Toward Unraveling Cyanopolyyne Surface Chemistry: A Preview on Isolated Systems from HC₃N to Ethyl Cyanide and Propylamine

Cyanopolyynes, a family of nitrogen-containing carbon chains, are common in the interstellar medium and possibly form the backbone of species relevant to prebiotic chemistry. Following their gas-phase formation, they are expected to freeze out on ice grains in cold interstellar regions. In this work, we present the hydrogenation reaction network of isolated HC₃N, the smallest cyanopolyyne, that consists of over-a-barrier radical-neutral reactions and barrierless radical-radical reactions. We employ density functional theory, coupled cluster, and multiconfigurational methods to obtain activation and reaction energies for the hydrogenation network of HC₃N. This work explores the reaction network of the isolated molecule and constitutes a preview of the reactions occurring on the ice grain surface. We find that the reactions where the hydrogen atom adds to the carbon chain at the carbon atom opposite of the cyano group give the lowest and narrowest barriers. Subsequent hydrogenation leads to the astrochemically relevant vinyl cyanide and ethyl cyanide. Alternatively, the cyano group can hydrogenate via radical-radical reactions, leading to the fully saturated propylamine. These results can be extrapolated to give insight into the general reactivity of carbon chains on interstellar ices.