The Reaction NCN + H: Quantum Chemical Calculations, Role of NCN Chemistry, and NCN Absorption Cross Section.

The NCN radical plays a key role for modeling prompt-NO formation in hydrocarbon flames. Recently, in a combined shock tube and flame modeling study, the so far neglected reaction NCN + H and the related chemistry of the main product HNCN turned out to be significant for NO modeling under fuel-rich conditions. In this study, the reaction has been thoroughly revisited by detailed quantum chemical rate constant calculations both for the singlet NCN and triplet NCN pathways. Optimized geometries and vibrational frequencies of reactants, products, and transition states were calculated on B3LYP/aug-cc-pVQZ level with single-point energy calculations carried out against the optimized structures using CASPT2/aug-cc-pVQZ. The determined rate constants for the NCN + H reaction as well as the newly measured high temperature absorption cross section of NCN made a reevaluation of the shock tube data of the previous work necessary, finally revealing quantitative agreement between experiment and theory. Moreover, the new directly measured Doppler-limited absorption cross section data, σ(NCN, λ = 329.1302 nm) = 2.63 × 10 × exp(-1.96 × 10 × /K) cm/mol (±23%, = 0 bar, = 870-1700 K), are in agreement with previously reported values based on detailed spectroscopic simulations. Hence, a long-standing debate about a reliable high temperature NCN absorption cross section has been resolved. Whereas NCN + H resembles a simple abstraction type reaction with the exclusive products HNCN + H, the singlet radical reaction is initiated by the insertion into the H-H bond. Up to pressures of 100 bar, the main products of the subsequent decomposition of the HNCN intermediate are HNCN + H as well, with minor contributions of CN + NH toward higher temperatures. Although much faster than the triplet reaction, the singlet radical insertion is actually rather slow, due to the necessary reorganization of the HOMO electron density in NCN that is equally distributed over the two N atom sites. In general, the distinct reactivity differences call for a separate treatment of NCN and NCN chemistry. However, as the main reaction products in case of the H reaction are the same and as the population of the NCN in thermal equilibrium remains low, a properly weighted effective rate constant (NCN + H → HNCN + H) = 2.62 × 10 × (/K) × exp(-97.6 kJ/mol/) cm mols(±30%, 800 K < < 3000 K, < 100 bar) is recommended for inclusion into flame models that, as yet, do not explicitly account for NCN chemistry.