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 CH _3 CN 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 CH _3 CN and the closely related imines from the vacuum ultraviolet irradiation of NH _3 :C _2 H _6 , NH _3 :C _2 H _4 , and NH _3 :C _2 H _2 ice mixtures at 10–50 K. CH _3 CN is formed in a subset of these experiments, with the highest yield of ∼5% with respect to the initial NH _3 abundance achieved at the lowest ice temperatures for the least saturated hydrocarbon ice mixture. We find that the imine CH _3 CH=NH serves as an intermediate for the production of CH _3 CN in all ices and its yield generally appears higher than that of CH _3 CN. If the investigated ice chemistry is an important formation pathway of nitriles, we should observe CH _3 CH=NH > CH _3 CN. The opposite is true toward the Galactic Center, while no published constraints on CH _3 CH=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 NH _3 :hydrocarbon ice photochemistry is an excellent candidate for efficient low-temperature interstellar imine production.

Keywords

• Astrochemistry
• Interstellar molecules
• Laboratory astrophysics
• Molecule formation
• Planet formation
• Pre-biotic astrochemistry