The reaction of nitrogen atoms with methyl radicals: Are spin-forbidden channels important?

A computational study of the N(4S) + CH3 reaction has been carried out. The reactants approach through an attractive potential surface leading to an intermediate, H3CN, whose formation does not involve any barrier. In agreement with the experimental results, the dominant channel for this reaction is H2CN+H. The theoretically estimated rate coefficient for the overall process at 298 K is 9.1 x 10(-12) cm3 s(-1) molecule(-1), which is nearly 1 order of magnitude lower than the experimental result, but also much larger than those computed for the reactions of ground-state nitrogen atoms with halomethyl radicals. The analysis of the singlet potential energy surface, and the corresponding computational kinetic study, shows that for the reaction of excited nitrogen atoms with methyl radicals, the preferred product from the kinetic point of view is also H2CN+H, but in this case production of HCN is significant (with branching ratios around 0.185). According to our calculations, spin-forbidden processes are highly unlikely for the N(4S) + CH3 reaction. However, further evolution of the preferred products, H2CN+H, might explain the experimental observation of hydrogen cyanide as a minor product in this reaction.