Identification of Dihydropentalenes as Products of the Molecular-Weight Growth Reaction of Cyclopentadienyl Plus Propargyl

The resonance-stabilized cyclopentadienyl (C₅H₅) and propargyl (C₃H₃) radicals are important precursors for polycyclic aromatic hydrocarbons (PAHs) and thus play a significant role in molecular-weight growth and soot formation processes under combustion conditions. In this work, we describe an experimental and theoretical investigation of the C₅H₅ + C₃H₃ reaction. Experimentally, we studied this reaction in a resistively heated microtubular SiC reactor at a controlled temperature of ∼1150 K and a pressure of 10–20 mbar. The reactants C₅H₅ and C₃H₃ were pyrolytically generated from anisole (C₆H₅OCH₃) and propargyl bromide (C₃H₃Br). We identified the reactants and the C₈H₈ products isomer-selectively utilizing photoion mass-selected threshold photoelectron spectroscopy (ms-TPES). The experimentally observed predominant formation of dihydropentalenes over the ring-enlargement reaction to styrene is consistent with our theoretical predictions of the kinetics on the newly calculated C₈H₈ potential energy surface. This work highlights dihydropentalenes as reactants in molecular-weight growth reactions and as potential building blocks in versatile routes for the formation of curved PAHs.