Ab Initio Study of the ClO + NH₂ Reaction:  Prediction of the Total Rate Constant and Product Branching Ratios^†

The mechanism for ClO + NH₂ has been investigated by ab initio molecular orbital and transition-state theory calculations. The species involved have been optimized at the B3LYP/6-311+G(3df,2p) level and their energies have been refined by single-point calculations with the modified Gaussian-2 method, G2M(CC2). Ten stable isomers have been located and a detailed potential energy diagram is provided. The rate constants and branching ratios for the low-lying energy channel products including HCl + HNO, Cl + NH₂O, and HOCl + ³NH (X³Σ^-) are calculated. The result shows that formation of HCl + HNO is dominant below 1000 K; over 1000 K, Cl + NH₂O products become dominant. However, the formation of HOCl + ³NH (X³Σ^-) is unimportant below 1500 K. The pressure-independent individual and total rate constants can be expressed as k₁(HCl + HNO) = 4.7 × 10^-8(T^-1.08) exp(^-129/_T), k₂(Cl + NH₂O) = 1.7 × 10^-9(T^-0.62) exp(^-24/_T), k₃(HOCl + NH) = 4.8 × 10^-29(T^5.11) exp(^-1035/_T), and k_total = 5.0 × 10^-9(T^-0.67) exp(^-1.2/_T), respectively, with units of cm³ molecule^-1 s^-1, in the temperature range of 200−2500 K.