A gas-phase kinetic study of the N(²D) + CH₃CCH and N(²D) + CH₃CN reactions

The chemistry of planetary atmospheres containing molecular nitrogen as a major atmospheric component is strongly influenced by the reactions of atomic nitrogen. Although nitrogen atoms in their ground electronic state N(⁴S) are mostly unreactive towards stable molecules, electronically excited nitrogen atoms N(²D) are much more reactive and could play an important role in the formation of nitriles and other nitrogen bearing organic molecules in planetary atmospheres such as Titan. Despite this, few kinetic studies of N(²D) reactions have been performed over the appropriate low temperature range. Here, we report the results of an experimental study of the reactions N(²D) + methylacetylene, CH₃CCH, and N(²D) + acetonitrile, CH₃CN, using a supersonic flow reactor at selected temperatures between 50 K and 296 K. N(²D) atoms, which were generated indirectly as a product of the C(³P) + NO reaction, were subsequently detected by laser induced fluorescence in the vacuum ultraviolet wavelength region. The measured rate constants are significantly larger than the estimated values in current photochemical models and do not display large variations as a function of temperature. The new rate constants are included in a 1D coupled ion-neutral model of Titan's atmosphere to test their influence on the simulated species abundances. In addition, the overall description of both reactions is improved by considering the results of recent experimental and theoretical work examining the product channels of these processes. These simulations indicate that while the N(²D) + CH₃CCH reaction has only a limited overall influence on Titan's atmospheric chemistry, the N(²D) + CH₃CN reaction could lead to the formation of significant relative abundances of cyanomethamine, HNCHCN, in the upper atmosphere.