The story of NCN as a key species in prompt-NO formation

International audience; The cyanonitrene radical, NCN, has been shown in the last two decades to play a crucial role in the formation of prompt-NO in combustion. This has stimulated a large number of experimental and theoretical studies on fundamental physico-chemical properties of NCN as well as on mechanistic and kinetic aspects of NCN reactions under combustion conditions. In this review, spectroscopic, thermodynamic, and kinetic data of NCN are collected and discussed. Methodic approaches for the detection of NCN in flames and in kinetic experiments are elucidated, and the suitability of cyanogen azide, NCN3, as a precursor for NCN in kinetic experiments is examined. Kinetic and mechanistic aspects of a number of NCN elementary chemical steps are extensively reviewed. Regarding prompt-NO formation, the role of the reaction network initialized by the reaction CH + N2 ⇌ NCN + H is examined by modeling measured flame profiles of NCN, HCN, and NO. In these simulations, the critical role of the product channel-branching of the NCN + H reaction, termed the prompt-NO switch, is confirmed. A particularly sensitive balance is observed between the product channels leading back to CH + N2 or forward to HCN + N. The roles of spin conservation and intersystem crossing processes under flame conditions and in kinetic experiments with NCN3 as NCN precursor are highlighted. A number of critical points and remaining open problems in NCN chemistry and prompt-NO formation are indicated.