A combined ab initio and photoionization mass spectrometric study of polyynes in fuel-rich flames

Abstract

Polyynic structures in fuel-rich low-pressure flames are observed using VUV photoionization molecular-beam mass spectrometry. High-level ab initio calculations of ionization energies for C2nH2 (n=1-5) and partially hydrogenated CnH4 (n=7-8) polyynes are compared with photoionization efficiency measurements in flames fuelled by allene, propyne, and cyclopentene. C2nH2 (n=1-5) intermediates are unambiguously identified, while HC[triple bond, length as m-dash]C-C[triple bond, length as m-dash]C-CH=C=CH2, HC[triple bond, length as m-dash]C-C[triple bond, length as m-dash]C-C[triple bond, length as m-dash]C-CH=CH2 (vinyltriacetylene) and HC[triple bond, length as m-dash]C-C[triple bond, length as m-dash]C-CH[double bond, length as m-dash]CH-C[triple bond, length as m-dash]CH are likely to contribute to the C7H4 and C8H4 signals. Mole fraction profiles as a function of distance from the burner are presented. C7H4 and C8H4 isomers are likely to be formed by reactions of C2H and C4H radicals but other plausible formation pathways are also discussed. Heats of formation and ionization energies of several combustion intermediates have been determined for the first time.

Chemically activated reactions on the C7H5 energy surface: propargyl + diacetylene, i-C5H3 + acetylene, and n-C5H3 + acetylene

“…In a sooting flame these radicals should isomerize relatively rapidly to form the fulvenallenyl radical, or react further with unsaturated hydrocarbons in molecular weight growth processes. As temperature increases the activated adduct has sufficient energy to isomerize to fulvenallenyl (9) Diactylene has been detected at peak mole fractions of around 0.001 to 0.003 in fuel-rich flames of unsaturated hydrocarbons, typically corresponding to around an order of magnitude less than the acetylene concentration, 2 and of similar magnitude to accepted concentrations for propargyl. The reaction of propargyl with diacetylene should proceed with a significant flux relative to the previously considered propargyl + acetylene reaction, given that this latter reaction proceeds at a similar (if not slower) rate.…”

“…They are known to occur in the interstellar medium (ISM) 1 and in flames, 2,3 and may also play a role in the atmospheric chemistry of Saturn's moon Titan. 4 Common among these disparate environments is the presence of polycyclic aromatic hydrocarbons (PAHs).…”

Detection and Identification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether

“…Nevertheless, it should be kept in mind that identification and separation of isomeric species, in this case performic and carbonic acid, can be challenging when the isomers' respective photoionization efficiency curves are unknown. 66,69,76 …”

Experimental and numerical methods for studying the flame structure

“…For this, laser measurements have been coupled with different variants of in situ mass spectrometry to provide quantitative concentrations of major species and intermediates [22,24,46]. Particularly useful is the possibility to separate isomers with the VUV-PI-MBMS technique [22]; also, small aromatic compounds can be sensitively detected using REMPI-MBMS [46].…”

“…This technique has been used, for example, in the analysis of the chemistry of fuel-rich hydrocarbon flames [22][23][24] and different classes of oxygencontaining fuels, including alcohols, ethers, aldehydes, and esters [25][26][27][28][29][30]. It has been instrumental in the characterization of blended hydrocarbon/oxygenate flames [31,32] and most recently in an investigation of the combustion of morpholine [33], a compound with both ether and amine functional groups.…”