Can cyanide radicals drive molecular backbone growth on interstellar icy grains?

Motivated by the value of CN-bearing molecules as tracers of interstellar physical conditions, we investigate the reactions of adsorbed CN radicals with acetylene and ethylene (C2H2 and C2H4) on interstellar dust-grain analogues using quantum chemical calculations. We find that reactivity is strongly controlled by the relative orientation of the reactants, with specific geometries either promoting or inhibiting reaction. We further show that, on ice, these reactions differ qualitatively from their gas-phase counterparts, stalling at the formation of the adduct complexes C2H2CN and C2H4CN and exhibiting newly emerged kinetic barriers for the neutral-radical association. We contextualize our calculations in the same reaction-diffusion framework that would be employed in astrochemical models, finding that, depending on the diffusion energy of the hydrocarbons, these reactions can be either negligible or efficient, highlighting the importance of the local ice structure in interstellar grain chemistry. These findings caution against the use of CN-based tracers that assume barrierless, bimolecular surface reactions involving CN radicals.