Formation pathways of polycyclic aromatic hydrocarbons (PAHs) in butane or butadiene flames

The reaction pathways from phenyl radicals to phenanthrene (A₃) and pyrene (A₄) via C₂H₃ and C₄H₄ additions were investigated using the G3(MP2, CC) method. Rate constants of elementary reactions were calculated. The influence of additions, H-abstraction ways and reactive sites on the reaction rates were considered. These polycyclic aromatic hydrocarbon (PAH) formation pathways were used to improve the combustion chemistry model for C₄ fuels, and the results from the improved model and the original model were compared with experimental data. H atoms are important for PAH formation owing to their influential roles in the production of aromatic radicals and stable aromatic structures. C₂H₃ and C₄H₄ addition reactions can occur at low temperature, and need less energy than C₂H₂ addition. The PAH formation pathways determined from G3 calculations, which were used to improve the model, were effective in promoting PAH formations in this model. Comparison of PAH formation in butane and butadiene flames showed both the C₂H₃ and C₄H₄ addition pathways included in this work can improve the formation of PAHs in butadiene and butane flames. C₄H₄ addition pathways in a butane flame were better for PAH formation than C₂H₃ addition.

The reaction pathways from phenyl radicals to phenanthrene (A₃) and pyrene (A₄) via C₂H₃ and C₄H₄ additions were investigated using the G3(MP2, CC) method.

A combined photoionization time-of-flight mass spectrometry and laser absorption spectrometry flash photolysis apparatus for simultaneous determination of reaction rates and product branching

In recent years, predictions of product branching for reactions of consequence to both combustion and atmospheric chemistry have outpaced validating experiments. An apparatus is described that aims to fill this void by combining several well-known experimental techniques into one: flash photolysis for radical generation, multiple-pass laser absorption spectrometry (LAS) for overall kinetics measurements, and time-resolved photoionization time-of-flight mass spectrometry (PI TOF-MS) for product branching quantification. The sensitivity of both the LAS and PI TOF-MS detection techniques is shown to be suitable for experiments with initial photolytically generated radical concentrations of ∼1 × 10¹² molecules cm^-3. As it is fast (μs time resolution) and non-intrusive, LAS is preferred for accurate kinetics (time-dependence) measurements. By contrast, PI TOF-MS is preferred for product quantification because it provides a near-complete picture of the reactor composition in a single mass spectrum. The value of simultaneous LAS and PI TOF-MS detection is demonstrated for the chemically interesting phenyl radical + propene system.

Highly efficient gas-phase reactivity of protonated pyridine radicals with propene

Small nitrogen containing heteroaromatics are fundamental building blocks for many biological molecules, including the DNA nucleotides. Pyridine, as a prototypical N-heteroaromatic, has been implicated in the chemical evolution of many extraterrestrial environments, including the atmosphere of Titan. This paper reports on the gas-phase ion-molecule reactions of the three dehydro-N-pyridinium radical cation isomers with propene. Photodissociation ion-trap mass spectrometry experiments are used to measure product branching ratios and reaction kinetics. Reaction efficiencies for 2-dehydro-N-pyridinium, 3-dehydro-N-pyridinium and 4-dehydro-N-pyridinium with propene are 70%, 47% and 41%, respectively. The m/z 106 channel is the major product channel across all cases and assigned 2-, 3-, and 4-vinylpyridinium for each reaction. The m/z 93 channel is also significant and assigned the 2-, 3-, and 4-N-protonated-picolyl radical cation for each case. H-Abstraction from propene is not competitive under experimental conditions. Potential energy schemes, at the M06-2X/6-31(2df,p) level of theory and basis set, are described to assist in rationalising observed product branching ratios and elucidating possible reaction mechanisms. Reaction barriers to the production of vinylpyridinium (m/z 106) + CH₃ are the lowest identified for the 3- and 4-dehydro-N-pyridinium reactions, in support of the observed dominance of the m/z 106 ion signal. Ethylene loss via ring-mediated H-transfer along the propyl group is found to be the lowest energy pathway for the 2-dehydro-N-pyridinium reaction, suggesting a preference toward m/z 93 (N-protonated-picolyl radical cation) over the experimentally observed products. Entropic bottle-necks along the m/z 93 pathway however, associated with ring-mediated H-atom transfer, are responsible for the dominance of m/z 106 in the 2-dehydro-N-pyridinium + propene reaction. For all three isomers, computed barriers for all observed reaction channels were below the entrance channel, suggesting these reactions can contribute to molecular weight growth in extraterrestrial environments with accelerated reaction rates in low temperature regions of space.

