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): kNCO+C3H4=(1.39±0.18), kNCO+C3H6=(4.29±0.20), kNCO+C4H8=(6.18±0.46) and kNCO+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.