CROSSED-BEAM REACTION OF ATOMIC CARBON, C(P-3(J)), WITH THE PROPARGYLRADICAL, C3H3((XB2)-B-2) - OBSERVATION OF DIACETYLENE, C4H2(X-1-SIGMA(G)(+))

The reaction of ground-state carbon, C(P-3(j)), with the propargyl rad ical, C3H3((XB2)-B-2), is investigated at an average collision energy of 42.0 kJmol(-1) employing the crossed molecular beams technique and a universal mass spectrometric detector. The laboratory angular distri bution and time-of-flight spectra of the C4H2 product are recorded at m/e = 50. Forward-convolution fitting of our data reveals the formatio n of diacetylene, HCCCCH, in its X-1 Sigma(g)(+) electronic ground sta te. The reaction dynamics are governed by an initial attack of C(P-3(j )) to the pi-electron density at the acetylenic carbon atom of the pro pargyl radical, followed by a [1,2]-hydrogen migration to the n-C4H3 i somer. A final carbon-hydrogen bond rupture yields atomic hydrogen and diacetylene through a tight exit transition state located 30-60 kJmol (-1) above the products. This first successful crossed molecular beams study of a reaction between an atom and a free radical marks the begi nning of the next generation of crossed beams experiments elucidating the formation of molecular species in combustion processes, chemical v apor deposition, in the interstellar medium, outflows of carbon stars, and hydrocarbon-rich planetary atmospheres via radical-radical reacti ons. (C) 1997 American Institute of Physics. [S0021-9606(97)04144-5].