Formation of bicyclic polycyclic aromatic hydrocarbons (PAHs) from the reaction of a phenyl radical with cis-3-penten-1-yne

The formation of PAHs within 4-, 5-, 6- and 7-membered rings on the C₆H₅ + C₅H₆ potential energy surface.

Phenyl radical + propene: a prototypical reaction surface for aromatic-catalyzed 1,2-hydrogen-migration and subsequent resonance-stabilized radical formation

H-Shifts in the alkyl chain catalyzed by an aromatic ring (green pathway).

Crossed beam polyatomic reaction dynamics: recent advances and new insights

This review summarizes the developments in polyatomic reaction dynamics, focusing on reactions of unsaturated hydrocarbons with O-atoms and methane with atoms/radicals.

Pressure-dependent rate constants for PAH growth: formation of indene and its conversion to naphthalene

Unraveling the mechanisms for growth of polycyclic aromatic hydrocarbons (PAHs) requires accurate temperature- and pressure-dependent rate coefficients for a great variety of feasible pathways. Even the pathways for the formation of the simplest PAHs, indene and naphthalene, are fairly complex. These pathways provide important prototypes for modeling larger PAH growth. In this work we employ the ab initio RRKM theory-based master equation approach to predict the rate constants involved in the formation of indene and its conversion to naphthalene. The reactions eventually leading to indene involve C₉H_x (x = 8–11) potential energy surfaces (PESs) and include C₆H₅ + C₃H₄ (allene and propyne), C₆H₆ + C₃H₃, benzyl + C₂H₂, C₆H₅ + C₃H₆, C₆H₆ + C₃H₅ and C₆H₅ + C₃H₅. These predictions allow us to make a number of valuable observations on the role of various mechanisms. For instance, we demonstrate that reactions which can significantly contribute to the formation of indene include phenyl + allene and H-assisted isomerization to indene of its major product, 3-phenylpropyne, benzyl + acetylene, and the reactions of the phenyl radical with propene and the allyl radical, both proceeding via the 3-phenylpropene intermediate. 3-Phenylpropene can be activated to a 1-phenylallyl radical, which in turn rapidly decomposes to indene. Next, indene can be converted to benzofulvene or naphthalene under typical combustion conditions, via its activation by H atom abstraction and methyl substitution on the five-membered ring followed by isomerization and decomposition of the resulting 1-methylindenyl radical, C₁₀H₉ → C₁₀H₈ + H. Alternatively, the same region of the C₁₀H₉ PES can be accessed through the reaction of benzyl with propargyl, C₇H₇ + C₃H₃ → C₁₀H₁₀ → C₁₀H₉ + H, which therefore can also contribute to the formation of benzofulvene or naphthalene. Benzofulvene easily transforms to naphthalene by H-assisted isomerization. An analysis of the effect of pressure on the reaction outcome and relative product yields is given, and modified Arrhenius fits of the rate constants are reported for the majority of the considered reactions. Ultimately, the implementation of such expressions in detailed kinetic models will help quantify the role of these reactions for PAH growth in various environments.

Formation of 5- and 6-methyl-1H-indene (C10H10) via the reactions of the para-tolyl radical (C6H4CH3) with allene (H2CCCH2) and methylacetylene (HCCCH3) under single collision conditions

Flux contour map for the reactions of the p-tolyl radical with allene-d4 and methylacetylene-d4 at collision energies of around 48 kJ mol⁻¹.

