Reaction dynamics calculations for the CN+H₂ →HCN+H reaction: Applications of the rotating-bond approximation

We present quantum dynamics studies of the CN+H₂→HCN+H reaction. An extended version of the rotating bond approximation (RBA), in which nonreactive CN stretch motion is taken into account in the dynamics explicitly, has been employed, and we have used a potential energy surface that was recently developed by ter Horst, Schatz, and Harding [J. Chem. Phys. 105, 558 (1996)]. This surface describes the HCN force field quite accurately and has significant coupling between CH and CN stretch vibrations in HCN. We find that neither CN vibration nor rotation affect the reaction cross section for the forward reaction significantly. This result is consistent with the fact that the forward reaction has an early barrier. On the other hand, for the reverse reaction HCN+H, HCN vibrational excitation significantly lowers the reactive threshold and enhances the cross section at energies above threshold. We find for the reverse reaction that all three modes of HCN (CH stretch, CN stretch and bend) reduce the threshold by an amount which equals the energy put into reagent vibration, but the enhancement in the cross section at a fixed energy above threshold is largest for energy put into the CH stretch. We also find that the HCN vibrational state distributions for the CN+H₂ reaction show significant population in both CH and CN stretching normal modes. These results indicate that the nonreactive CN bond in the CN+H₂ reaction, as well as its reverse, cannot be described totally by a simple spectator model, and that coupling between CH and CN stretch vibrations plays an important role in the dynamics.