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

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.

Gas Phase Formation of the Interstellar Molecule Methyltriacetylene

Methyltriacetylene - the largest methylated polyacetylene detected in deep space - has been synthesized in the gas phase via the bimolecular reaction of the 1-propynyl radical with diacetylene under single-collision conditions. The barrier-less route to methyltriacetylene represents a prototype of a polyyne chain extension through a radical substitution mechanism and provides a novel low temperature route, in which the propynyl radical piggybacks a methyl group to be incorporated into methylated polyynes. This mechanism overcomes a key obstacle in previously postulated reactions of methyl radicals with unsaturated hydrocarbon, which fail the inclusion of methyl groups into hydrocarbons due to insurmountable entrance barriers thus providing a fundamental understanding on the electronic structure, chemical bonding, and formation of methyl-capped polyacetylenes. These species are key reactive intermediates leading to carbonaceous nanostructures in molecular clouds like TMC-1.

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.

Naphthalene formation pathways from phenyl radical via vinyl radical (C2H3) and vinylacetylene (C4H4): computational studies on reaction mechanisms and kinetics

The reaction mechanisms of PAH formation from phenyl radical (C6H5) to naphthalene via C2H3 (C2H3-Path) and C4H4 (C4H4-Path) were investigated by the G3(MP2, CC) method. The hydrogen abstraction, ring closure, cis–trans isomerization, and disproportionation reactions were considered, as well as their occurred sequence. The results showed that H-abstraction reactions occurred more easily than H-dissociation reactions. The cis–trans conversion reactions in sub-routes of C2H3-Path and C4H4-Path provided the largest barriers of 51, 53, and 36 kcal/mol along their routes, which illustrated that the cis–trans isomerization was energetically costly in the PAH formation process. The entrance barriers of C2H2-Path, C2H3-Path, and C4H4-Path are 6, 8, and 3 kcal/mol, respectively, which indicates that it is easier to add C4H4 to C6H5 compared with adding C2H2 to C2H3. C2H3 additions were highly exothermic with reaction energies greater than 110 kcal/mol, and compared with C2H2 additions, C2H3 additions were irreversible. However, C2H2-Path, C2H3-Path and C4H4-Path involved energy barriers of 20, 32, and 36 kcal/mol, respectively. Considering the high temperature in combustion and the approximate concentrations of C2H3 and C4H4, all three of these pathways could lead to naphthalene in some combustion flames.

Oxidation of the para-Tolyl Radical by Molecular Oxygen under Single-Collison Conditions: Formation of the para-Toloxy Radical

Crossed molecular beam experiments were performed to elucidate the chemical dynamics of the para-tolyl (CH₃C₆H₄) radical reaction with molecular oxygen (O₂) at an average collision energy of 35.3 ± 1.4 kJ mol^-1. Combined with theoretical calculations, the results show that para-tolyl is efficiently oxidized by molecular oxygen to para-toloxy (CH₃C₆H₄O) plus ground-state atomic oxygen via a complex forming, overall exoergic reaction (experimental, -33 ± 16 kJ mol^-1; computational, -42 ± 8 kJ mol^-1). The reaction dynamics are analogous to those observed for the phenyl (C₆H₅) plus molecular oxygen system which suggests the methyl group is a spectator during para-tolyl oxidation and that application of phenyl thermochemistry and reaction rates to para-substituted aryls is likely a suitable approximation.