Pyrolysis of Cyclopentadienone: Mechanistic Insights from a Direct Measurement of Product Branching Ratios

The thermal decomposition of cyclopentadienone (C₅H₄=O) has been studied in a flash pyrolysis continuous flow microreactor. Passing dilute samples of o-phenylene sulfite (C₆H₄O₂SO) in He through the microreactor at elevated temperatures yields a relatively clean source of C₅H₄=O. The pyrolysis of C₅H₄=O was investigated over the temperature range 1000-2000 K. Below 1600 K, we have identified two decomposition channels: (1) C₅H₄=O (+ M) → CO + HC≡C-CH=CH₂ and (2) C₅H₄=O (+ M) → CO + HC≡CH + HC≡CH. There is no evidence of radical or H atom chain reactions. To establish the thermochemistry for the pyrolysis of cyclopentadienone, ab initio electronic structure calculations (AE-CCSD(T)/aug-cc-pCVQZ//AE-CCSD(T)/cc-pVQZ and anharmonic FC-CCSD(T)/ANO1 ZPEs) were used to find Δ_fH₀(C₅H₄=O) to be 16 ± 1 kcal mol^-1 and Δ_fH₀(CH₂=CH-C≡CH) to be 71 ± 1 kcal mol^-1. The calculations predict the reaction enthalpies Δ_rxnH₀(1) to be 28 ± 1 kcal mol^-1 (Δ_rxnH₂₉₈(1) is 30 ± 1 kcal mol^-1) and Δ_rxnH₀(2) to be 66 ± 1 kcal mol^-1 (Δ_rxnH₂₉₈(2) is 69 ± 1 kcal mol^-1). Following pyrolysis of C₅H₄=O, photoionization mass spectrometry was used to measure the relative concentrations of HCC-CHCH₂ and HCCH. Reaction 1 dominates at low pyrolysis temperatures (1000-1400 K). At temperatures above 1400 K, reaction 2 becomes the dominant channel. We have used the product branching ratios over the temperature range 1000-1600 K to extract the ratios of unimolecular rate coefficients for reactions 1 and 2. If Arrhenius expressions are used, the difference of activation energies for reactions 1 and 2, E₂ - E₁, is found to be 16 ± 1 kcal mol^-1 and the ratio of the pre-exponential factors, A₂/A₁, is 7.0 ± 0.3.