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

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 (H₂O/D₂O) 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 D₂O, by the velocity map imaging measurements of H₂O⁺/D₂O⁺ products. Using the average dipole orientation capture model, we estimate the orientation angles of C_2v axis of H₂O/D₂O 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 H₂O⁺ and D₂O⁺ around the resonant charge transfer.