Low temperature (39–298 K) kinetics study of the reactions of the C 4H radical with various hydrocarbons observed in Titan's atmosphere

Abstract

The reaction kinetics of the butadinyl radical, C 4H, with various hydrocarbons detected in the atmosphere of Titan (methane, ethane, propane, acetylene, ethene and methylacetylene) are studied over the temperature range of 39–298 K using the Rennes CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus. Kinetic measurements were made using the pulsed laser photolysis—laser induced fluorescence technique. The rate coefficients, except for the reaction with methane, all show a negative temperature dependence and can be fitted with the following expressions over the temperature range of this study: k C 2 H 6 = 0.29 × 10 −10 exp ( − 25.6 K / T ) × ( T / 298 K ) −1.24 cm 3 molecule −1 s −1 ; k C 3 H 8 = 1.06 × 10 −10 exp ( − 56.3 K / T ) × ( T / 298 ) −1.35 cm 3 molecule −1 s −1 ; k C 2 H 2 = 1.82 × 10 −10 exp ( 65.8 K / T ) × ( T / 298 K ) −1.06 cm 3 molecule −1 s −1 , k C 2 H 4 = 1.95 × 10 −10 exp ( − 9.5 K / T ) × ( T / 298 K ) −0.40 cm 3 molecule −1 s −1 , k CH 3 C 2 H = 3.21 × 10 −10 exp ( − 47.2 K / T ) × ( T / 298 K ) −0.82 cm 3 molecule −1 s −1 . These expressions are not intended to be physically meaningful but rather to provide an easy way to introduce experimental results in photochemical models. They are only valid over the temperature range of the experiments. Possible channels of these reactions are discussed as well as possible consequences of these results for the production of large molecules and hazes in the atmosphere of Titan. These results should also be considered for the photochemistry of Giant Planets.