A semiempirical capture model for fast neutral reactions at low temperature.

The description of Titan's chemistry relies strongly on reaction rate data collected at room temperature or above. However, the temperature in the atmosphere of Titan ranges from 70 to 200 K. We describe here a simple theoretical method to extend the available measurements toward those temperatures. It is based on the long-range capture theory combined with room temperature data. First results are presented for 28 neutral reactions involving atomic carbon, CN, CH, and C(2)H radicals and are compared to low-temperature Cinétique de Réaction en Ecoulement Supersonique Uniforme measurements. A good agreement is observed, to within a factor of 2, for most reactions in the temperature range 13-295 K. Predictions are made for reactions of the CN radical with hydrocarbons and nitriles of particular importance in Titan's atmosphere.