Photochemical Reactions in Molecular Solids Containing CH₃CN and NH₃ Studied by UV Laser Photodesorption of Surface Molecules

Abstract

UV light from an expanded laser at 193 nm or a low pressure mercury lamp (254 nm +185 nm) was irradiated on the solid composed of CH₃CN and NH₃ at 77∼130 K. Primary photochemical processes were studied by the photodesorption of product molecules on the solid surface. A pulsed laser at 248 nm (∼20 mJ/cm2) was used as a photodesorption light source and the mass numbers and translational energies of the photoproducts were measured by a quadrupole mass spectrometer. Formation of C₂H₂N₂ and C₂H₄ from CH₃CN has been ascribed to a disproportionation reaction of two acetonitrile molecules, which is the main reaction in a pure acetonitrile solid. Aminoacetonitrile (NH₂CH₂CN) and acetamidine (CH₃C(NH₂) =NH) were formed through a bimolecular condensation reaction of CH₃CN and NH₃ and the addition reaction of the two parent molecules, respectively.
The product analysis of the CH₃CN + NH₃ solid irradiated with a low pressure mercury lamp at 77∼130 K was performed by the use of a liquid chromatograph and a high resolution mass spectrometer. As well as the primary products, many large molecules such as DAMN (diaminomaleonitrile), AICN (5-amino-4-imidazolecarbonitrile) and other HCN oligomers, were detected in secondary photoproducts, which are known as precursor molecules in the prebiotic synthesis of purines, amino acids and peptides.