Mode-specific chemistry in the H + HCN and H + N₂O reactions

We present a theoretical study of the effect of reagent vibrational excitation on cross sections and rate constants for the reactions H + HCN (v₁v₂v₃) → H₂ + CN and H + N₂O(V₁v₂v₃) → N₂ + OH, NH + NO. For H + HCN, we study the states (000), (004) and (302), and we find that C-H stretch excitation is much more effective than C-N stretch in lowering the reactive threshold energy and thus enhancing the rate constant. For H + N₂O we study the states (000), (100), (01¹0) and (001). Here N-N stretch excitation is more effective in enhancing the cross section at low energy where the reaction mechanism involves HNNO complex formation. At higher energy (3.3 kcal/mol above threshold and higher), N-O stretch excitation is more effective in enhancing the reaction cross section, due to the dominance of a direct mechanism involving a NN-O-H transition state.