Unimolecular Fragmentation of CH3NH2: Towards a Mechanistic Description of HCN Formation

Electron induced reactions in condensed mixtures of methane and ammonia

We demonstrate the efficient formation of carbon-nitrogen bonds starting from CH₄ and NH₃ on a metal surface at cryogenic temperatures. Electrons in the energy range of 1-90 eV are used to initiate chemical reactions in mixed molecular films of CH₄ and NH₃ at ∼15 K, and the products are detected by performing temperature programmed desorption (TPD). Extensive dehydrogenation occurs at all energies giving the products CH₂NH and HCN in preference to CH₃NH₂. This is likely to do with the energetics of the reactions and the subsequent stability of these species in the condensed film. Thermal processing of the irradiated mixture favours dehydrogenation as indicated by the results of using different desorption rates. Electron impact excitation and subsequent dissociation into radicals is the reaction-initiating step rather than ionization of CH₄ and NH₃, as inferred from the yield of products as a function of electron energy. This could give insight into the important catalytic process of the industrial scale synthesis of HCN from CH₄ and NH₃ over Pt. This may also be a relevant pathway in the astrochemical environment where CN and HCN are abundant and low-energy electrons are found ubiquitously.