A VUV Photoionization Study of the Combustion-Relevant Reaction of the Phenyl Radical (C₆H₅) with Propylene (C₃H₆) in a High Temperature Chemical Reactor

Abstract

We studied the reaction of phenyl radicals (C 6 H 5 ) with propylene (C 3 H 6 ) exploiting a high temperature chemical reactor under combustion-like conditions (300 Torr, 1,200-1,500 K). The reaction products were probed in a supersonic beam by utilizing tunable vacuum ultraviolet (VUV) radiation from the Advanced Light Source and recording the photoionization efficiency (PIE) curves at mass-to-charge ratios of m/z = 118 (C 9 H 10 + ) and m/z = 104 (C 8 H 8 + ). Our results suggest that the methyl and atomic hydrogen losses are the two major reaction pathways with branching ratios of 86 ± 10 % and 14 ± 10 %. The isomer distributions were probed by fitting the recorded PIE curves with a linear combination of the PIE curves of the individual C 9 H 10 and C 8 H 8 isomers. Styrene (C 6 H 5 C 2 H 3 ) was found to be the exclusive product contributing to m/z = 104 (C 8 H 8 + ), whereas 3-phenylpropene, cis-1-phenylpropene, and 2-phenylpropene with branching ratios of 96 ± 4 %, 3 ± 3 %, and 1 ± 1 % could account for signal at m/z = 118 (C 9 H 10 + ).Although searched for carefully, no evidence of the bicyclic indane molecule could be provided.The reaction mechanisms and branching ratios are explained in terms of electronic structure calculations nicely agreeing with a recent crossed molecular beam study on this system.

Reactivity of the Indenyl Radical (C9H7) with Acetylene (C2H2) and Vinylacetylene (C4H4)

“…The identification of the 1‐indenyl radical suggests that if the 2‐, 6‐, and 7‐indenyl radicals are formed via homolytic carbon‐bromine bond rupture, these radicals effectively isomerize within a few tens of microseconds, i. e. the residence time of the radicals within our chemical reactor . An alternative explanation for the non‐observation of the 2‐, 6‐, and 7‐indenyl radicals would be that they do not actually form in the first place from the bromoindenes used in the present experiment.…”

“…The experiments were conducted at the Chemical Dynamics Beamline (9.0.2) of the Advanced Light Source exploiting a resistively‐heated silicon‐carbide (SiC) chemical reactor interfaced to a molecular beam apparatus operated with a reflectron time‐of‐flight mass spectrometer (Re‐TOF‐MS) . First, we determine the nature of the indenyl radical isomer(s) formed (and potential isomerization processes); thereafter we study the reaction of the characterized indenyl radical(s) with acetylene (C 2 H 2 ) and vinylacetylene (C 4 H 4 ).…”

“…The quasi‐continuous tunable VUV light from the Advanced Light Source intersects the neutral molecular beam perpendicularly in the extraction region of a Wiley‐McLaren Re‐TOF‐MS. VUV single photon ionization represents basically a fragment‐free ionization technique and hence is considered as a soft ionization method compared to electron impact ionization . The ions generated in the photoionization process are extracted and fed onto a microchannel plate detector through an ion lens.…”

Vacuum Ultraviolet Photoionization of Complex Chemical Systems

“…This is nicely illustrated in the case of SiC2H2 where Figure 5 Since there has been controversy in the measurements of the IE of the silicon dimer and no direct measurements of the higher clusters, the IEs of Sin (n=1-7) using VUV-PIMS coupled to laser ablation of silicon in a supersonic molecular beam were recorded (90). The extracted AIEs obtained by fitting a FC simulation to the experimental PIE curves represented a significant 22 improvement in the literature values with error bars ranging from ±0.05 to 0.1 eV depending on cluster size. In the case of the silicon dimer, a feature in the PIE spectra, coupled to a FC simulation allowed for the observation of the ionization of the X 3 Ʃgneutral state to the a 2 Πu excited state of the dimer cation.…”

“…Pressures up to few hundred torr, timescales of microseconds, and temperatures up to 1500 C became accessible in a flow reactor (hot nozzle) coupled to molecular beams (17; 18). Photoionization mass spectrometry (PIMS) studies of the unimolecular decomposition pathways of acetaldehyde (17) , furan (19) , the benzyl radical (20), and cyclopentadienone (21) provide glimpses to processes relevant in renewable biofuel processing while the reactions of the phenyl radical with propyne (18), allene, propylene (22), 1,3-butadiene (23), acetylene (24), oxygen (25) and the reactions of benzyl (26), and napthyl (27) radicals with acetylene, shed light on molecular growth mechanisms relevant to soot formation. For instance, the phenyl radical with acetylene study (24) provided first experimental evidence of the HACA (hydrogen abstraction followed by acetylene addition) mechanism, postulated over three decades ago, to be operative leading to PAH formation (naphthalene) under combustion relevant conditions.…”