Shock Tube Measurements of the Rate Constant of the Reaction NCN + O 2

The rate constant of the comparably slow bimolecular NCN radical reaction NCN + O 2 has been measured for the first time under combustion relevant conditions using the shock tube method. The thermal decomposition of cyanogen azide (NCN 3 ) served as a clean high‐temperature source of NCN radicals. NCN concentration–time profiles have been detected by narrow‐bandwidth laser absorption atν ̃ = 30383.11 cm −1 . The experiments behind incident shock waves have been performed with up to 17% O 2 in the reaction gas mixture. At such high O 2 mole fractions, it was necessary to take O 2 relaxation into account that caused a gradual decrease of the temperature during the experiment. Moreover, following fast decomposition of NCN 3 and collision‐induced intersystem crossing of the initially formed singlet NCN to its triplet ground state, an unexpected and slow additional formation of triplet NCN has been observed on a 100‐μs timescale. This delayed NCN formation was attributed to a fast recombination of 1 NCN with O 2 forming a 3 NCNOO adduct acting as a reservoir species for NCN. Rate constant data for the reaction NCN + O 2 have been measured at temperatures between 1674 and 2308 K. They are best represented by the Arrhenius expressionk 2 / (cm 3mol− 1s − 1 ) = 1.3 × 10 12 exp ( − 97kJmol − 1R T ) , ( ± 57 % ) . No pressure dependence has been observed at pressures between 216 and 706 mbar.