Formation of three C4H3N isomers from the reaction of CN (X2Σ+) with allene, H2CCCH2 (X1A1), and methylacetylene, CH3CCH (X1A1):: A combined crossed beam and ab initio study

Crossed molecular beam reactions of cyano radicals, CN (X(2)Sigma(+)), with two C3H4 isomers-methylacetylene, CH3CCH (X(1)A(1)), and allene. H2CCCH2 (X(1)A(1))-have been investigated at six collision energies between 13.4 and 36.7 kJ mol(-1) to elucidate the chemical reaction dynamics to form three C4H3N isomers-1-cyanomethylacetylene, CH3CCCN (X(1)A(1)), cyanoallene, H2CCCH(CN) (X(1)A'), and 3-cyanomethylacetylene, CH2(CN)CCH (X(1)A')-under single collision conditions. The forward-convolution fitting of the laboratory angular and time-of-flight distributions combined with ab initio calculations reveal that both reactions have no entrance barrier, proceed via indirect (complex-forming) reaction dynamics, and are initiated by addition of CN(X(2)Sigma(+)) to the pi electron density of the unsaturated hydrocarbon at the terminal carbon atom to form long-lived CH3CCH(CN) (methylacetylene reaction) and H2CCCH2(CN) (allene reaction) intermediates. Both complexes fragment via exit transition states located 8-19 0 mol-1 above the products to form 1-cyanomethylacetylene, CH3CCCN (X(1)A(1)), cyanoallene, H2CCCHCN (X(1)A'), and 3-cyanomethylacetylene, CH2(CN)CCH (X(1)A'). Because both reactions are barrierless and exothermic and the exit transition states lie below the energy of the separated reactants, all hydrogen-deficient nitriles identified in our investigation can be synthesized in the atmosphere of Saturn's moon Titan and in molecular clouds holding temperatures as low as 10 K.