The Reaction of Nitrogen Atoms with Methyl Radicals:  Are Spin-Forbidden Channels Important?

Abstract

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.

Neutral production of hydrogen isocyanide (HNC) and hydrogen cyanide (HCN) in Titan’s upper atmosphere

“…These authors suggest that HCN formation is associated with H 2 , however HCN + H 2 production is spinforbidden and needs intersystem crossing to occur. Additionally recent ab-initio calculations (Cimas & Largo 2006) found almost 100% of H 2 CN production in good agreement with previous calculations (Nguyen et al 1996). As H 2 CN may have enough internal energy (153 kJ mol −1 ) to overcome the dissociation barrier for C-H dissociation (130 kJ mol −1 ), some HCN + H + H may be produced (Nguyen et al 1996).…”

CRITICAL REVIEW OF N, N + , N + 2 , N ++ , And N ++ 2 MAIN PRODUCTION PROCESSES AND REACTIONS OF RELEVANCE TO TITAN'S ATMOSPHERE

“…However, the (HCN + H 2 ) production is spinforbidden and needs an intersystem crossing process to occur. Additionally, recent ab initio calculations (Cimas & Largo 2006) found almost 100% of H 2 CN production, in good agreement with previous calculations (Nguyen et al 1996). As H 2 CN may have enough internal energy to overcome the dissociation barrier for C-H dissociation, some (HCN + H + H) may be produced (Nguyen et al 1996).…”

A Consistent Reduced Network for HCN Chemistry in Early Earth and Titan Atmospheres: Quantum Calculations of Reaction Rate Coefficients

“…A computational study of the CH 3 + N − −→ products reaction shows a preference for the CH 3 + 4 N − −→ H 2 CN + H pathway 64 . This study finds the CH 3 + 2 N − −→ HCN + H 2 channel to be negligible.…”

“…In this case study, we analyze the three suggested main branches for CH 3 + N − −→ products using CVT (see the methods section for full details). Computational studies show that CH 3 + 4 N − −→ products reactions first proceed through a barrierless reaction to H 3 CN on the triplet surface 64,143 . We confirm this barrierless reaction (CH 3 + 4 N − −→ 3 H 3 CN) and calculate its rate coefficients at the BHandHLYP/aug-cc-pVDZ level of theory to be 3.3×10 −11 cm 3 s −1 .…”