Investigation of the reactions NCN + CH3, NCN + OH, and CH3 + OH behind shock waves

Abstract

The NCN radical plays a key role for modeling prompt-NO formation in hydrocarbon flames. As both CH3 and OH radicals are present at high mole fraction level in the reaction zone, their radical-radical cross-reactions with NCN are of special interest, but no experimental rate constants are available. In contrast to the OH reaction, the reaction with CH3 is not considered in common detailed flame mechanisms as yet. It is shown in this study, however, that the NCN + CH3 reaction proceeds close to collision limit and therefore acts as one of the most important reactions to describe the fate of NCN in flames. NCN and OH concentration-time profiles were recorded behind shock waves. The thermal decomposition of cyanogen azide (NCN3) and tert‑Butyl hydroperoxide (TBHP) served as quantitative source of NCN and CH3/OH radicals. Representing a secondary reaction under the applied experimental conditions, the rate constant of the reaction CH3 + OH has been first determined from OH profiles with TBHP as precursor. With k(CH3 + OH) = 4.34 × 10–10 × (T / K)6.38 exp(+68.0 kJ mol–1 / RT) cm3 mol–1 s–1 (±40%, 925 – 1800 K, p ≈ 393 mbar) it was found to be in good agreement with available literature results. Rate constants of the reaction NCN + CH3 were then determined from NCN profiles by shock-heating gas mixtures with TBHP and NCN3. According to previous as well as our own statistical rate theory calculations, the only relevant product channel is the formation of CH2NH + CN. In the temperature range 857 – 1817 K and at a pressure of p ≈ 314 mbar, a high value for the rate constant k(NCN + CH3) = 5.40 × 1013 exp(+1.77 kJ mol–1 / RT) cm3 mol–1 s–1 (±30%) was found. For future implementation into flame models, we recommend the rate constant expression k(NCN + CH3) = 8.44 × 1019 (T / K) –1.76 exp(–15.2 kJ mol–1 / RT) cm3 mol–1 s–1 (±40%, 800 – 4000 K), which is based on a theoretically assisted extrapolation of the experimental data. Finally, OH profiles in the presence of CH3 and NCN have been analyzed. In agreement with previous theoretical estimates, the reaction is indeed comparably slow and therefore only a rough upper limit of k(NCN + OH) < 8 × 1012 cm3mol–1s–1 (918 – 1808 K, p ≈ 415 mbar) could be inferred from the experiments.