Polycyclic Aromatic Hydrocarbon Growth by Diradical Cycloaddition/Fragmentation

Abstract

The recent theoretical and experimental investigations on the growth of polycyclic aromatic hydrocarbons in pyrolytic environments highlight the possible role of the 1,4-cycloaddition/fragmentation (1,4-CAF) steps in the formation of PAH intermediates and consequently soot. The present theoretical study explores the possibility to generalize such mechanism to reactions involving various diradical compounds and stable multiring structures. The calculations were performed using the uB3LYP/6-311G(d,p) method and different composite methods, when possible, for more accurate energy estimates. First, the complex potential energy surface for the reactions between o-benzyne and naphthalene was investigated, including the 1,4-CAF mechanism to form anthracene and acetylene through the dibenzobicyclo[2.2.2]octatriene intermediate. Moreover, the products of the addition reactions to the α- and β-carbons and to the ring-junction atoms were determined. The energies for the optimized CAF structures, which constitute the most-favorable pathway from an energetic point of view, were calculated using CBS-QB3, G3(MP2)B3, and G3B3 methods and compared to the corresponding values for the o-benzyne + benzene reactions. Additional calculations were focused on the possible CAF reactions between o-benzyne and larger multiring structures, such as anthracene, phenanthrene, pyrene, and the four-ring PAHs. The results indicate how the energetics of such reactions is influenced by both the size of the PAH compound and the position of the carbon atoms involved. In the second part of the study, the energy barriers necessary to form multiring diradicals from the corresponding radical molecules were analyzed at a G3(MP2)B3 level of theory. Such calculations are preliminary for the subsequent study on the CAF reactions between the different diradical intermediates and benzene. While the size of the diradical does not affect significantly the energy barriers, the position of the diradical site is critical. The concerted Diels-Alder reactions between the naphthynes and naphthalene were also studied in order to further clarify the analogies between the reactions involving different diradicals. Based on these results, kinetic considerations were provided based on the comparison with the simpler o-benzyne + benzene system, although further higher-level calculations and master equation kinetic analyses will be required to derive the general kinetic rules.

Predicting polycyclic aromatic hydrocarbon formation with an automatically generated mechanism for acetylene pyrolysis

“…The phenyl + benzene and benzyne + benzene surfaces have been previously reported by Comandini et al. at the uCCSD(T)/cc‐pVDZ//uB3LYP/6‐311+G(d,p) level of theory 27 . Here, we recalculated the addition and H‐loss reactions using CBS‐QB3 to obtain more accurate energies.…”

“…[21] at the uCCSD(T)/cc-pVDZ//uB3LYP/6-311+G(d,p) level of theory. 27 Here, we recalculated the addition and H-loss reactions using CBS-QB3 to obtain more accurate energies. The two surfaces for first and second addition of acetylene to naphthalenyl have been previously reported by Kislov et al 17 and Frenklach et al 18 For the C 14 H 11 surface one-dimensional (1D) hindered-rotor scans were performed, and for the C 12 H 9 surface hindered-rotor scans were obtained from Frenklach et al…”

“…21 Vinylacetylene addition also provides a route to adding another aromatic ring, which has been calculated for phenyl 22 and naphthalenyl. 23 Finally, addition of phenyl [24][25][26] and benzyne 26,27 have also been studied as a direct pathway to an additional aromatic ring.…”

The Reaction of o‐Benzyne with Vinylacetylene: An Unexplored Way to Produce Naphthalene

“…Since the C 6 H 5 +C 2 H 2 reaction is the key “acetylene addition” step in the HACA mechanism [13] of the PAH growth by an extra aromatic ring, it is important to see what alternative pathways o ‐C 6 H 4 can provide for this process. Thus, the potential of o ‐benzyne to contribute to the formation of large aromatic structures in combustion reactions has been recently recognized [14,15] . In particular, several studies considered the reactions of o ‐C 6 H 4 with resonantly stabilized free radicals (RSFR) as representing alternative pathways to PAH; the reaction with propargyl radicals was suggested to yield the indenyl radical, [16] whereas the reaction with allyl radicals was shown experimentally to almost cleanly form indene using the photoelectron photoion coincidence spectroscopy (PEPICO), which was supported by the theoretical electronic structure/rate theory calculations [17] .…”

Formation Mechanism of Benzo(a)pyrene: One of the Most Carcinogenic Polycyclic Aromatic Hydrocarbons (PAH)

“…Altogether, based on the above described mechanisms, it can be stated that in the last few decades the examinations of the molecular growth of small precursor molecules to large PAHs has become one of the central subjects in combustion chemistry [39,40]. The resultant numerous concepts were based mostly on experimental studies [14,29,41,42,43].…”