A combined crossed beam and ab initio investigation on the reaction of carbon species with C4H6 isomers.: I.: The 1,3-butadiene molecule, H2CCHCHCH2(X1A′)

The reaction between ground state carbon atoms, C(P-3(j)), and 1,3-butadiene, H2CCHCHCH2, was studied at three averaged collision energies between 19.3 and 38.8 kJmol(-1) using the crossed molecular beam technique. Our experimental data combined with electronic structure calculations show that the carbon atom adds barrierlessly to the rr-orbital of the butadiene molecule via a loose, reactantlike transition state located at the centrifugal barrier. This process forms vinylcyclopropylidene which rotates in a plane almost perpendicular to the total angular momentum vector J around its C-axis. The initial collision complex undergoes ring opening to a long-lived vinyl-substituted triplet allene molecule. This complex shows three reaction pathways, Two distinct H atom loss channels form 1- and 3-vinylpropargyl radicals, HCCCHC2H3(X(2)A") and H2CCCC2H3(X(2)A"), through tight exit transition states located about 20 kJmol(-1) above the products; the branching ratio of 1- versus 3-vinylpropargyl radical is about 8:1. A minor channel of less than 10% is the formation of a vinyl, C2H3(X(2)A'), and propargyl radical C3H3((XB2)-B-2). The unambiguous identification of two C5H5 chain isomers under single collision has important implications to combustion processes and interstellar chemistry. Here, in denser media such as fuel flames and in circumstellar shells of carbon stars, the linear structures can undergo a collision-induced ring closure followed by a hydrogen migration to cyclic C5H5 isomers such as the cyclopentadienyl radical-a postulated intermediate in the formation of polycyclic aromatic hydrocarbons (PAHs). (C) 2000 American Institute of Physics. [S0021-9606(00)00338-X].