Temperature dependence of the reactions of CH radicals with NO, NH 3 and N 2O in the range 200–1300 K

Abstract

Bimolecular rate constants for the reactions of CH(X 2Π) radicals with nitric oxide, ammonia and nitrous oxide were measured under pseudo-first-order conditions at 4 Torr total pressure, as a function of temperature in the range 300–1300 K for the CH+NO and CH + NH 3 reaction and in the range 200–1250 K for the CH + N 2O reaction. CH radicals were generated by excimer laser photolysis of CHClBr 2/Ar mixtures and were detected by laser-induced flourescence. The rate constant of the reaction CH+NO was found to be independent of temperature. The measured rate constants of the reactions CH + N 2O and CH + NH 3 exhibit negative temperature dependences which are described by the following Arrhenius equations (with E A in units of kJ/mol): k CH + N 2O ( T) = (3.09 ± 0.07) × 10 −11 exp[(2.14 ± 0.08)/ RT] cm 3 s −1, k CH + NH 3 ( T) = (7.23 ± 0.17) × 10 −11 exp[(2.64 ± 0.10)/ RT] cm 3 s −1 and k CH + NO ( T) = (1.87 ± 0.06) × 10 −10 exp[(0.00 ± 0.15)/ RT] cm 3 s −1. All reactions were found to be very fast, with rate constants close to the gas kinetic limit. The negative temperature dependences for the reactions CH + N 2O and CH + NH 3 are in accord with an addition/decomposition mechanism and are similar to those observed for the reactions of CH radicals with saturated and unsaturated hydrocarbons.