Formation of Three C4H3N Isomers from the Reaction of CN (X2Σ+) with Allene, H2CCCH2 (XA1), and Methylacetylene, CH3CCH (X1A1): A Combined Crossed Beam and Ab Initio Study

Crossed molecular beam reactions of cyano radicals, CN (X 2 Σ + ), with two C 3 H 4 isomers methylacetylene, CH 3 CCH (X 1 A 1 ), and allene, H 2 CCCH 2 (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 C 4 H 3 N isomers 1-cyanomethylacetylene, CH 3 CCCN (X 1 A 1 ), cyanoallene, H 2 CCCH(CN) (X 1 A‘), and 3-cyanomethylacetylene, CH 2 (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 Σ + ) to the π electron density of the unsaturated hydrocarbon at the terminal carbon atom to form long-lived CH 3 CCH(CN) (methylacetylene reaction) and H 2 CCCH 2 (CN) (allene reaction) intermediates. Both complexes fragment via exit transition states located 8−19 kJ mol -1 above the products to form 1-cyanomethylacetylene, CH 3 CCCN (X 1 A 1 ), cyanoallene, H 2 CCCHCN (X 1 A‘), and 3-cyanomethylacetylene, CH 2 (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.