Formation of a Resonance-Stabilized Radical Intermediate by Hydroxyl Radical Addition to Cyclopentadiene

The reaction of the OH radical with cyclopentadiene (C₅H₆) was investigated at room temperature using multiplexed photoionization mass spectrometry. OH radicals in their ground electronic state were generated in the gas phase by 248 nm photolysis of H₂O₂ or 351 nm photolysis of HONO. Analysis of photoion spectra and temporal profiles reveal that at room temperature and over the 4-8 Torr pressure range, the resonance-stabilized 5-hydroxycyclopent-2-en-1-yl (C₅H₆OH) is the main observed reaction product. Abstraction products (C₅H₅) were not detected. The C₅H₆OH potential energy surface calculated at the CCSD(T)/cc-pVTZ//M06-2X/6-311++G** level of theory suggests that the resonance-stabilized radical product is formed through barrierless addition of the OH radical onto cyclopentadiene's π system to form a van der Waals complex. This weakly bound adduct isomerizes through a submerged energy barrier to the resonance-stabilized addition adduct. Master Equation calculations, including two OH-addition entrance pathways, predict that 5-hydroxycyclopent-2-en-1-yl remains the sole addition product up to 500 K. The detection of an OH-containing resonance-stabilized radical at room temperature further highlights their importance in carbon- and oxygen-rich environments such as combustion, planetary atmospheres, and the interstellar medium.