Photochemical and Discharge-Driven Pathways to Aromatic Products from 1,3-Butadiene

Abstract

A detailed study of the photochemical and discharge-driven pathways taken by gas-phase 1,3-butadiene has been carried out. Photolysis or discharge excitation was initiated inside a short reaction tube attached to the outlet of a pulsed valve. Bath gas temperatures near 100 K were achieved in the reaction tube by the constrained expansion of the gas mixture into the tube, simulating temperatures of relevance in Titan's atmosphere. Photolysis of 1,3-butadiene was initiated at 218 nm with a laser pulse that counter-propagated the reaction tube. Discharge excitation was carried out using discharge electrodes imbedded in the reaction tube walls, enabling the study of the photochemical and discharge products under similar conditions. Products were detected using either single-photon VUV photoionization (118 nm = 10.5 eV) or resonant two-photon ionization (R(2)PI) spectroscopy in a time-of-flight mass spectrometer. Emphasis was placed on characterization of the aromatic products formed, since these may be of particular relevance to Titan's atmosphere, where benzene has been positively identified and 1,3-butadiene is projected as the principle pathway to its formation. Consistent with previous studies of the photodissociation of 1,3-butadiene, C(3)H(3) + CH(3) is the dominant primary product formed. Under the temperature-pressure conditions present in the reaction tube (T approximately 75-100 K, P = 50 mbar), C(6)H(6) is the dominant secondary photochemical product formed. A 1:1 C(4)H(6):C(4)D(6) mixture was used to prove that the C(6)H(6) product was formed by recombination of two C(3)H(3) radicals; however, a careful search for benzene revealed none, indicating that less than 1% of the C(6)H(6) formed in the reaction tube is benzene. This is consistent with expectations for these temperatures and pressures based on previous modeling of propargyl recombination. Two aromatic products were observed from the photochemistry: ethylbenzene and 3-phenylpropyne. Plausible pathways leading to these products are proposed. In the discharge, C(3)H(3) + CH(3) are also identified as significant primary neutral products and C(6)H(6) as a dominant higher-mass product. In this case, the C(6)H(6) was identified as benzene via its R2PI spectrum, appearing with intensity about 10 times larger than any other aromatic formed in the discharge. R2PI spectra of a total of about 15 aromatic products were recorded from the 1,3-butadiene discharge, among them toluene; styrene; phenylacetylene; o-, m-, and p-xylene; ethylbenzene; indane; indene; beta-methylstyrene; and naphthalene. Previously unidentified spectra in the m/z 142 and 144 mass channels were positively identified as the 1,3- and 1,4-isomers of phenylcyclopentadiene and the analogous 1-phenylcyclopentene.

Formation of fulvene in the reaction of C2H with 1,3-butadiene

“…In combustion environments, propargyl (C 3 H 3 ) recombination is generally thought to be one of the main benzene formation pathways [4,[11][12][13]. However, several studies have shown that benzene is a minor, or non-existent, product of propargyl recombination at low temperatures and pressures [12,14]. The C 3 H 3 + C 3 H 3 route to C 6 H 6 can only proceed by relatively inefficient radiative association in the very low pressures of astrochemical environments and so alternative mechanisms must be invoked.…”

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

“…24 The aromatic, bicyclic indane molecule as well as its α-methylstyrene (2-phenylpropene) and 1-phenylpropene isomers are considered as important reaction intermediates and toxic byproducts in the combustion of fossil fuel ( Figure 1). 11,12,[25][26][27][28][29][30][31][32] The reaction of the phenyl radcial (C 6 H 5 ) with propylene (C 3 H 6 ) presents a potential synthetic routes to synthesize C 9 H 10 isomers and to access the C 9 H 11 potential energy surface.…”

Spectroscopy and ionization thresholds of π-isoelectronic 1-phenylallyl and benzylallenyl resonance stabilized radicals

“…Propargyl radical (C 3 H 3 ), one of the smallest RSRs, has been of great interest in the formation of benzene both experimentally 1 and in theoretical models. 2 In recent ground-breaking work, Schmidt and co-workers spectroscopically characterized the doubly resonance-stabilized 50 1-phenylpropargyl radical, 5,6 with its benzylic radical site linking the phenyl and ethynyl groups.…”

“…As a doubly resonance-stabilized radical, 1-phenylallyl possesses the same energetic benefits as [1][2][3][4][5][6][7][8][9][10] phenylpropargyl, and therefore may play a role of similar importance in pathways that lead from benzene to indene, naphthalene and beyond to larger PAHs. From an electronic structure viewpoint, 1-phenylallyl is also a close analog of the 1-hydronaphthyl radical, 3, 4 which is !-isoelectronic with 1-15 phenylallyl in its cis form.…”