title: Identification and chemistry of C4H3 and C4H5 isomers in fuel-rich flames.
abstract: Quantitative identification of isomers of hydrocarbon radicals in flames is critical to understanding soot formation. Isomers of C4H3 and C4H5 in flames fueled by allene, propyne, cyclopentene, or benzene are identified by comparison of the observed photoionization efficiencies with theoretical simulations based on calculated ionization energies and Franck-Condon factors. The experiments combine molecular-beam mass spectrometry (MBMS) with photoionization by tunable vacuum-ultraviolet synchrotron radiation. The theoretical simulations employ the rovibrational properties obtained with B3LYP/6-311++G(d,p) density functional theory and electronic energies obtained from QCISD(T) ab initio calculations extrapolated to the complete basis set limit. For C4H3, the comparisons reveal the presence of the resonantly stabilized CH2CCCH isomer (i-C4H3). For C4H5, contributions from the CH2CHCCH2 (i-C4H5) and some combination of the CH3CCCH2 and CH3CHCCH isomers are evident. Quantitative concentration estimates for these species are made for allene, cyclopentene, and benzene flames. Because of low Franck-Condon factors, sensitivity to n-isomers of both C4H3 and C4H5 is limited. Adiabatic ionization energies, as obtained from fits of the theoretical predictions to the experimental photoionization efficiency curves, are within the error bars of the QCISD(T) calculations. For i-C4H3 and i-C4H5, these fitted adiabatic ionization energies are (8.06 +/- 0.05) eV and (7.60 +/- 0.05) eV, respectively. The good agreement between the fitted and theoretical ionization thresholds suggests that the corresponding theoretically predicted radical heats of formation (119.1, 76.3, 78.7, and 79.1 kcal/mol at 0 K for i-C4H3, i-C4H5, CH3CCCH2, and CH3CHCCH, respectively) are also quite accurate.
authors.affiliation: Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551, USA. nhansen@sandia.gov
authors.forename: Nils
authors.initials: N
authors.lastname: Hansen
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Klippenstein
authors.forename: Craig A
authors.initials: CA
authors.lastname: Taatjes
authors.forename: James A
authors.initials: JA
authors.lastname: Miller
authors.forename: Juan
authors.initials: J
authors.lastname: Wang
authors.forename: Terrill A
authors.initials: TA
authors.lastname: Cool
authors.forename: Bin
authors.initials: B
authors.lastname: Yang
authors.forename: Rui
authors.initials: R
authors.lastname: Yang
authors.forename: Lixia
authors.initials: L
authors.lastname: Wei
authors.forename: Chaoqun
authors.initials: C
authors.lastname: Huang
authors.forename: Jing
authors.initials: J
authors.lastname: Wang
authors.forename: Fei
authors.initials: F
authors.lastname: Qi
authors.forename: Matthew E
authors.initials: ME
authors.lastname: Law
authors.forename: Phillip R
authors.initials: PR
authors.lastname: Westmoreland
mesh_terms: D017554:Carbon Compounds, Inorganic / Q000737:chemistry*; D055598:Chemical Phenomena; D002621:Chemistry; D005740:Gases / Q000378:metabolism; D007536:Isomerism; D013058:Mass Spectrometry; D015394:Molecular Structure; D011522:Protons; D011725:Pyridines / Q000737:chemistry*; D053260:Soot / Q000737:chemistry*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
chemical_list: D017554:Carbon Compounds, Inorganic; D005740:Gases; D011522:Protons; D011725:Pyridines; D053260:Soot