Kinetic investigation of NCO radicals reacting with selected hydrocarbons

Abstract

Bimolecular rate coefficients for the reactions of isocyanate radicals (NCO) with ethane (C₂H₆), allene (C₃H₄), propene (C₃H₆), but-1-ene (C₄H₈) and but-1-yne (C₄H₆) were measured in argon as carrier gas. NCO radicals have been formed using excimer laser photolysis of chlorine isocyanate (ClNCO) and were detected in their electronic ground state, ²Π, by laser-induced fluorescence. The NCO+C₂H₆ reaction was investigated at 20 Torr total pressure in the temperature range 297–899 K. The reaction exhibits a positive temperature dependence which is well described by the following modified Arrhenius equation: In addition, measurements carried out at 651 K in the pressure range 5–528 Torr showed no pressure dependence, which supports the view that the NCO+C₂H₆ reaction proceeds as a simple H atom abstraction. For the reaction of NCO radicals with allene (C₃H₄), propene (C₃H₆), but-1-ene (C₄H₈) and but-1-yne (C₄H₆) bimolecular rate coefficients were determined at room temperature and at 20 Torr total pressure. Additionally, the reactions of NCO with C₃H₄ and C₄H₆ were studied at different total pressures. For both reactions, no pressure dependence was observed. The following rate coefficients were obtained at 298 K (in units of 10^-11 cm³ s^-1): k_NCO+C3H4=(1.39±0.18), k_NCO+C3H6=(4.29±0.20), k_NCO+C4H8=(6.18±0.46) and k_NCO+C4H6=(1.41±0.14). A correlation between the measured bimolecular rate coefficients of the NCO reactions with unsaturated hydrocarbons and their ionisation potentials was established.