On the formation of pyridine in the interstellar medium

The nitrogen bearing aromatic molecule pyridine (C₅H₅N) is revealed to form in high temperature environments simulating conditions in carbon-rich circumstellar envelopes via the reaction of the cyano vinyl radical with vinyl cyanide.

Reaction dynamics of the 4-methylphenyl radical (p-tolyl) with 1,2-butadiene (1-methylallene): are methyl groups purely spectators?

The reactions of the 4-tolyl radical (C6H4CH3) and of the D7-4-tolyl radical (C6D4CD3) with 1,2-butadiene (C4H6) have been probed in crossed molecular beams under single collision conditions at a collision energy of about 54 kJ mol(-1) and studied theoretically using ab initio G3(MP2,CC)//B3LYP/6-311G** and statistical RRKM calculations. The results show that the reaction proceeds via indirect scattering dynamics through the formation of a van-der-Waals complex followed by the addition of the radical center of the 4-tolyl radical to the C1 or C3 carbon atoms of 1,2-butadiene. The collision complexes then isomerize by migration of the tolyl group from the C1 (C3) to the C2 carbon atom of the 1,2-butadiene moiety. The resulting intermediate undergoes unimolecular decomposition via elimination of a hydrogen atom from the methyl group of the 1,2-butadiene moiety through a rather loose exit transition state leading to 2-para-tolyl-1,3-butadiene (p4), which likely presents the major reaction product. Our observation combined with theoretical calculations suggest that one methyl group (at the phenyl group) acts as a spectator in the reaction, whereas the other one (at the allene moiety) is actively engaged in the underlying chemical dynamics. On the contrary to the reaction of the phenyl radical with allene, which leads to the formation of indene, the substitution of a hydrogen atom by a methyl group in allene essentially eliminates the formation of bicyclic PAHs such as substituted indenes in the 4-tolyl plus 1,2-butadiene reaction.

A VUV photoionization study of the multichannel reaction of phenyl radicals with 1,3-butadiene under combustion relevant conditions

We studied the reaction of phenyl radicals (C(6)H(5)) with 1,3-butadiene (H(2)CCHCHCH(2)) exploiting a high temperature chemical reactor under combustion-like conditions (300 Torr, 873 K). The reaction products were probed in a supersonic beam by utilizing VUV radiation from the Advanced Light Source and by recording the experimental PIE curves at mass-to-charge ratios of m/z = 130 (C(10)H(10)(+)), 116 (C(9)H(8)(+)), and 104 (C(8)H(8)(+)). Our data suggest that the atomic hydrogen (H), methyl (CH(3)), and vinyl (C(2)H(3)) losses are open with estimated branching ratios of about 86 ± 4%, 8 ± 2%, and 6 ± 2%, respectively. The isomer distributions were probed further by fitting the experimentally recorded PIE curves with a linear combination of the PIE curves of individual C(10)H(10), C(9)H(8), and C(8)H(8) isomers. These fits indicate the formation of three C(10)H(10) isomers (trans-1,3-butadienylbenzene, 1,4-dihydronaphthalene, 1-methylindene), three C(9)H(8) isomers (indene, phenylallene, 1-phenyl-1-methylacetylene), and a C(8)H(8) isomer (styrene). A comparison with results from recent crossed molecular beam studies of the 1,3-butadiene-phenyl radical reaction and electronic structure calculations suggests that trans-1,3-butadienylbenzene (130 amu), 1,4-dihydronaphthalene (130 amu), and styrene (104 amu) are reaction products formed as a consequence of a bimolecular reaction between the phenyl radical and 1,3-butadiene. 1-Methylindene (130 amu), indene (116 amu), phenylallene (116 amu), and 1-phenyl-1-methylacetylene (116 amu) are synthesized upon reaction of the phenyl radical with three C(4)H(6) isomers: 1,2-butadiene (H(2)CCCH(CH(3))), 1-butyne (HCCC(2)H(5)), and 2-butyne (CH(3)CCCH(3)); these C(4)H(6) isomers can be formed from 1,3-butadiene via hydrogen atom assisted isomerization reactions or via thermal rearrangements of 1,3-butadiene involving hydrogen shifts in the high temperature chemical reactor.