Singlet Oxygen Insertion into Hydrocarbons: The Role of First- and Second-Generation Pathways in Astronomically Relevant Ices

Abstract

Complex organic molecules are widespread in different areas of the interstellar medium, including cold areas such as molecular clouds where chemical reactions occur in ice. The underlying molecular mechanisms responsible for the observed rich chemistry are still not understood. O(1D) atom reactions provide a pathway for chemical complexity even in cold areas, as the reactions are typically barrierless, and O(1D) is a photofragmented product of astronomically relevant ices such as CO2. In this work, we use quantum chemistry methods to model reactions in astronomical ices containing oxygen with small C1 and C2 hydrocarbons in the presence of UV radiation. Our results demonstrate that the underlying molecular mechanism of reactions in ice includes the oxygen insertion reaction (first-generation reactions), photofragmentation of products and radical recombination reactions (second-generation reactions). The mechanism explains the formation of formaldehyde in methane ice, acetaldehyde in ethane ice, CO in acetylene ice and the consumption of alcohol in all systems. This work demonstrates the important role of first- and second-generation reactions in the unique chemical processes in astronomical ices; where basic molecular building blocks are fragmented and recombined into new molecules resulting in enhanced chemical complexity.