Crossed beam reaction of cyano radicals with hydrocarbon molecules.: IV.: Chemical dynamics of cyanoacetylene (HCCCN; X 1Σ+) formation from reaction of CN(X 2Σ+) with acetylene, C2H2(X 1Σg+)

The chemical reaction dynamics to form cyanoacetylene, HCCCN (X (1)Sigma (+)), via the radical-neutral reaction of cyano radicals, CN(X (2)Sigma (+); nu =0), with acetylene, C2H2(X (1)Sigma (+)(g)), are unraveled in crossed molecular beam experiments at two collision energies of 21.1 and 27.0 kJ mol(-1). Laboratory angular distributions and time-of-flight spectra of the HCCCN product are recorded at m/e=51 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 pi electron density of the acetylene molecule to form a doublet cis/trans HCCHCN collision complex on the (2)A' 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(-1) above the cyanoacetylene, HCCCN (X (1)Sigma (+)) and H(S-2(1/2)) products (microchannel 1). To a minor amount (15%) trans HCCHCN shows a 1,2-H shift via a 177 kJ mol(-1) barrier to form a doublet H2CCCN radical, which is 46 kJ mol(-1) more stable than the initial reaction intermediate (microchannel 2). The H2CCCN complex decomposes via a rather loose exit transition state situated only 7 kJ mol(-1) above the reaction products HCCCN (X (1)Sigma (+)) and H(S-2(1/2)). In both cases the geometry of the exit transition states is reflected in the observed center-of-mass angular distributions showing a mild forward/sideways peaking. The explicit identification of the cyanoacetylene as the only reaction product represents a solid background for the title reaction to be included in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan. (C) 2000 American Institute of Physics. [S0021-9606(00)01136-3].