Inverse Isotope Kinetic Effect of the Charge Transfer Reactions of Ar + with H 2 O and D 2 O.

Hydrogen isotopic effect, as the key to revealing the origin of Earth's water, arises from the H/D mass difference and quantum dynamics at the transition state of reaction. The ion-molecule charge-exchange reaction between water (H 2 O/D 2 O) and argon ion (Ar + ) proceeds spontaneously and promptly, where there is no transition-state or intermediate complex. In this energetically resonant process, we find an inverse kinetic isotope effect (KIE) leading to the higher charge transfer rate for D 2 O, by the velocity map imaging measurements of H 2 O + /D 2 O + products. Using the average dipole orientation capture model, we estimate the orientation angles of C 2v axis of H 2 O/D 2 O relative to the Ar + approaching direction and attribute to the difference of stereodynamics. According to the long-distance Landau-Zener charge transfer model, this inverse KIE could be also attributed to the density-of-state difference of molecular bending motion between H 2 O + and D 2 O + around the resonant charge transfer.