Laboratory kinetics of C 2H radical reactions with ethane, propane, and n-butane at T = 96–296 K: implications for Titan

Abstract

The kinetics of the reactions of C 2H radical with ethane ( k 1), propane ( k 2), and n-butane ( k 3) are studied over the temperature range of T = 96–296 K with a pulsed Laval nozzle apparatus that utilizes a pulsed laser photolysis–chemiluminescence technique. The C 2H decay profiles in the presence of both the alkane reactant and O 2 are monitored by the CH(A 2Δ) chemiluminescence tracer method. The results, together with available literature data, yield the following Arrhenius expressions: k 1( T) = (0.51 ± 0.06) × 10 −10 exp[(−76 ± 30)K/ T] cm 3 molecule −1 s −1 ( T = 96–800 K), k 2( T) = (0.98 ± 0.32) × 10 −10exp[(−71 ± 60)K/ T] cm 3 molecule −1 s −1 ( T = 96–361 K), and k 3( T) = (1.23 ± 0.26) × 10 −10 cm 3 molecule −1 s −1 ( T = 96–297 K). At T = 296 K, k 1 is measured as a function of total pressure and has little or no pressure dependence. The results from this work support a direct hydrogen abstraction mechanism for the title reactions. Implications to the atmospheric chemistry of Titan are discussed.