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

Abstract

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 11et. 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.

Atomic Structure, Electronic Properties, and Reactivity of In-Plane Heterostructures of Graphene and Hexagonal Boron Nitride

“…We calculated the value of 0.75 eV, in a good agreement with results from the literature. In accordance with recent reports, the enhanced electron density at the edges gives rise to stronger H binding at the polycyclic aromatic hydrocarbons compared to an ideal graphene sheet. , The DFT studies of H adsorption on the defect-free h-BN layer indicate a weak adsorption on the B sites, with the binding energy smaller than 0.05 eV. , The H adsorption on the N site is very unfavorable with the negative binding energy of −0.57 eV . Our calculations confirm these findings: we calculated the binding energies of −0.01 and −0.81 eV, for H chemisorption at B and N sites, respectively.…”

Generalized Reaction Mechanism for the Selective Aerobic Oxidation of Aryl and Alkyl Alcohols over Nitrogen-Doped Graphene

“…Fukui functions − were calculated using the Dmol software package , at the B3LYP , /DND level of theory. In our previous investigation, we used coronene as a model to mimic graphene due to computational restrictions; however, numerous computational studies have been previously undertaken to study the properties of graphene using different model systems. − For example, Krishnamurty et al have studied the site selectivity of graphene nanoflakes using various model structures and reactivity descriptors . In this study, various models of graphene as reported in previous studies were taken to benchmark the catalytic properties of graphene. ,, The models chosen for the present study are presented in Figure .…”

“…64 In this study, various models of graphene as reported in previous studies were taken to benchmark the catalytic properties of graphene. 61,63,65 The models chosen for the present study are presented in Figure 4. Both hydrogenation energies (E H ) and band gap of graphene were used as the screening parameter to select suitable graphene model for further investigation.…”

Electronic Properties and Stability of Graphene Oxyradical Systems

“…7) Some first-principle studies showed that edge chemical modification alter the electronic structure, stability and magnetic properties of graphene nanodots. 46) And some other studies demonstrated electron density and spin density being enhanced at the edge of graphene.…”

“…7) Table 1 and 2 summarized the energies of doublet, quarter and sextet spin multiplicity states for different isomers of 4a4z-O, 5a5z-O and 6a6z-O graphene oxyradicals. For the O-bonding to armchair edge, the doublet state is the lowest state in energy for a1-a5 sites of 4a4z-O system by comparing the energies of different states.…”