A quasiclassical trajectory study of the Cl+HCN→HCl+CN reaction dynamics.: Microscopic reaction mechanism of the H(Cl)+HCN→H2(HCl)+CN reactions

The Cl + HCN --> HCl + CN reaction dynamics has been studied using the quasiclassical trajectory method. The potential energy surface is taken from an accurate global surface for the HHCN system. Cl + HCN and H + HCN have very similar energetics, so the present calculation provides a test of whether the Cl + HCN dynamics is captured by a model in which the only difference is provided by the mass of the attacking atom. We find generally good agreement with experimental studies of the Cl + HCN reaction, including CN product rovibrational distributions and the relative rate coefficients for HCN initially in highly excited vibrational states. The results correctly describe the differences between Cl and H attack, so apparently the differences in the reactivity of these two reactions are a kinematic effect. A detailed analysis of the microscopic reaction mechanism of the H + HCN --> H-2 + CN and Cl + HCN --> HCl + CN reactions is also provided. This shows that the H and Cl reactions are both dominated by direct dynamics; however, the direct reaction with Cl frequently involves secondary collisions in which the Cl interacts with the CN fragment of HCN before abstracting the H atom, while the H atom reaction rarely does this. This allows the CN stretch mode to interact more strongly with reaction coordinate motions in Cl + HCN than in H + HCN, leading to greater CN vibrational excitation for initial HCN states that have no C-N stretch excitation, in agreement with observations.