Understanding hydrogen scrambling and infrared spectrum of bare CH5+ based on ab initio simulations

Abstract

A comprehensive study of the properties of protonated methane obtained from ab initio molecular dynamics simulations is presented. Comparing computed infrared spectra to the measured one gives further support to the high fluxionality of bare CH₅⁺. The computational trick to partially freezing out large-amplitude motion, in particular hydrogen scrambling and internal rotation of the H₂ moiety, leads to an understanding of the measured IR spectrum despite the underlying rapid hydrogen scrambling motion that interconverts dynamically structures of different symmetry and chemical bonding pattern. In particular, the fact that C–H stretching modes involving the carbon nucleus and those protons that form the H₂ moiety and the CH₃ tripod, respectively, result in distinct peaks is arguably experimental support for three-center two-electron bonding being operative at experimental conditions. It is proposed that hydrogen scrambling is associated with the softening of a mode that involves the bending of the H₂ moiety relative to the CH₃ tripod, which characterizes the C_s ground-state structure. The potential energy surface that is mapped on to a two dimensional subspace of internal coordinates provides insight into the dynamical mechanism for exchange of hydrogens between CH₃ tripod and the three-center bonded H₂ moiety that eventually leads to full hydrogen scrambling.