A low temperature investigation of the N(2D) + CH4, C2H6 and C3H8 reactions.

The gas-phase reactions between metastable nitrogen atoms, N(2D) and saturated hydrocarbons CH4, C2H6 and C3H8 have been investigated using a supersonic flow reactor over the 296-75 K temperature range. N(2D) was generated as a product of the C(3P) + NO → N(2D) + CO reaction, with C(3P) atoms created in situ by pulsed laser photolysis of CBr4. The kinetics of N(2D) loss were followed by vacuum ultraviolet laser induced fluorescence. The measured rate constants for the N(2D) + CH4 reaction are in good agreement with earlier work and extend the available kinetic data for this process down to 127 K. The measured rate constants for the N(2D) + C2H6 and N(2D) + C3H6 reactions are in reasonable agreement with previous work at room temperature and extend the available kinetic data for these processes down to 75 K. The rate constants for all three reactions decrease as the temperature falls, indicating the presence of activation barriers for these processes. While the recommended values for the low temperature rate constants of the N(2D) + CH4 reaction are close to those reported here, the previous recommendations for the other saturated hydrocarbon reactions significantly overestimate the rate constants for these processes. The effects of the new rate constants on a coupled ion-neutral photochemical model of Titan's atmosphere are discussed.