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 Δf H 0(C5H4O) to be 16 ± 1 kcal mol–1 and Δf H 0(CH2CHCCH) to be 71 ± 1 kcal mol–1. The calculations predict the reaction enthalpies Δrxn H 0(1) to be 28 ± 1 kcal mol–1 (Δrxn H 298(1) is 30 ± 1 kcal mol–1) and Δrxn H 0(2) to be 66 ± 1 kcal mol–1 (Δrxn H 298(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 dominates at low pyrolysis temperatures (1000–1400 K). At temperatures above 1400 K, reaction 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 and . If Arrhenius expressions are used, the difference of activation energies for reactions and , E 2 – E 1, is found to be 16 ± 1 kcal mol–1 and the ratio of the pre-exponential factors, A 2/A 1, is 7.0 ± 0.3.