Direct measurements of the rate constants of the reactions NCN + NO and NCN + NO2 behind shock waves

Article abstract of DOI:10.1021/jp208715c

The high-temperature rate constants of the reactions NCN + NO and NCN + NO2 have been directly measured behind shock waves under pseudo-first-order conditions. NCN has been generated by the pyrolysis of cyanogen azide (NCN ₃) and quantitatively detected by sensitive difference amplification laser absorption spectroscopy at a wavelength of 329.1302 nm. The NCN ₃ decomposition initially yields electronically excited ¹NCN radicals, which are subsequently transformed to the triplet ground state by collision-induced intersystem crossing (CIISC). CIISC efficiencies were found to increase in the order of Ar < NO₂ < NO as the collision gases. The rate constants of the NCN + NO/NO₂ reactions can be expressed as k_NCN+NO/(cm³ mol ^-1s^-1) = 1.9 × 10¹² exp[- 26.3 (kJ/mol)/RT] (±7%, ΔE_a = ± 1.6 kJ/mol, 764 K < T < 1944 K) and k_NCN+NO2/(cm³ mol^-1s ^-1) = 4.7 × 10¹² exp[- 38.0(kJ/mol)/RT] (±19%,ΔE_a = ± 3.8 kJ/mol, 704 K < T < 1659 K). In striking contrast to reported low-temperature measurements, which are dominated by recombination processes, both reaction rates show a positive temperature dependence and are independent of the total density (1.7 × 10^-6 mol/cm < ρ < 7.6 × 10 mol/cm³). For both reactions, the minima of the total rate constants occur at temperatures below 700 K, showing that, at combustion-relevant temperatures, the overall reactions are dominated by direct or indirect abstraction pathways according to NCN + NO f CN + N₂O and NCN + NO₂ → NCNO + NO. (Figure presented)