On the Mechanism of Soot Nucleation. III. The Fate and Facility of the E-Bridge.

Rotationally excited dimerization of aromatic moieties is a mechanism proposed recently to explain the initial steps of soot particle inception in combustion and pyrolysis of hydrocarbons. The product of such dimerization, termed E-bridge, is an angled molecular structure composed of two aromatic rings sharing a common bond. The present study explores the immediate fate of the E-bridge. The performed theoretical analysis indicates that abstraction of a bridge H atom by a gaseous H leads to a rapid transformation of the angled to planar structure. The implications of this result is that the collisionally activated E-bridge formation followed by its flattening effectively increases the size of "planar" aromatic precursors by combining two aromatic moieties with essentially collisional rates, instead of a slower "atom-by-atom" buildup. The faster growth speeds up PAH reaching a size when physical dimerization takes over. The dimerization can be further assisted by the biradicaloid valence structure of the flattened E-bridge.

Transformation of an Embedded Five-Membered Ring in Polycyclic Aromatic Hydrocarbons via the Hydrogen-Abstraction-Acetylene-Addition Mechanism: A Theoretical Study.

Five-membered rings are constituents of many polycyclic aromatic hydrocarbons (PAHs), and their presence on the edges of large PAHs has been repeatedly observed experimentally. However, modern kinetic combustion models often do not consider the growth of PAHs through the transformation of the five-membered rings. In connection with the above, we carried out a theoretical study of the mechanism of hydrogen-abstraction-acetylene-addition (HACA) transformation of an embedded five-membered ring on the armchair PAH edge to a six-membered ring, considering cyclopenta[d,e,f]phenanthrene (4,5-methylenephenanthrene) as a prototype system for this process. The potential energy surface for the reactions of cyclopenta[d,e,f]phenanthrenyl radicals produced by direct H abstractions from cyclopenta[d,e,f]phenanthrene with acetylene has been compiled at the G3(MP2,CC)//B3LYP/6-311G(d,p) level of theory including zero-point vibrational energy corrections. The computed energies and molecular parameters were then used to solve the Rice-Ramsperger-Kassel-Marcus master equation in order to calculate the reaction rate at various pressures and temperatures, which were fitted to the modified Arrhenius equation for further kinetic modeling. The results show that the HACA transformation of the embedded five-membered ring to a six-membered ring is possible, albeit slow. The most viable reaction mechanism involves the R2 + C2H2 reaction, where the acetylene molecules add to a σ-radical in the six-membered ring adjacent to the five-membered ring via a low entrance barrier. The predominant product of R2 + C2H2 is predicted to be 3-ethynyl-4H-cyclopenta[def]phenanthrene Pr5 via immediate H elimination from the initial addition complex. Next, Pr5 undergoes H-assisted isomerization to 4aH-pentaleno[4,3,2,1-cdef]phenanthrene Pr4, and the latter adds a H-atom eventually forming the 1-pyrenylmethyl radical Pr3: R2 + C2H2 ⇆ 3-ethynyl-4H-cyclopenta[def]phenanthrene (Pr5) + H or 4aH-pentaleno[4,3,2,1-cdef]phenanthrene (Pr4) + H; Pr5 + H ⇆ Pr4 + H; Pr4 + H → 1-pyrenylmethyl (Pr3). This HACA sequence may be competitive with the methyl radical addition to the R1 radical formed by H abstraction from the CH2 group in the five-membered ring of cyclopenta[d,e,f]phenanthrene, which provides a pathway to pyrene following two H-atom losses. Relative contributions of the two mechanisms of the five- to six-membered ring transformation would strongly depend on the branching ratios of the R1 and R2 radicals produced by the H abstractions and the available concentration of C2H2 versus CH3 and hence differ in different flames.

On the mechanism of soot nucleation. II. E-bridge formation at the PAH bay.

A recently proposed mechanism of soot nucleation (M. Frenklach and A. M. Mebel, Phys. Chem. Chem. Phys., 2020, 22, 5314-5331) based upon the formation of a rotationally-activated dimer in the collision of an aromatic molecule and a radical leading to a stable, doubly-bonded E-bridge between them, rooted in the existence of a five-membered ring on the molecule edge, has been further investigated with a focus on the 5-6 E-bridge forming in the reaction of a PAH cyclopenta group with a bay site of another PAH. As a prototype reaction of this kind, we examined the reaction between 4-phenanthrenyl and acenaphthylene and, to project these results to larger aromatic structures, we also explored the size effect of the E-bridge forming reactions by computing the 1-naphthyl + acenaphthylene system and comparing these results with the prior results for pyrenyl + acepyrene. The two systems have been studied through high-level G3(MP2,CC)//B3LYP/6-311G(d,p) ab initio calculations of their potential energy surfaces combined with the RRKM/Master Equation calculations of reaction rate constants. With PAH monomers of similar sizes involved, the formation of E-bridge structures at the bay radical sites appeared to be faster and lead to increased nucleation rates as compared to the zigzag-forming ones. A model combining both the bay and zigzag rotationally-induced formation of E-bridges successfully reaches the level of nucleation fluxes comparable to those of the irreversible pyrene dimerization, thus affirming the rotationally-activated dimerization as a feasible mechanism for soot particle nucleation.

Mechanism and rate constants of the CH2 + CH2 CO reactions in triplet and singlet states: A theoretical study.

Ab initio and density functional CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/6-311G** calculations have been performed to unravel the reaction mechanism of triplet and singlet methylene CH2 with ketene CH2 CO. The computed potential energy diagrams and molecular properties have been then utilized in Rice-Ramsperger-Kassel-Marcus-Master Equation (RRKM-ME) calculations of the reaction rate constants and product branching ratios combined with the use of nonadiabatic transition state theory for spin-forbidden triplet-singlet isomerization. The results indicate that the most important channels of the reaction of ketene with triplet methylene lead to the formation of the HCCO + CH3 and C2 H4 + CO products, where the former channel is preferable at higher temperatures from 1000 K and above. In the C2 H4 + CO product pair, the ethylene molecule can be formed either adiabatically in the triplet electronic state or via triplet-singlet intersystem crossing in the singlet electronic state occurring in the vicinity of the CH2 COCH2 intermediate or along the pathway of CO elimination from the initial CH2 CH2 CO complex. The predominant products of the reaction of ketene with singlet methylene have been shown to be C2 H4 + CO. The formation of these products mostly proceeds via a well-skipping mechanism but at high pressures may to some extent involve collisional stabilization of the CH3 CHCO and cyclic CH2 COCH2 intermediates followed by their thermal unimolecular decomposition. The calculated rate constants at different pressures from 0.01 to 100 atm have been fitted by the modified Arrhenius expressions in the temperature range of 300-3000 K, which are proposed for kinetic modeling of ketene reactions in combustion. © 2018 Wiley Periodicals, Inc.