The formation of isomers as potential key intermediates C5H5 to polycyclic aromatic hydrocarbon-like molecules

The reactions of ground-state atomic carbon with three C4H6 isomers, dimethylacetylene, 1,2- butadiene, and 1,3-butadiene, are studied at relative collision energies between 19.3 and 48.6 kJ mol(-1) in crossed molecular beam experiments to elucidate the reaction products and chemical dynamics to form C5H5 isomers-possible precursor radicals to PAH-like species-in the circumstellar envelopes of carbon-rich asymptotic giant branch (AGB) stars. Our combined experimental and computational studies show that reactions are dominated by the C(P-3(j)) versus hydrogen atom exchange to form the 1-methylbutatrienyl radical, H2CCCCCH3 (X(2)A"), as well as 1- and 3-vinylpropargyl radicals, HCCCHC2H3 (X(2)A") and H2CCCC2H3 (X(2)A"), under single-collision conditions. The methyl (CH3) and vinyl radical (C2H3) loss channels to yield the propargyl radical C3H3 ((XB2)-B-2) and n-C4H3 (X(2)A') are only minor pathways. All reactions have no entrance barrier and are dominated by an addition of the carbon atom to the pi electron density of the second reactant via a complex-forming reaction mechanism on the triplet potential energy surface (PES). The decomposing complexes are long lived and reside in a deep potential energy well bound by 360-410 kJ mol(-1) with respect to the separated reactants. Tight exit transition states of about 20 kJ mol(-1) above the products and strong exothermic reactions of 180-240 kJ mol(-1) are common features of the involved PESs. These characteristics make the reactions of atomic carbon with C4H6 isomers a compelling candidate to form C5H5 isomers in the outflow of AGB stars.