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

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• 作者：Thomas K. Ormond ; Adam M. Scheer ; Mark R. Nimlos ; David J. Robichaud ; Tyler P. Troy ; Musahid Ahmed ; John W. Daily ; Thanh Lam Nguyen ; John F. Stanton ; G. Barney Ellison
• 刊名：Journal of Physical Chemistry A
• 出版年：2015
• 出版时间：July 16, 2015
• 年：2015
• 卷：119
• 期：28
• 页码：7222-7234
• 全文大小：665K
• ISSN：1520-5215

文摘

The thermal decomposition of cyclopentadienone (C₅H₄鈺怬) 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鈺怬. The pyrolysis of C₅H₄鈺怬 was investigated over the temperature range 1000鈥?000 K. Below 1600 K, we have identified two decomposition channels: (1) C₅H₄鈺怬 (+ M) 鈫?CO + HC鈮鈥擟H鈺怌H₂ and (2) C₅H₄鈺怬 (+ M) 鈫?CO + HC鈮H + HC鈮H. 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 螖_fH₀(C₅H₄鈺怬) to be 16 卤 1 kcal mol^鈥? and 螖_fH₀(CH₂鈺怌H鈥擟鈮H) to be 71 卤 1 kcal mol^鈥?. The calculations predict the reaction enthalpies 螖_rxnH₀(1) to be 28 卤 1 kcal mol^鈥? (螖_rxnH₂₉₈(1) is 30 卤 1 kcal mol^鈥?) and 螖_rxnH₀(2) to be 66 卤 1 kcal mol^鈥? (螖_rxnH₂₉₈(2) is 69 卤 1 kcal mol^鈥?). Following pyrolysis of C₅H₄鈺怬, photoionization mass spectrometry was used to measure the relative concentrations of HCC鈥擟HCH₂ and HCCH. Reaction 1 dominates at low pyrolysis temperatures (1000鈥?400 K). At temperatures above 1400 K, reaction 2 becomes the dominant channel. We have used the product branching ratios over the temperature range 1000鈥?600 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 鈥?E₁, is found to be 16 卤 1 kcal mol^鈥? and the ratio of the pre-exponential factors, A₂/A₁, is 7.0 卤 0.3.