic temperatures using MP2 level ab initio molecular dynamics (AIMD) simulations. The vertical electron detachment energy (VDE) of these clusters varies in the 150–550 meV range in good agreement with experiments. The dominant characteristic pattern of the electron binding sites consists of a double hydrogen bond acceptor water molecule with two dangling hydrogen atoms in direct contact with the excess electron. In addition to surface localized excess electron clusters, we examine the dynamics of a hexamer model of the bulk hydrated electron. We analyze correlations between the binding strength of the excess electron and geometrical and spectroscopic properties, in particular, the radius of the excess electron and the bending frequencies of the electron binding water units. Our investigations were extended to the evaluation of nuclear quantum effects on the physical properties of the clusters by performing path integral molecular dynamics simulations on a neural network based potential energy surface and also using the generalized smoothed trajectory analysis method. Nuclear quantum effects at these low temperatures were found to be significant, as demonstrated by structural, energetic, and spectroscopic characteristics of the clusters. Most strikingly, the half-width of the quantum distributions of the VDE or the radius of the electron increases by a stunning factor of ∼5–10 relative to the classical ones. Molecular dynamics trajectories also reveal that, while all investigated isomers persist at 10 K in AIMD simulations, nuclear quantum effects promote isomerizations to more stable, lower lying minima.