Effects of zero point vibration on the reaction dynamics of water dimer cations following ionization.

Reactions of water dimer cation (H2O)2+ following ionization have been investigated by means of a direct ab initio molecular dynamics method. In particular, the effects of zero point vibration and zero point energy (ZPE) on the reaction mechanism were considered in this work. Trajectories were run on two electronic potential energy surfaces (PESs) of (H2O)2+: ground state ((2) A″-like state) and the first excited state ((2) A'-like state). All trajectories on the ground-state PES lead to the proton-transferred product: H2 O(+) (Wd)-H2 O(Wa) → OH(Wd)-H3 O(+) (Wa), where Wd and Wa refer to the proton donor and acceptor water molecules, respectively. Time of proton transfer (PT) varied widely from 15 to 40 fs (average time of PT = 30.9 fs). The trajectories on the excited-state PES gave two products: an intermediate complex with a face-to-face structure (H2 O-OH2 )(+) and a PT product. However, the proton was transferred to the opposite direction, and the reverse PT was found on the excited-state PES: H2 O(Wd)-H2 O(+) (Wa) → H3 O(+) (Wd)-OH(Wa). This difference occurred because the ionizing water molecule in the dimer switched between the ground and excited states. The reaction mechanism of (H2O)2+ and the effects of ZPE are discussed on the basis of the results. © 2017 Wiley Periodicals, Inc.