THEORETICAL-STUDY OF THE INTRAMOLECULAR ISOTOPE EFFECT IN THE REACTION OF O(P-3)+HD - THE IMPORTANCE OF MOLECULAR-REORIENTATION

A quasiclassical trajectory study has been made of the intramolecular isotope effect for the reaction of O(P-3) + HD (v = 0, J = 0) on the potential energy surface of Johnson and Winter for relative energies E between 15 and 150 kcal/mol. In this energy range OD + H is the favored product channel. During each trajectory the OH and OD bond lengths were monitored to determine which end of the HD molecule was ''hit'' by the O atom. The results show unambiguously that the O atom exerts a torque on the molecule that rotates the H atom away from O, increasing the likelihood that 0 will hit and react with D. The effect of this torque increases as the HD molecule stretches during the reactive process. The reorientation of HD is important even at collision energies as high as E = 100 kcal/mol. Plots of the potential energy surface for this reaction confirm the importance of the intermolecular torque exerted on HD by O.