Thermal Decomposition of NCN: Shock-Tube Study, Quantum Chemical Calculations, and Master-Equation Modeling

The thermal decomposition of cyanonitrene, NCN, was studied behind reflected shock waves in the temperature range 1790–2960 K at pressures near 1 and 4 bar. Highly diluted mixtures of NCN₃ in argon were shock-heated to produce NCN, and concentration–time profiles of C atoms as reaction product were monitored with atomic resonance absorption spectroscopy at 156.1 nm. Calibration was performed with methane pyrolysis experiments. Rate coefficients for the reaction ³NCN + M → ³C + N₂ + M (R1) were determined from the initial slopes of the C atom concentration–time profiles. Reaction was found to be in the low-pressure regime at the conditions of the experiments. The temperature dependence of the bimolecular rate coefficient can be expressed with the following Arrhenius equation: <i>k</i>₁^bim = (4.2 ± 2.1) × 10¹⁴ exp­[−242.3 kJ mol^–1/(<i>RT</i>)] cm³ mol^–1 s^–1. The rate coefficients were analyzed by using a master equation with specific rate coefficients from RRKM theory. The necessary molecular data and energies were calculated with quantum chemical methods up to the CCSD­(T)/CBS//CCSD/cc-pVTZ level of theory. From the topography of the potential energy surface, it follows that reaction proceeds via isomerization of NCN to CNN and subsequent C–N bond fission along a collinear reaction coordinate without a tight transition state. The calculations reproduce the magnitude and temperature dependence of the rate coefficient and confirm that reaction is in the low-pressure regime under our experimental conditions