Kinetics of the Self Reaction of Cyclopentadienyl Radicals

The kinetics of the self-reaction of cyclopentadienyl radicals (c-C₅H₅) was studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants were obtained in direct real-time experiments in the temperature region 304–600 K and at bath gas densities of (3.00–12.0) × 10¹⁶ molecules cm^–3. The room-temperature value of the rate constant, (3.98 ± 0.41) × 10^–10 cm³ molecule^–1 s^–1, is significantly higher than the rate constants for most hydrocarbon radical–radical reactions and coincides with the estimated collision rate. The observed overall c-C₅H₅ + c-C₅H₅ rate constant demonstrates an unprecedented strong negative temperature dependence: <i>k</i>₁ = 2.9 × 10^–12 exp­(+1489 K/T) cm³ molecule^–1 s^–1, with estimated uncertainty increasing with temperature, from 13% at 304 to 32% at 600 K. Formation of C₁₀H₁₀ as the primary product of cyclopentadienyl self-reaction was observed. In additional experiments performed at the temperature of 800 K, formation of C₁₀H₁₀, C₁₀H₉, and C₁₀H₈ was observed. Final product analysis by gas chromatography/mass spectrometry detected two isomers of C₁₀H₈ at 800 K: naphthalene (major) and azulene (minor)