Mechanism of formation of four-ring polycyclic aromatic hydrocarbons in the self-recombination of indenyl

The geometric structures, vibration frequencies and relative energies of reactants, products, intermediates, and transients involved in the self-recombination of the indenyl radical were determined using G3(MP2,CC) // B3LYP/6-311G${}$ quantum chemical calculations. The barrierless association of a pair of indenyl radicals forms the $\mathrm{C}_{18}\mathrm{H}_{14}$ complex. The subsequent set of isomerizations of the complex is divided into five reaction channels, which in all cases end in $\mathrm{H}$ abstraction but with different four-ring isomers $\mathrm{C}_{18}\mathrm{H}_{14}$: in the form of condensed rings-tetraphene, tetracene, chrysene, dibenzoazulene; with an associated internal bond of the rings-dibenzofulvalene. The yield of chrysene prevails since the energy barriers encountered on the pathway of its formation are lower than the barriers on the formation pathways of other products.