Shock Tube Measurements of the Rate Constant of the Reaction NCN + O2

Abstract

ABSTRACTThe rate constant of the comparably slow bimolecular NCN radical reaction NCN + O2 has been measured for the first time under combustion relevant conditions using the shock tube method. The thermal decomposition of cyanogen azide (NCN3) served as a clean high‐temperature source of NCN radicals. NCN concentration–time profiles have been detected by narrow‐bandwidth laser absorption at cm−1. The experiments behind incident shock waves have been performed with up to 17% O2 in the reaction gas mixture. At such high O2 mole fractions, it was necessary to take O2 relaxation into account that caused a gradual decrease of the temperature during the experiment. Moreover, following fast decomposition of NCN3 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 1NCN with O2 forming a 3NCNOO adduct acting as a reservoir species for NCN. Rate constant data for the reaction NCN + O2 have been measured at temperatures between 1674 and 2308 K. They are best represented by the Arrhenius expression . No pressure dependence has been observed at pressures between 216 and 706 mbar.