Intramolecular Vibrational Energy Redistribution in the Reaction H₃⁺ + CO → H₂ + HCO⁺

An <i>ab initio</i> direct molecular dynamics (MD) calculation at the RS2/aug-cc-pVQZ level, followed by vibration mapping, has been applied to the H₃⁺ + CO → H₂ + HCO⁺ reaction to elucidate the intramolecular vibrational energy redistribution (IVR) processes during the reaction. Direct MD calculations were carried out for 20 K (a typical temperature for interstellar dark clouds) and 330 K (a typical translational temperature for ions in a glow discharge). Under the <i>C</i>_<i>s</i> symmetry constraint, the approach of H₃⁺ turned out to be the H-C stretching mode of the [H···CO]⁺ part, which invoked the C-O stretching and then the H-C-O bending modes. Under no symmetry constraint, a strong bending mode was first invoked, and the intensities of the subsequent H-C and C-O stretching modes were kept relatively small. The detailed analyses of the IVR during the reaction, in terms of vibration mixing, gave a clue to understanding experimentally observed anomalies in the bending modes, such as population inversion at some bending states. In the MD simulation at 20 K, less than two-thirds of the reaction energy was converted to the vibrational energy of the resultant HCO⁺ part and one-third to the translational and rotational energies of the leaving H₂ molecule. These direct MD simulations, when combined with the experimental spectroscopy data, shed light on a clear understanding of the reaction mechanism, including the IVR during the reaction.