A theoretical study on the vibrationally state-selected hydrogen transfer reaction: NH +3 (ν) + NH 3 → NH +4 + NH 2. An ab initio MR-SD-CI and classical trajectory approach

Abstract

Ab initio MR-SD-CI and classical trajectory calculations have been performed to elucidate the vibrational mode specificity of the title reaction, whose reactive cross section is enhanced by vibrational excitation of the ν 2 umbrella-bending mode of NH + 3. Potential energy surfaces (PESs) of the reaction have been obtained for vibrationally groun and excited states (vibrational quantum numbers, ν = 0 and 2, respectively) by assuming a hydrogen-bonded structure with fixed bending angles. The MO calculations show that a hydrogen transfer is composed of two elementary steps: (1) an electron transfer from NH 3 to NH + 3 at the avoided crossing region on the entrance PES, and (2) a proton transfer in the (NH 3 · NH 3) + intermediate complex region. The PESs show that the avoided crossing point shifts to larger intermolecular separation due to vibrational excitation. Using the ab initio fitted PESs, the classical trajectory calculations elucidate the reaction dynamics. The maximum value of the impact parameter ( b max) for the reaction is increased by the vibrational excitation. Based on these theoretical results, a simple reaction model has been proposed, in which the electron capturing volume of NH + 3 increases with increasing vibrational quantum number ν.