The Formation of C5H5Isomers as Potential Key Intermediates to Polycyclic Aromatic Hydrocarbon–like Molecules

The reactions of ground-state atomic carbon with three C 4 H 6 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 C 5 H 5 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( 3 P j ) versus hydrogen atom exchange to form the 1-methylbutatrienyl radical, H 2 CCCCCH 3 ( X 2 A ''), as well as 1- and 3-vinylpropargyl radicals, HCCCHC 2 H 3 ( X 2 A '') and H 2 CCCC 2 H 3 ( X 2 A ''), under single-collision conditions. The methyl (CH 3 ) and vinyl radical (C 2 H 3 ) loss channels to yield the propargyl radical C 3 H 3 ( X 2 B 2 ) and n -C 4 H 3 ( 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 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 C 4 H 6 isomers a compelling candidate to form C 5 H 5 isomers in the outflow of AGB stars.