Full dimensional ab-initio dynamics calculations of electron capture processes by the H3O+ ion

Dynamics of H3O following the electron capture by H3O+, which plays an impo rtant role in the H2O formation in interstellar cloud, have been investigat ed by means of ab-initio direct dynamics calculations. The full dimensional potential energy surface, calculated at the HF/6-311+G(d,p) level, was use d in the dynamics calculation throughout. The reaction proceeds on the grou nd state of H3O with zero excess energy at time zero, while H3O is formed b y vertical electron capture by H3O+ without structural change. A total of 2 00 trajectories were run from an initial geometrical configuration generate d by ab-initio molecular dynamics calculation for H3O+ at 300 K. The dynami cs calculations for H3O showed that two reaction channels are involved in t he decay processes of H3O, formed by electron capture by H3O+: one is a dis sociation channel in which the reaction proceeds via a short-lived complex, H3O* (channel 1), and the other channel is one of complex formation in whi ch the reaction proceeds via a long-lived complex, H3O* (channel 2). The li fetimes of the complexes for channels 1 and 2 are calculated to be about 5- 60 fs and > 200 fs, respectively, although the branching ratio for channel 2 is negligibly small. The relative translational energy between H2O and H is distributed in the range 150-200 kJ mol(-1) with an average of 185 kJ mo l(-1), which is 90% of the total available energy. The mechanism of the rea ction has been discussed on the basis of the theoretical results.