title: Pyrolysis of Cyclopentadienone: Mechanistic Insights from a Direct Measurement of Product Branching Ratios.
abstract: 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 ΔfH0(C5H4═O) to be 16 ± 1 kcal mol(-1) and ΔfH0(CH2═CH-C≡CH) to be 71 ± 1 kcal mol(-1). The calculations predict the reaction enthalpies ΔrxnH0(1) to be 28 ± 1 kcal mol(-1) (ΔrxnH298(1) is 30 ± 1 kcal mol(-1)) and ΔrxnH0(2) to be 66 ± 1 kcal mol(-1) (ΔrxnH298(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.
authors.affiliation: †National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States.|‡Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States.
authors.forename: Thomas K
authors.initials: TK
authors.lastname: Ormond
authors.affiliation: §Combustion Research Facility, Sandia National Laboratory, PO Box 969, Livermore, California 94551-0969, United States.
authors.forename: Adam M
authors.initials: AM
authors.lastname: Scheer
authors.affiliation: †National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States.
authors.forename: Mark R
authors.initials: MR
authors.lastname: Nimlos
authors.affiliation: †National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States.
authors.forename: David J
authors.initials: DJ
authors.lastname: Robichaud
authors.affiliation: ∥Chemical Sciences Division, Lawrence Berkeley National Laboratories, Berkeley, California 94720, United States.
authors.forename: Tyler P
authors.initials: TP
authors.lastname: Troy
authors.affiliation: ∥Chemical Sciences Division, Lawrence Berkeley National Laboratories, Berkeley, California 94720, United States.
authors.forename: Musahid
authors.initials: M
authors.lastname: Ahmed
authors.affiliation: ⊥Center for Combustion and Environmental Research, Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427, United States.
authors.forename: John W
authors.initials: JW
authors.lastname: Daily
authors.affiliation: #Institute for Theoretical Chemistry, Department of Chemistry, University of Texas, Austin, Texas 78712, United States.
authors.forename: Thanh Lam
authors.initials: TL
authors.lastname: Nguyen
authors.affiliation: #Institute for Theoretical Chemistry, Department of Chemistry, University of Texas, Austin, Texas 78712, United States.
authors.forename: John F
authors.initials: JF
authors.lastname: Stanton
authors.affiliation: ‡Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States.
authors.forename: G Barney
authors.initials: GB
authors.lastname: Ellison