Ring-Polymer Molecular Dynamics Calculations of Thermal Rate Coefficients and Branching Ratios for the Interstellar H 3 + + CO → H 2 + HCO + /HOC + Reaction and Its Deuterated Analogue.

The reaction between H 3 + and CO is important in understanding the H 3 + destruction mechanism in the interstellar medium. In this work, thermal rate coefficients for the H 3 + + CO and D 3 + + CO reactions are calculated using ring-polymer molecular dynamics (RPMD) on a high-level machine-learning potential energy surface. The RPMD results agree well with the classical molecular dynamics results, where nuclear quantum effects are completely ignored, whereas the agreement between the RPMD results and the previous quasi-classical trajectory is good only at low temperatures. The calculated [HCO + ]/[HOC + ] product branching ratios decrease as the temperature increases, and the product branching is exclusively determined by the initial collisional orientation, which governs the formation of an ion-dipole complex, H 3 + ···CO or H 3 + ···OC, that dissociates into H 2 + HCO + or H 2 + HOC + , respectively, via a direct mechanism. However, the contribution of the indirect mechanism via the rearrangement between H 3 + ···CO and H 3 + ···OC increases as the temperature increases, although its absolute fraction is small.