Isomer-Resolved High-Temperature Chemistry of the Molecular-Weight Growth Reaction of Phenyl Plus Propargyl

Radical-radical reactions play a crucial role in the molecular-weight growth that leads to the formation of polycyclic aromatic hydrocarbons (PAHs) and ultimately soot. In this study, we experimentally investigated the reaction between C₆H₅ (phenyl) and C₃H₃ (propargyl) at pressures around 30 Torr and combustion-relevant temperatures (∼1200 K). The reactants were generated through flash pyrolysis of nitrosobenzene and propargyl bromide in a resistively heated SiC tube. We identified the reaction intermediates and products using mass-selected threshold photoelectron spectroscopy (ms-TPES) with the photoelectron-photoion coincidence (PEPICO) instrument at the vacuum-ultraviolet (VUV) beamline of the Swiss Light Source at the Paul Scherrer Institute. Our findings indicate that C₆H₅ associates with C₃H₃ to form C₉H₈, which partially decomposes via hydrogen loss to yield C₉H₇ radicals. The C₆H₅ + C₃H₃ reaction is complex and yields more than just the most stable indene isomer. The experimental threshold photoelectron spectrum provides clear spectroscopic evidence of five isomers: indene, phenylallene, 1-phenyl-1-propyne, 1-phenyl-3-propyne, and cycloprop-2-en-1-ylbenzene. The inclusion of a sixth isomer, 3aH-indene, provides an even better fit to the experimental spectrum, although its presence should not be considered conclusive. All of these species correspond to minima on the known C₉H₈ potential energy surface [Selby et al., J. Phys. Chem. A, 2023, 127(11), 2577-2590 and Morozov et al., Phys. Chem. Chem. Phys. 2020, 22(13), 6868-6880]. Many of these isomers are not included in kinetic mechanisms that seek to describe the chemical pathways leading to PAHs.