A Brief Story of H₅⁺: Spectroscopy and Dynamics from Theory

As a mysterious existence in the interstellar medium, H₅⁺ has been proposed as the intermediate of the proton transfer reaction, H₃⁺ + H₂ → H₅⁺ → H₂ + H₃⁺. Although products appear identical to reactants, they can possess different nuclear quantum states due to extensive proton scrambling in H₅⁺ driven by large-amplitude motions (LAMs). These LAMs bring challenges to nuclear quantum mechanics as the conventional rigid rotor-harmonic oscillator (RRHO) approximation fails. Herein we introduced diffusion Monte Carlo (DMC) and its extensions to account for the non-rigid and anharmonic properties of H₅⁺. We evaluated the energetics and wave functions for selected nuclear energy levels along proton transfer reaction paths and analyzed the roles of LAMs. We discovered strong couplings among intramolecular modes of H₅⁺, including those between proton hops and dissociations, and between internal and overall rotations. These results managed to decipher and predict the exotic behaviors of H₅⁺ in its rovibrational spectroscopy and reaction dynamics.