On the hydrogen atom transfer in the reaction of O−, with H20. Ab initio calculations of the potential energy along two reaction paths

The geometry corresponding to the minimum potential energy of an O−, H2O cluster is calculated at the second order Møller-Plesset approximation level as a function of the O-O− distance (path 1) and H-O− distance (path 2). Along path 1 the oxygen atoms are inequivalent for O-O− distances larger than 2 A and equivalent for smaller distances. The finding of clusters with equivalent oxygen atoms is in accordance with the observation of equal probability of proton and hydrogen atom transfer in collisions in the gas phase between O−, and H2O at thermal energies. Along path 2 the oxygen atoms of the cluster remain inequivalent for all H-O− distances. The comparison of energies of selected configurations along the two paths offers a qualitative explanation for the findings that the reaction O−+ H2O → OH → OH- in aqueous solution proceeds predominantly by H+ ion transfer and that the contribution from H atom transfer increases with increasing temperature.