A Barrierless Pathway Accessing the C(9)H(9) and C(9)H(8) Potential Energy Surfaces via the Elementary Reaction of Benzene with 1-Propynyl

The crossed molecular beams reactions of the 1-propynyl radical (CH(3)CC; X(2)A(1)) with benzene (C(6)H(6); X(1)A(1g)) and D6-benzene (C(6)D(6); X(1)A(1g)) were conducted to explore the formation of C(9)H(8) isomers under single-collision conditions. The underlying reaction mechanisms were unravelled through the combination of the experimental data with electronic structure and statistical RRKM calculations. These data suggest the formation of 1-phenyl-1-propyne (C(6)H(5)CCCH(3)) via the barrierless addition of 1-propynyl to benzene forming a low-lying doublet C(9)H(9) intermediate that dissociates by hydrogen atom emission via a tight transition state. In accordance with our experiments, RRKM calculations predict that the thermodynamically most stable isomer – the polycyclic aromatic hydrocarbon (PAH) indene – is not formed via this reaction. With all barriers lying below the energy of the reactants, this reaction is viable in the cold interstellar medium where several methyl-substituted molecules have been detected. Its underlying mechanism therefore advances our understanding of how methyl-substituted hydrocarbons can be formed under extreme conditions such as those found in the molecular cloud TMC-1. Implications for the chemistry of the 1-propynyl radical in astrophysical environments are also discussed.