DIFFUSION MONTE CARLO CALCULATIONS OF MINIMUM ENERGY PATHS FOR THE ISOTOPIC VARIANTS OF THE CH$_{3}^{+}$ + H$_{2}$ $\leftrightarrow$ CH$_{5}^{+}$ $\leftrightarrow$ CH$_{3}^{+}$ + H$_{2}$ REACTION

Abstract

Protonated methane is of interest to astrochemists due to its presumed importance as a reaction intermediate in the reaction involving CH$_{3}^{+}$ + HD within the interstellar medium. However, within the interstellar medium there is a nonstatistical H/D isotopic abundance observed for the isotopologues of CH$_{3}^{+}$. Kinetic studies performed by Gerlich and co-workers determined that the reactions \begin{equation} \mathrm{CH_{3-n}D_{n}^{+} + HD \rightarrow CH_{4-n}D_{n+1}^{+} \rightarrow CH_{2-n}D_{n+1}^{+} + H_2} \end{equation} have identical net rate constants regardless of the value of n.{} We have calculated zero-point corrected energies and wave functions of the CH$_{3}^{+}$ + H$_{2}$ system{} and its deuterated analogs as functions of the center of mass separation between CH$_{3}^{+}$ and H$_{2}$, $R$. We can divide these simulations into distinct ranges of $R$; long-range interactions, complexation, and intermediate distances. Analysis of the wave functions associated with these three ranges of $R$ allows us to study how zero-point energy influences the approach geometries that are sampled during low-energy collisions.