Theoretical study on the mechanism and kinetics for the reaction of HNCO with CN radical: HNCO + CN → HCN + NCO or HNCO + CN → HNCN + CO?

The mechanism for the HNCO + CN reaction was investigated by considering the possible channels of the C atom and N atom of the CN radical attacking the H, N, C, and O atoms of HNCO based on the B3LYP/6-311+G(2d,p) method; and the CCSD/6-31+G(d,p) method was adopted to optimize the geometries of stationary points of the main paths for further kinetics calculation. The energies of all the stationary points were refined with an accurate multilevel method. The energetically most favorable channel for the HNCO + CN reaction was predicted to be the addition reaction of the C atom of CN radical to the N atom of HNCO, producing the HNCN + CO (P2) products, which is different from most of the HNCO-radical reactions in which the hydrogen abstraction channel is dominant. The thermal rate constants were calculated using the conventional transition state theory with Eckart tunneling correction. The results showed that the C–N addition channel (HNCO + CN → HNCN + CO) is dominant at T > 273 K, whereas the hydrogen abstraction channel is more competitive at T < 273 K. As compared to the previously over-estimated temperature independent reaction rate constant of 2.5 × 10−11 cm3 molecule−1 s−1, the calculated rate constants of the C–N addition channel and the hydrogen abstraction channel are positively temperature dependent, and are only 7.22 × 10−16 and 5.34 × 10−16 cm3 molecule−1 s−1 at 298 K, respectively.