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

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 ang ular distributions of the C3H3N products have been recorded at mass-to-char ge ratios 53-50. Forward-convolution fitting of the data combined with ab i nitio calculations show that the reaction has no entrance barrier, is indir ect (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 ex it transition state located 16 kJ mol(-1) above the products via H atom eli mination 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 l oose exit transition state located only 3 kJ mol(-1) above the separated pr oducts. The experimentally observed mild "sideways scattering" at lower col lision energy verifies the electronic structure calculations depicting a hy drogen atom loss in both exit transition states almost parallel to the tota l angular momentum vector J and nearly perpendicular to the C2H3CN molecula r plane. Since the reaction has no entrance barrier, is exothermic, and all the involved transition states are located well below the energy of the se parated reactants, the assignment of the vinylcyanide reaction product soun dly implies that the title reaction can form vinylcyanide, C2H3CN, as obser ved in the atmosphere of Saturn's moon Titan and toward dark, molecular clo uds holding temperatures as low as 10 K. In strong agreement with our theor etical calculations, the formation of the C2H3NC isomer was not observed. ( C) 2000 American Institute of Physics. [S0021-9606(00)01036-9].