Less-Dominant Resonance Configuration of Propargyl Radical Leads to a Growth Mechanism for Polycyclic Aromatic Hydrocarbons that Preserves the Cyclopenta Ring

Understanding the growth of polycyclic aromatic hydrocarbons (PAHs) is essential for combustion, astrochemistry, and carbon-based nanomaterial synthesis. This study presents theory-guided experiments on radical-radical combination reactions of propargyl (^•C₃H₃). The addition of ^•C₃H₃ to three cyclopenta-fused PAH radicals─1-indenyl (^•1-C₉H₇), acenaphthenyl (^•C₁₂H₉), and 4<i>H</i>-cyclopenta[<i>def</i>]phenanthrenyl (^•C₁₅H₉)─revealed that the reaction between the dominant propyne-3-yl resonance configuration of ^•C₃H₃ and the three radicals consistently produces PAHs with all hexagonal rings, while the reaction between the less dominant allene-1-yl resonance configuration of ^•C₃H₃ and the three radicals selectively preserves the cyclopenta ring and forms a new hexagonal ring. Elusive intermediates and isomeric products were observed and identified by combining molecular beam-sampling synchrotron photoionization mass spectrometry with gas chromatography-mass spectrometry. The complementary results suggest a high selectivity of the allene-1-yl addition pathway, which is thermodynamically controlled. The findings presented here are based on a combination of experimental capabilities, and they provide new mechanisms and insights into the selective formation of bowl-shaped PAHs, serving as templates for fullerene and nanotube structures. The high selectivity of the allene-1-yl pathway provides a rational synthetic strategy for cyclopenta-fused PAHs, bearing barrierless and facile radical-radical reaction pathways in various environments, including high-temperature combustion, circumstellar envelopes, and cold molecular clouds.