The Formation of C5H5 Isomers as Potential Key Intermediates 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(3Pj) versus hydrogen atom exchange to form the 1-methylbutatrienyl radical, H2CCCCCH3 (X2A''), as well as 1- and 3-vinylpropargyl radicals, HCCCHC2H3 (X2A'') and H2CCCC2H3 (X2A''), under single-collision conditions. The methyl (CH3) and vinyl radical (C2H3) loss channels to yield the propargyl radical C3H3 (X2B2) and n-C4H3 (X2A') are only minor pathways. All reactions have no entrance barrier and are dominated by an addition of the carbon atom to the π 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.