An Optimized Semidetailed Submechanism of Benzene Formation from Propargyl Recombination

The isomer distribution of C6H6 products from the propargyl radical gas-phase recombination investigated by threshold-photoelectron spectroscopy

The resonance-stabilization of the propargyl radical (C3H3) makes it among the most important reactive intermediates in extreme environments and grants it a long enough lifetime to recombine in both terrestrial combustion media and cold molecular clouds in space. This makes the propargyl self-reaction a pivotal step in the formation of benzene, the first aromatic ring, to eventually lead to polycyclic aromatic hydrocarbons in a variety of environments. In this work, by producing propargyl radicals in a flow tube where propyne reacted with F atoms and probing the reaction products by mass-selected threshold-photoelectron spectroscopy (TPES), we identified eight C6H6 products in total, including benzene. On top of providing the first comprehensive measurements of the branching ratios of the eight identified C6H6 isomers in the propargyl self reaction products (4 mbar, 298 K conditions), this study also highlights the advantages and disadvantages of using isomer-selective TPES to identify and quantify reaction products.

From six to eight Π-electron bare rings of group-XIV elements and beyond: can planarity be deciphered from the "quasi-molecules" they embed?

Ab initio molecular orbital theory is used to study the structures of six and eight π-electron bare rings of group-XIV elements, and even larger [n]annulenes up to C₁₈H₁₈, including some of their mono-, di-, tri-, and tetra-anions. While some of the above rings are planar, others are nonplanar. A much spotlighted case is cyclo-octatetraene (C₈H₈), which is predicted to be nonplanar together with its heavier group-XIV analogues Si₈H₈ and Ge₈H₈, with the solely planar members of its family having the stoichiometric formulas C₄Si₄H₈ and C₄Ge₄H₈. A similar situation arises with the six π-electron bare rings, where benzene and substituted ones up to C₃Si₃H₆ or so are planar, while others are not. However, the explanations encountered in the literature find support in ab initio calculations for such species, often rationalized from distinct calculated features. Using second-order Møller-Plesset perturbation theory and, when affordable (particularly tetratomics, which may allow even higher levels), the coupled-cluster method including single, double, and perturbative triple excitations, a common rationale is suggested based on a novel concept of quasi-molecules or the (3+4)-atom partition scheme. Any criticism of tautology is therefore avoided. The same analysis has also been successfully applied to even larger [n]annulenes, to their mixed family members involving silicon and germanium atoms, and to the C₁₈ carbon ring. Furthermore, it has been extended to annulene anions to check the criteria of the popular Hückel rule for planarity and aromaticity. Exploratory work on cycloarenes is also reported. Besides a partial study of the involved potential energy surfaces, equilibrium geometries and harmonic vibrational frequencies have been calculated anew, for both the parent and the actual prototypes of the quasi-molecules.

A Conical Intersection Influences the Ground State Rearrangement of Fulvene to Benzene

The rearrangement of fulvene to benzene is believed to play an important role in the formation of soot during hydrocarbon combustion. Previous work has identified two possible mechanisms for the rearrangement─a unimolecular path and a hydrogen-atom-assisted, bimolecular path. Computational results to date have suggested that the unimolecular mechanism faces a barrier of about 74 kcal/mol, which makes it unable to compete with the bimolecular mechanism under typical combustion conditions. This computed barrier is about 10 kcal/mol higher than the experimental value, which is an unusually large discrepancy for modern electronic structure theory. In the present work, we have reinvestigated the unimolecular mechanism computationally, and we have found a second transition state that is approximately 10 kcal/mol lower in energy than the previously identified one and, therefore, in excellent agreement with the experimental value. The existence of two transition states for the same rearrangement arises because there is a conical intersection between the two lowest singlet states which occurs in the vicinity of the reaction coordinates. The two possible paths around the cone on the lower adiabatic surface give rise to the two distinct saddle points. The lower barrier for the unimolecular mechanism now makes it competitive with the bimolecular one, according to our calculations. In support of this conclusion, we have reanalyzed some previous experimental results on anisole pyrolysis, which leads to benzene as a significant product and have shown that the unimolecular and bimolecular mechanisms for fulvene → benzene must be occurring competitively in that system. Finally, we have identified that similar conical intersections arise during the isomerizations of benzofulvene and isobenzofulvene to naphthalene.

On the role of resonantly stabilized radicals in polycyclic aromatic hydrocarbon (PAH) formation: pyrene and fluoranthene formation from benzyl–indenyl addition

Resonantly stabilized benzyl and indenyl radicals play a role in the formation of pyrene and fluoranthene.

