Polycyclic Aromatic Hydrocarbon Growth by Diradical Cycloaddition/Fragmentation

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.