Inverse Isotope Kinetic Effect of the Charge Transfer Reactions of Ar+ with H2O and D2O

Abstract 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.