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

Abstract

Crossed molecular beam reactions of cyano radicals, CN (X2Σ+), with two C3H4 isomersmethylacetylene, CH3CCH (X1A1), and allene, H2CCCH2 (X1A1)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 isomers1-cyanomethylacetylene, CH3CCCN (X1A1), cyanoallene, H2CCCH(CN) (X1A‘), and 3-cyanomethylacetylene, CH2(CN)CCH (X1A‘)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(X2Σ+) to the π 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 kJ mol-1 above the products to form 1-cyanomethylacetylene, CH3CCCN (X1A1), cyanoallene, H2CCCHCN (X1A‘), and 3-cyanomethylacetylene, CH2(CN)CCH (X1A‘). 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.