Density functional B3LYP/6-31G(d) and ab initio G3(MP2,CC) calculations have been carried out to determine thermal rate constants of direct H abstraction reactions from four- and five-ring polycyclic aromatic hydrocarbons (PAH) chrysene and benzo[a]pyrene by various radicals abundant in combustion flames, such as H, CH, CH, and OH, using transition state theory. The results show that the H abstraction reactions with OH have the lowest barriers of ∼4 kcal mol, followed by those with H and CH with barriers of 16-17 kcal mol, and then with propargyl radicals with barriers of 24-26 kcal mol. Thus, the OH radical is predicted to be the fastest H abstractor from PAH. Even at 2500 K, the rate constant for H abstraction by H is still 34% lower than the rate constant for H abstraction by OH. The reaction with H is calculated to have rate constants 35-19 times higher than those for the reaction with CH due to a more favorable entropic factor. The reactions of H abstraction by CH are predicted to be orders of magnitude slower than the other reactions considered and their equilibrium is strongly shifted toward the reactants, making propargyl an inefficient H abstractor from the aromatics. The calculations showed strong similarity of the reaction energetics in different H abstraction positions of benzo[a]pyrene and chrysene within armchair and zigzag edges in these molecules, but clear distinction between the armchair and zigzag sites. The zigzag sites appear to be more reactive, with H abstraction rate constants by H, CH, and OH being respectively 37-42%, a factor of 2.1, and factors of 8-9 higher than the corresponding rate constants for the H abstraction reactions from armchair sites. Although the barrier heights for the two types of edges are similar, the entropic factor makes zigzag sites more favorable for H abstraction. Rate expressions have been generated for all studied reactions with the goal to rectify current combustion kinetics mechanisms.

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The mechanism of the reaction of ketene with methyl radical has been studied by ab initio CCSD(T)‐F12/cc‐pVQZ‐f12//B2PLYPD3/6‐311G** calculations of the potential energy surface. Temperature‐ and pressure‐dependent reaction rate constants have been computed using the Rice–Ramsperger–Kassel–Marcus (RRKM)–Master Equation and transition state theory methods. Three main channels have been shown to dominate the reaction; the formation of the collisionally stabilized CH3COCH2 radical and the production of the C2H5 + CO and HCCO + CH4 bimolecular products. Relative contributions of the CH3COCH2, C2H5 + CO, and HCCO + CH4 channels strongly depend on the reaction conditions; the formation of thermalized CH3COCH2 is favored at low temperatures and high pressures, HCCO + CH4 is dominant at high temperatures, whereas the yield of C2H5 + CO peaks at intermediate temperatures around 1000 K. The C2H5 + CO channel is favored by a decrease in pressure but remains the second most important reaction pathway after HCCO + CH4 under typical flame conditions. The calculated rate constants at different pressures are proposed for kinetic modeling of ketene reactions in combustion in the form of modified Arrhenius expressions. Only rate constant to form CH3COCH2 depends on pressure, whereas those to produce C2H5 + CO and HCCO + CH4 appeared to be pressure independent.

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The recently proposed mechanism of soot nucleation based upon the formation of a rotationally-activated dimer in collision of aromatic molecule and radical leading to a stable, doubly-bonded E-bridge between them,...

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 + C 2 H 2 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 + C 2 H 2 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 + C 2 H 2 ⇆ 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 CH 2 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 fiveto 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 C 2 H 2 versus CH 3 and hence differ in different flames.