The Reaction NCN + H₂: Quantum Chemical Calculations, Role of ¹NCN Chemistry, and ³NCN Absorption Cross Section

Abstract

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 + H2 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 1NCN and triplet 3NCN 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 1NCN + H2 reaction as well as the newly measured high temperature absorption cross section of 3NCN 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(3NCN, λ = 329.1302 nm) = 2.63 × 109 × exp­(−1.96 × 10–3 × T/K) cm2/mol (±23%, p = 0 bar, T = 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 3NCN absorption cross section has been resolved. Whereas 3NCN + H2 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 H2NCN intermediate are HNCN + H as well, with minor contributions of CN + NH2 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 1NCN that is equally distributed over the two N atom sites. In general, the distinct reactivity differences call for a separate treatment of 1NCN and 3NCN chemistry. However, as the main reaction products in case of the H2 reaction are the same and as the population of the 1NCN in thermal equilibrium remains low, a properly weighted effective rate constant k(NCN + H2 → HNCN + H) = 2.62 × 104 × (T/K)2.78 × exp­(−97.6 kJ/mol/RT) cm3 mol–1s–1(±30%, 800 K < T < 3000 K, p < 100 bar) is recommended for inclusion into flame models that, as yet, do not explicitly account for 1NCN chemistry.

Self-consistent modeling of microwave activated N2/CH4/H2 (and N2/H2) plasmas relevant to diamond chemical vapor deposition

“…Additional sources specific to the N/C/H plasmas include the N2 decomposition reactions CHx + N2 → products (x = 0, 1), which serve as significant sources in the hot plasma core (reactions (34)- (36) in Table 2, where the reverse reaction rate coefficients (k-35 and k-36) are presented as approximations of the relevant ki values calculated from thermochemical data. Following Faheber et al [73] we treat the CH + N2 interaction as two reversible reactions CH + N2 ↔ NCN + H and NCN + H ↔ HCN + N, but the calculated concentrations of the NCN intermediate product never exceed 10 8 cm -3 under the conditions of present interest.…”

Quantitative and Sensitive Mid-Infrared Frequency Modulation Detection of HCN behind Shock Waves

“…In order to resolve this issue, measurements of the branching ratio by monitoring HCN concentration-time profiles as a direct product of the reaction NCN + H are required. However, in contrast to quantitative detection schemes for the educt species NCN [12][13][14], reliable high-temperature detection of HCN is difficult and suitable detection schemes are sparse. Ex-situ detection of absolute HCN concentrations in lowpressure flames using near-infrared cavity ring down spectroscopy has been performed by Lamoureux et al [15,16].…”