Polyradicals of Polycyclic Aromatic Hydrocarbons as Finite Size Models of Graphene: Highly Open-Shell Nature, Symmetry Breaking, and Enhanced-Edge Electron Density

Properties of polyradicals (all CH bonds dissociated) of benzene and certain polycyclic aromatic hydrocarbons (PAHs) were studied. The occurrence of symmetry breaking is revealed in going from benzene and the PAHs to their polyradicals. Polyradicals would serve as finite size models of graphene with unpassivated edges in a more realistic way than the PAHs. Monoradicals (one CH bond dissociated) of benzene and all of the PAHs and higher radicals of benzene and one PAH (two to all CH bonds successively dissociated) were also investigated. Reliability of the methodology employed was ascertained by a comparison of our calculated single CH bond dissociation energy of benzene with the available previous experimental and theoretical results. Besides ground-state geometries, the aspects studied include single and successive CH bond dissociation energies, and electron density, molecular electrostatic potential (MEP), and spin density distributions. All of the monoradicals studied were found to have doublet spin multiplicity, while polyradicals with 4 to 16 rings and zigzag or mixed-type edges were found to have spin multiplicities varying from triplet to I let. Bond lengths and bond angles of rings located at the edges are appreciably modified in going from PAHs to polyradicals. Electron density and spin density are found to be enhanced at the edges of monoradicals and polyradicals of PAHs, as found previously for PAHs. However, MEP maps of polyradicals have significantly different features from those of monoradicals and PAHs, which has a significant implication.