Extensive ab initio Gaussian-3-type calculations of potential energy surfaces (PES), which are expected to be accurate within 1-2 kcal/mol, combined with statistical theory calculations of reaction rate constants have been applied to study various possible pathways in the hydrogen abstraction acetylene addition (HACA) mechanism of naphthalene and acenaphthalene formation as well as Diels-Alder pathways to acenaphthalene, phenanthrene, and pyrene. The barrier heights; reaction energies; and molecular parameters of the reactants, products, intermediates, and transition states have been generated for all types of reactions involved in the HACA and Diels-Alder mechanisms, including H abstraction from various aromatic intermediates, acetylene addition to radical sites, ring closures leading to the formation of additional aromatic rings, elimination of hydrogen atoms, H disproportionation, C2H2 cycloaddition, and H2 loss. The reactions participating in various HACA sequences (e.g., Frenklach's, alternative Frenklach's, and Bittner and Howard's routes) are demonstrated to have relatively low barriers and high rate constants under combustion conditions. A comparison of the significance of different HACA mechanisms in PAH growth can be made in the future using PES and molecular parameters obtained in the present work. The results show that the Diels-Alder mechanism cannot compete with the HACA pathways even at high combustion temperatures, because of high barriers and consequently low reaction rate constants. The calculated energetic parameters and rate constants have been compared with experimental and theoretical data available in the literature.

Chemically accurate ab initio Gaussian-3-type calculations of the C(10)H(9) potential energy surface (PES) for rearrangements of the 9-H-fulvalenyl radical C(5)H(5)-C(5)H(4) have been performed to investigate the formation mechanisms of polycyclic aromatic hydrocarbons (PAHs) originated from the recombination of two cyclopentadienyl radicals (c-C(5)H(5)) as well as from the intermolecular addition of cyclopentadienyl to cyclopentadiene (c-C(5)H(6)) under combustion and pyrolytic conditions. Statistical theory calculations have been applied to obtain high-pressure-limit thermal rate constants, followed by solving kinetic equations to evaluate relative product yields. At the high-pressure limit, naphthalene, fulvalene, and azulene have been shown as the reaction products in rearrangements of the 9-H-fulvalenyl radical, with relative yields depending on temperature. At low temperatures (T < 1000 K), naphthalene is predicted to be the major product (>50%), whereas at higher temperatures the naphthalene yield rapidly decreases and the formation of fulvalene becomes dominant. At T > 1500 K, naphthalene and azulene are only minor products accounting for less than 10% of the total yield. The reactions involving cyclopentadienyl radicals and cyclopentadiene have thus been shown to give only a small contribution to the naphthalene production on the C(10)H(9) PES at medium and high combustion temperatures. The high yields of fulvalene at these conditions indicate that cyclopentadienyl radical and cyclopentadiene more likely represent significant sources of cyclopentafused PAHs, which are possible fullerene precursors. Our results agree well with a low-temperature cyclopentadiene pyrolysis data, where naphthalene has been identified as the major reaction product together with indene. Azulene has been found to be only a minor product in 9-H-fulvalenyl radical rearrangements, with branching ratios of less than 5% at all studied temperatures. The production of naphthalene at low combustion temperatures (T < 1000 K) is governed by the spiran mechanism originally suggested by Melius et al. At higher temperatures, the alternative C-C bond scission route, which proceeds via the formation of the cis-4-phenylbutadienyl radical, is competitive with the spiran pathway. The contributions of the previously suggested methylene walk pathway to the production of naphthalene have been calculated to be negligible at all studied temperatures.

Chemically accurate ab initio Gaussian-3-type calculations of various rearrangements on the C10H11 potential energy surface have been performed to investigate the indene formation mechanism originating from the reactions of two abundant cyclic C5 species, cyclopentadiene and cyclopentadienyl radicals. Using the accurate ab initio data, statistical theory calculations have been applied to obtain high-pressure-limit thermal rate constants within the 300-3000 K temperature range, followed by calculations of relative product yields. Totally, 12 reaction pathways leading to indene and several azulene precursors, 1,5-, 1,7-, 1,8a-, and 1,3a-dihydroazulene, have been mapped out, and the relative contributions of each pathway to the formation of reaction products have been estimated. At temperatures relevant to combustion, the indene has been found as the major reaction product (>50%) followed by 1,5-dihydroazulene (25-35%), whereas all other products demonstrate either minor or negligible yields. The results of the present study have been combined with our previous data for rearrangements of the 9-H-fulvalenyl radical on the C10H9 potential energy surface to draw the detailed picture of radical-promoted reaction mechanisms leading from c-C5 species to the production of indene, naphthalene, azulene, and fulvalene in combustion. The suggested mechanism and computed product yields are consistent with the experimental data obtained in the low-temperature pyrolysis of cyclopentadiene, where indene and naphthalene have been found as the major reaction products.