The Role of Methylaryl Radicals in the Growth of Polycyclic Aromatic Hydrocarbons: The Formation of Five-Membered Rings

Abstract

The regions of the C13H11 potential energy surface (PES) related to the unimolecular isomerization and decomposition of the 1-methylbiphenylyl radical and accessed by the 1-/2-methylnaphthyl + C2H2 reactions have been explored by ab initio G3­(MP2,CC)//B3LYP/6-311G­(d,p) calculations. The kinetics of these reactions relevant to the growth of polycyclic aromatic hydrocarbons (PAH) under high-temperature conditions in circumstellar envelopes and in combustion flames has been studied employing the RRKM-Master Equation approach. The unimolecular reaction of 1-methylbiphenylyl proceeding via a five-membered ring closure followed by H elimination is predicted to be very fast, on a submicrosecond scale above 1000 K and to result in the formation of an embedded five-membered ring in the 9H-fluorene product. The 1-/2-methylnaphthyl + C2H2 reaction mechanism involves acetylene addition to the radical on the methylene group followed by a six- or five-membered ring closure and aromatization via an H atom loss. Despite of the complexity of the C13H11 PES, these straightforward pathways are dominant in the high-temperature regime (above ∼1000 K), with the prevailing products being phenalene, with a significant contribution of 1H-cyclopenta­(a)­naphthalene, for 1-methylnaphthyl + C2H2, and 1H-cyclopenta­(b)­naphthalene and 3H-cyclopenta­(a)­naphthalene, for 2-methylnaphthyl + C2H2. The methylnaphthyl reactions with acetylene represent a clean source of the three-ring PAHs, but they are relatively slow owing to the high entrance barriers of ∼10 kcal/mol, with the rate constants of about an order of magnitude lower as compared to those for naphthyl + allene and σ-aryl + C2H2. The 1-methylnaphthyl + C2H2 and 2-methylnaphthyl + C2H2 reactions represent prototypes for PAH growth by an extra six- and five-membered ring on a zigzag edge or a corner of PAH and the generated modified Arrhenius expressions are recommended for kinetic modeling of PAH expansion by the mechanism of acetylene addition to methylaryl radicals.

A unified reaction network on the formation of five-membered ringed polycyclic aromatic hydrocarbons (PAHs) and their role in ring expansion processes through radical–radical reactions

“…These processes involve barriers to addition of 41 and 42 kJ mol −1 , respectively. 48 This barrier is slightly below the barrier to addition of acetylene to the benzyl radical (C 6 H 5 CH 2 ˙) of 51 kJ mol −1 as explored previously. 1 Intermediates i1 and i2 undergo cyclization to i3 and i4/i5 , respectively, passing barriers between 45 and 52 kJ mol −1 .…”

“…3H-cyclopenta[a]naphthalene (p2)/1H-cyclopenta[a]naphthalene (p1) and 3H-cyclopenta[a]naphthalene (p2)/1Hcyclopenta[b]naphthalene (p3), respectively, suggests an initial addition of the acetylene molecule with one carbon atom to the radical center at the -CH 2 moiety leading to two distinct C 13 H 11 radical intermediates i1 and i2 carrying each a C3 side chain. These processes involve barriers to addition of 41 and 42 kJ mol À1 , respectively 48. This barrier is slightly below the barrier to addition of acetylene to the benzyl radical (C 6 H 5 CH 2 )of 51 kJ mol À1 as explored previously 1.…”

On the Mechanism of Soot Nucleation. IV. Molecular Growth of the Flattened E-Bridge

“…14 The use of B3LYP geometries and vibrational frequencies in conjunction with higher-level chemically accurate single-point energies has been repeatedly shown to provide kinetically accurate rate constants for PAH growth reactions, see e.g., refs ( 15 ) and ( 16 ). In a recent paper, 17 we have also tested the performance of B3LYP for a related ring-closure reaction vs a modern ωB97XD functional which includes dispersion corrections. It appeared that the B3LYP- and ωB97XD-optimized geometries were nearly identical, and the difference in vibrational frequencies of local minima and transition states was under 4% and only ∼1% on average.…”

An extensive theoretical study on the thermochemistry of aromatic compounds: from electronic structure to group additivity values

“…However, the thermochemistry study of aromatics mainly focuses on polycyclic aromatic hydrocarbons, [11][12][13][14] with relevant studies of mono-substituted alkylbenzenes and derived aromatics being limited. Mebel and his coworkers [15][16][17][18][19] have carried out a series of ab initio calculations about the potential energy surface and rate constants for the formation mechanism of polycyclic aromatic hydrocarbons, including indene and phenanthrene. In their papers, they provide geometries, frequencies, single-point energies (SPE) and zero-point energy corrections (ZPE) of relevant aromatic species; however the thermochemistry data are not included.…”