A theoretical study for H₂ + CN ↔ HCN + H reaction and its kinetic isotope effects with variational transition state theory

We present variational transition state theory (VTST) calculations for the H₂ + CN → HCN + H (R1) and H₂ + CN → DCN + D (R2) reactions and their reverses based on a global many-body expansion potential energy surface (PES) for ground-state H₂CN (ter Horst MA, Schatz GC, Harding LB, J Chem Phys 105:658, 1996). It is found that the tunneling effects are negligible over the 200-2000 K temperature range and non-negligible over 100-200 K for R1 and R2 reactions. The C-N bond acts almost as a spectator for both reactions. The present VTST rate constants are in good agreement with the available experimental results and the previous theoretical predictions for R1 and R2 reactions except for the overestimation of rate constants by VTST at lower temperatures that may be caused by recrossing effect. Additionally, the kinetic isotope effects are important for the forward R1 and R2 reactions, but not for the reverses of R1 and R2.