Crossed beam reaction of cyano radicals with hydrocarbon molecules.: III.: Chemical dynamics of vinylcyanide (C2H3CN;X 1A') formation from reaction of CN(X 2Σ+) with ethylene, C2H4(X 1Ag)

The neutral-neutral reaction of the cyano radical, CN(X (2)Sigma (+)), with ethylene, C2H4(X (1)A(g)), has been performed in a crossed molecular beams setup at two collision energies of 15.3 and 21.0 kJ mol(-1) to investigate the chemical reaction dynamics to form vinylcyanide, C2H3CN(X (1)A') under single collision conditions. Time-of-flight spectra and the laboratory angular distributions of the C3H3N products have been recorded at mass-to-charge ratios 53-50. Forward-convolution fitting of the data combined with ab initio calculations show that the reaction has no entrance barrier, is indirect (complex forming reaction dynamics), and initiated by addition of CN(X (2)Sigma (+)) to the pi electron density of the olefin to give a long-lived CH2CH2CN intermediate. This collision complex fragments through a tight exit transition state located 16 kJ mol(-1) above the products via H atom elimination to vinylcyanide. In a second microchannel, CH2CH2CN undergoes a 1,2 H shift to form a CH3CHCN intermediate prior to a H atom emission via a loose exit transition state located only 3 kJ mol(-1) above the separated products. The experimentally observed mild "sideways scattering" at lower collision energy verifies the electronic structure calculations depicting a hydrogen atom loss in both exit transition states almost parallel to the total angular momentum vector J and nearly perpendicular to the C2H3CN molecular plane. Since the reaction has no entrance barrier, is exothermic, and all the involved transition states are located well below the energy of the separated reactants, the assignment of the vinylcyanide reaction product soundly implies that the title reaction can form vinylcyanide, C2H3CN, as observed in the atmosphere of Saturn's moon Titan and toward dark, molecular clouds holding temperatures as low as 10 K. In strong agreement with our theoretical calculations, the formation of the C2H3NC isomer was not observed. (C) 2000 American Institute of Physics. [S0021-9606(00)01036-9].