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

Abstract

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.

Temperature Effect on Formation of Polycyclic Aromatic Hydrocarbons in Acetylene Pyrolysis

“…This is mainly due to the presence of the embedded five-membered ring structure, which reduces the stability of π electrons in the bay area and makes them more likely to be activated by losing H in the H-abstraction process. [41,67,68] Therefore, in a sense, the energy required for the activation of the HACA mechanism is reduced and the process of the mechanism is accelerated. This factor also exists in the process of acenaphthene forming acephenanthrylene.…”

“…As can be seen from the flux diagram (Figure 8), it is easier for the HACA mechanism to form the six‐membered ring structures than the acetylene direct addition to forming the five‐membered ring structures. This is mainly due to the presence of the embedded five‐membered ring structure, which reduces the stability of π electrons in the bay area and makes them more likely to be activated by losing H in the H‐abstraction process [41,67,68] . Therefore, in a sense, the energy required for the activation of the HACA mechanism is reduced and the process of the mechanism is accelerated.…”

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

“…The MESS software package was utilized for the RRKM-ME calculations, where partition functions and densities of states for local mimima and numbers of states for transition states were computed within the Rigid-Rotor, Harmonic-Oscillator (RRHO) model. The Lennard-Jones parameters were estimated using the general expressions for ε and σ depending on the molecular mass proposed by Wang and Frenklach for PAH molecules, whereas the parameters for the bath gas (N 2 ) were taken from the papers by Vishnyakov et al , Our previous calculations showed that moderate changes in the Lennard-Jones parameters within the ranges corresponding to four- to six-ring PAHs result in insignificant changes of the calculated pressure-dependent rate constants of less than 10% . The collisional energy transfer parameters in RRKM-ME calculations were described within the “exponential down” model, where the temperature dependence of the parameter α for the deactivating wing of the energy transfer function was expressed as α­( T ) = α 300 ( T /300) n , where n = 0.85 and α 300 = 247 cm –1 are “universal” values proposed by Jasper and Miller for hydrocarbons …”

Cleavage of an aromatic ring and radical migration

“…17,18 For uniformity, we kept the same α 300 = 424 cm −1 and n = 0.62 values for pyrenyl, but it is our experience that for the reactions involving PAH, differences in pressure-dependent rate constants using these values and with the “universal” values for hydrocarbons proposed by Jasper and Miller, 19 α 300 = 247 cm −1 and n = 0.85, do not exceed 10%. 20…”