Electron transfer controlled by hydrogen bond donor/acceptor exchange in [H2O]6−

Abstract

We present results from Path Integral Molecular Dynamics simulations that describe the characteristics of the exchange between the hydrogen-bond donor/acceptor roles of two key water molecules in the most stable isomer of the H2O6−, at cryogenic conditions. We investigated a reactive path described in terms of geared rotations of the water pair, which leads to a substantial reorganization of the localization of the excess negative charge. In the reactant and product states, the electron exhibits a surface solvation structure, lying in the vicinity of the acceptor partner of the pair involved in the HB exchange and spanning a spatial domain on the order of 10 Å, away from the cluster boundaries. In contrast, at transition states, the excess charge extends along a spatial realm that encompasses the locations of both partners in the dimer unit, which participate in the electron solvation on an equal footing. The introduction of quantum fluctuations in the treatment of the nuclear coordinates promotes important modifications in the otherwise classical double-well free-energy profile associated with the exchange. The most relevant changes manifest in the appearance of a plateau-like transition-state regime, which is clearly associated with the onset of proton tunneling. Differences in the activation energies between the isolated neutral dimer case and the anionic water hexamer are discussed. The quantum description of the nuclei also promotes a reduction in the predicted vertical detachment energies at reactant/product states; this result contrasts with the increment observed at transition states. In addition, tunneling effects are also manifested in the modifications operative in the electron delocalization at transition states.