More π, please: What drives the formation of unsaturated molecules in the interstellar medium?†

Abstract

We present a computational investigation into the fragmentation pathways of ethanolamine (C₂H₇NO, EtA), propanol (C₃H₈O, PrO), butanenitrile (C₄H₇N, BuN), and glycolamide (C₂H₅NO₂, GlA)—saturated organic molecules detected in the interstellar medium (ISM), particularly in the molecular cloud complex Sagittarius B2 (Sgr B2) and its molecular cloud G+0.693-0.027. Using electron-impact ionization data and Born–Oppenheimer molecular dynamics simulations, we investigate how cosmic rays, cosmic-ray-induced UV fields, and shock-induced heating can induce the fragmentation of these molecules, resulting in the formation of unsaturated species with extended π-bond networks. Despite the attenuation of external UV radiation in G+0.693-0.027, these energetic processes are capable of driving partial transformations of saturated into unsaturated molecules, supporting the coexistence of species like EtA and GlA alongside unsaturated nitriles such as cyanoacetylene (HC₃N), cyanopropyne (CH₃C₃N), and cyanoallene (CH₂CCHCN). Our findings underscore the significance of high-energy mechanisms in enhancing chemical complexity within molecular clouds and offer insights into the pathways that govern the evolution of organic molecules in the ISM.