An Ab initio determination of the rate constant for H2 + CN → H + HCN

The reactants, products, and saddle point for the reaction H2 + CN → H + HCN have been studied by ab initio calculations. The computed structures, frequencies, and energetics are compared directly to available measurements and, indirectly, to experimental rateconstants. The theoretical rate constants used in the comparison are calculated with conventional transition state theory. By reduction of the computed reaction barrier to 4.1 kcal mol,−1 good agreement with experimental rate constants is obtained over a 3250‐K temperature range. This computed rate constant is well represented by the form 4.9 × 10−18 T2.45 e−1, 126/T over the temperature range of 250 K–3500 K. Substantial reaction rate curvature is found due to low‐frequency bending modes at the saddle point. The results for this reaction are compared to other ion reactions involving H atom transfer to identify correlations between reaction exothermicity and both ion barriers and reaction rate curvature.