Ab initio chemical kinetics for the reactions of N₂ with singlet and triplet C₂O radicals

The possible energy pathways for the reactions of N₂ with singlet and tripled C₂O have been investigated at the CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) level of theory. Our results show that the rate-controlling transition states for the formation of ³CNN + CO, NCO + CN and ³NCN + CO through triplet surface have 36.2, 57.7 and 60.5 kcal/mol barriers relative to the reactants ³C₂O + N₂. Formation of ¹CNN + CO and ¹NCN + CO via the singlet surface needs to overcome 43.7 and 66.9 kcal/mol barriers. The dominant products are ^1,3CNN + CO and cyc-¹NCN + CO, their rate constants in cm³ molecule^-1 s^-1 can be presented as k₁ (³CNN + CO) = 3.5 × 10^-11 exp(-36.8 kcal/mol/RT), k₂ (¹CNN + CO) ≤ 2.9 × 10^-12 exp(-33.2 kcal/mol/RT) and k₃ (cyc-¹NCN + CO) = 6.86 × 10^-20exp(-27.7 kcal/mol/RT), which are significantly lower than those assumed in the literature. The rate constants for the formation of ³NCN + CO and NCO + CN are too small to be important due to their high exit barriers. The predicted heats of reaction for formation of products NCO + CN, ³CNN + CO and ³NCN + CO are 45.9, 18.1 and -10.7 kcal/mol, which agree excellently with the experimental values, 45.8, 17.7 and -10.7 kcal/mol. Our results imply that the reaction of C₂O with N₂ cannot compete with the CH + N₂ reaction for prompt-NO formation in hydrocarbon combustion.