Theoretical investigation of the mechanism and product branching ratios of the reactions of cyano radical with 1- and 2-butyne and 1,2-butadiene.

Ab initio CCSD(T)/cc-pVTZ(CBS)//B3LYP/6-311g(d,p) calculations of the C(5)H(6)N potential energy surface have been performed to investigate the reaction mechanism of cyano radical (CN) with C(4)H(6) isomers 1- and 2-butyne and 1,2-butadiene. They were followed by RRKM calculations of the reaction rate constants and product branching ratios under single-collision conditions in the 0-5 kcal/mol collision energy range. With the assumption of equal probabilities of the barrierless terminal and central addition of the cyano radical to 1-butyne, 2-cyano-1,3-butadiene + H, and cyanoallene + CH(3) are predicted to be the major reaction products with a branching ratio of ∼2:1. The terminal CN addition to C(1) favors the formation of cyanoallene + CH(3), whereas the central CN addition to C(2) enhances the formation of 2-cyano-1,3-butadiene + H. For the CN + 2-butyne reaction, the dominant product is calculated to be 1-cyano-prop-1-yne + CH(3), and the CH(3) loss occurs directly from the initial adduct formed by the barrierless CN addition to either of the two acetylenic carbon atoms. A small amount of the H loss product, 3-cyano-1,2-butadiene (1-cyano-1-methylallene), can be also formed as was observed in earlier crossed molecular beam experiments. Three different products are predicted for the CN + 1,2-butadiene reaction, which also occurs without entrance barriers. If various initial complexes formed by the CN addition to C(1), C(2), C(3), or to the C═C double bonds in 1,2-butadiene are produced in the entrance channel with equal probabilities, the dominating product (70-60%) is 2-cyano-1,3-butadiene + H, and the other significant products include 1-cyano-prop-3-yne + CH(3) (19-25%) favored by the initial CN addition to C(1) and cyanoallene + CH(3) (11-15%) preferred for the CN addition to C(3). The H abstraction HCN + C(4)H(5) products may also be formed either from the initial CN addition adducts through a CN roaming mechanism or via certain trajectories directly from the initial reactants, but their yield is not expected to be significant, at least at low temperatures. The energetics, mechanisms, and product branching ratios of the cyano radical reactions with various C(4)H(6) isomers and their analogous isoelectronic C(2)H + C(4)H(6) reactions have been summarized and compared.