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

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 Cs 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.