Crossed beam reaction of cyano radicals with hydrocarbon molecules . IV . Chemical dynamics of cyanoacetylene „ HCCCN ; X 1 S

The chemical reaction dynamics to form cyanoacetylene, HCCCN (X S), via the radical–neutral reaction of cyano radicals, CN(X S;n50), with acetylene, C2H2(X Sg ), are unraveled in crossed molecular beam experiments at two collision energies of 21.1 and 27.0 kJ mol. Laboratory angular distributions and time-of-flight spectra of the HCCCN product are recorded at m/e551 and 50. Experiments were supplemented by electronic structure calculations on the doublet C3H2N potential energy surface and RRKM investigations. Forward-convolution fitting of the crossed beam data combined with our theoretical investigations shows that the reaction has no entrance barrier and is initiated by an attack of the CN radical to the p electron density of the acetylene molecule to form a doublet cis/trans HCCHCN collision complex on the A8 surface via indirect reactive scattering dynamics. Here 85% of the collision complexes undergo C–H bond rupture through a tight transition state located 22 kJ mol above the cyanoacetylene, HCCCN (X S) and H(S1/2) products ~microchannel 1!. To a minor amount ~15%! trans HCCHCN shows a 1,2-H shift via a 177 kJ mol barrier to form a doublet H2CCCN radical, which is 46 kJ mol 21