Isomer-Specific Branching Ratios in the Formation of Cyanopropene (C ₃ H ₅ CN) through the C ₃ H ₆ + CN Reaction under Interstellar Conditions

We investigated the reaction of propene (C₃H₆) with the cyano radical (CN) in light of the recent detection of five cyanopropene isomers in TMC-1. To provide reliable branching ratios, we characterized the stationary points on the potential energy surface using the high-accuracy jun-ChS-F12 method. The resulting energetics were then employed to derive temperature-dependent rate constants. Our calculations show that the formation of all observed cyano derivatives is feasible through this reaction, although it is secondary compared to the dominant formation channel of vinyl cyanide (C₂H₃CN). The predicted branching ratios are in good agreement with the observations, with the discrepancies prompting further investigation on the destruction mechanisms of the different isomers. Overall, this work supports a scenario in which these cyano derivatives in TMC-1 arise primarily from pure gas-phase chemistry.