Kinetic Study of the Thermal Decomposition of Isocyanic Acid in Shock Waves

Thermal decomposition of isocyanic acid HNCO diluted to less than 2.0 mol% in argon was studied behind incident shock waves over the temperature range 2100–2500 K. The decomposition course was followed by monitoring the light absorption of HNCO and NH(3Σ−) at 206 and 336 nm, respectively. It is confirmed that the primary step of the decomposition is a bimolecular process HNCO+Ar→NH(3Σ−)+CO+Ar, ΔH°0=337 kJ mol−1, with the low-pressure limit rate constants k=1017.23±0.36exp[−(402±17) kJ mol−1/RT] cm3 mol−1 s−1. The siglet-to-triplet crossing point is estimated on the basis of the RRKM low-pressure-limit rate constant calculations. The overall decomposition mechanism is suggested and its validity is confirmed by computer simulation of the time–concentration profiles of NH(3Σ−) at varying temperature.