The Formation of Imines and Nitriles during VUV Photoirradiation of NH3:C2H x Ice Mixtures

Abstract

Nitriles are key reactants in prebiotic synthesis networks of RNA bases and amino acids. The detection of CH3CN and other complex nitriles in planet-forming disks suggests that such molecules are regularly delivered to nascent planets, increasing the likelihood of origins of life outside of Earth. In this paper, we investigate the formation of CH3CN and the closely related imines from the vacuum ultraviolet irradiation of NH3:C2H6, NH3:C2H4, and NH3:C2H2 ice mixtures at 10–50 K. CH3CN is formed in a subset of these experiments, with the highest yield of ∼5% with respect to the initial NH3 abundance achieved at the lowest ice temperatures for the least saturated hydrocarbon ice mixture. We find that the imine CH3CH=NH serves as an intermediate for the production of CH3CN in all ices and its yield generally appears higher than that of CH3CN. If the investigated ice chemistry is an important formation pathway of nitriles, we should observe CH3CH=NH > CH3CN. The opposite is true toward the Galactic Center, while no published constraints on CH3CH=NH exist in disks. Such constraints are needed to distinguish between the formation pathway presented in this work and other possible gas and ice nitrile formation pathways in different astrophysical environments. In the meantime, we conclude that NH3:hydrocarbon ice photochemistry is an excellent candidate for efficient low-temperature interstellar imine production.