Experimental and theoretical studies into the formation of C4-C6 products in partially chlorinated hydrocarbon pyrolysis systems: a probabilistic approach to congener-specific yield predictions

This work presents a study of the pyrolytic formation of vinylacetylene and benzene congeners formed from chlorinated hydrocarbon precursors, a complex, multipath polymerization system formed in a monomer-rich environment. (Co-)pyrolyses of dichloro- and trichloroethylene yield a rich array of products, and assuming a single dominant underlying growth mechanism, this (on comparing expected and observed products) allows a number of potentially competing channels to C4 and C6 products to be ruled out. Poor congener/isomer descriptions rule out even-carbon radical routes, and the absence of C3 and C5 products rule out odd-carbon processes. Vinylidenes appear unable to describe the increased reactivity of acetylenes with chlorination noted in our experiments, leaving molecular acetylene dimerization processes and, in C6 systems, the closely related Diels-Alder cyclization as the likely reaction mechanism. The feasibility of these routes is further supported by ab initio calculations. However, some of the most persuasive evidence is provided by congener-specific yield predictions enabled by the construction of a probability tree analogue of kinetic modeling. This approach is relatively quick to construct, provides surprisingly accurate predictions, and may be a very useful tool in screening for important reaction channels in poorly understood congener- or isomer-rich reaction systems.

Molecular mechanisms in the pyrolysis of unsaturated chlorinated hydrocarbons: formation of benzene rings. 2. Experimental and kinetic modeling studies

The mechanism of formation of benzene rings during the pyrolysis of dichloro- and trichloroethylenes has been investigated by the method of laser powered homogeneous pyrolysis coupled with product analysis by gas chromatography. Additionally, selected (co)pyrolyses between the chlorinated ethylenes, CH2Cl2, C4Cl4, C4Cl6, and C2H2 have been performed to explicitly probe the roles of 2C3 and C4/C2 reaction pairs in aromatic growth. The presence of odd-carbon products in neat C4Cl6 pyrolyses indicates that 2C3 processes are operative in these systems; however, comparison with product yields from C2HCl3 suggests that C4/C2 processes dominate most other systems. This is further evidenced by an absence of C3 and other odd-carbon species in (co)pyrolyses with dichloromethane which should seed C3-based growth. The reactions of perchlorinated C4 species C4Cl5, C4Cl3, and C4Cl4 with C2Cl2 were subsequently explored through extensive kinetic simulations of the possible reaction pathways based on previous kinetic models and the exhaustive quantum chemical investigations of our preceding work. The experimental and theoretical results strongly suggest that, at moderate temperatures, aromatic ring formation from chlorinated ethylenes normally follows a Diels-Alder coupling of C4 and C2 molecular units followed by internal shifts; the one exception is the C4Cl4 + C2Cl2 system, where steric factors lead to the formation of nonaromatic products. There is little evidence for radical-based routes in these systems.

Shock tube pyrolysis of 1,2,4,5-hexatetraene

1,2,4,5-Hexatetraene (1245HT) is, according to theory, a key intermediate to benzene from propargyl radicals in a variety of flames; however, it has only been experimentally observed once in previous studies of the C3H3 + C3H3 reaction. To determine if it is indeed an intermediate to benzene formation, 1245HT was synthesized, via a Grignard reaction, and pyrolysized in a single-pulse shock tube at two nominal pressures of 22 and 40 bar over a temperature range from 540 to 1180 K. At temperatures T < 700 K, 1245HT converts efficiently to 3,4-dimethylenecyclobutene (34DMCB) with a rate constant of k = 10(10.16) x exp(-23.4 kcal/RT), which is in good agreement with the one calculated by Miller and Klippenstein. At higher temperatures, various C6H6 isomers were generated, which is consistent with theory and earlier experimental studies. Thus, the current work strongly supports the theory that 1245HT plays a bridging role in forming benzene from propargyl radicals. RRKM modeling of the current data set has also been carried out with the Miller-Klippenstein potential. It was found that the theory gives reasonably good predictions of the experimental observations of 1245HT, 1,5-hexadiyne (15HD), and 34DMCB in the current study and in our earlier studies of 15HD pyrolysis and propargyl recombination; however, there is considerable discrepancy between experiment and theory for the isomerization route of 1,2-hexadien-5-yne (12HD5Y) --> 2-ethynyl-1,3-butadiene (2E13BD) --> fulvene.