Theoretical Approaches To Study Graphene-like Nanopatches And Model Systems

The polycyclic aromatic hydrocarbons (PAHs) have been the focal point of research activities related to hydrocarbon fuels for the last thirty years. The interest is great because incomplete combustion of organic fuels is a costly side effect resulting in discharge into atmosphere of harmful particles of carbon black and soot. By understanding the underlying reaction mechanisms of combustion one could improve on the efficiency of the carbon fuel burning process and at the same time use less of this precious natural resource. Graphene, a two dimensional sheet of sp2-hybridized carbons regularly spaced from each other in honeycomb-like hexagons, has attracted much attention of both experimental and theoretical physicists and chemists because of its unique electronic properties and potential applications. Graphene is related to many systems and processes of high applied and fundamental relevance, e.g., oxidation of fossil fuels, basic processes of battery electrodes and conductivity in two-dimensional systems.In order to investigate oxidation reactions of very large polyaromatic hydrocarbons the least computationally expensive yet reliable theoretical approach has been established. Several commonly used DFT and MP2 methods with various basis sets have been benchmarked against experimental and high level theory data available for small benzoid molecules: benzene, phenol, toluene, naphthalene, naphthol and their dehydrogenated and oxygenated radicals. The properties tested were: energies, geometries, frequencies, zero-point vibrational energies and bond dissociation energies. The best overall results are demonstrated by B3LYP method. This method was successfully tested on larger PAHs for its capability to predict thermodynamic stability of PAHs oxidation intermediates. In addition, it correctly described two phenomena large graphene like PAHs show: decreasing of the band gap and ground electronic state as a multiradical state. The relative stability of linear pentacene oxyradicals can be explained by fragmentation of the delocalizedπ-electron system of the precursor pentacene molecule. The relative energies of oxyradicals with different placement of O depend on the amount of locallyπ-aromatic fragments formed and their nature. The fragments formed can be readily related to the reference aromatic hydrocarbons of benzene (prototypical system with odd number of six-atom rings) and naphthalene (prototypical system with even number of six-atom rings). Relative energies of linear oxyradicals show linear dependency of the cumulative HOMA aromaticity measure. This relation can be useful for quickly assessing the thermodynamic stability of oxyradicals for arbitrary-size graphene edges.A family of small graphene patches, i.e., rectangular polyaromatic hydrocarbons (PAHs), that have both zigzag and armchair edges is investigated to establish their ground state electronic structure. Broken symmetry density functional theory (DFT) and plane wave DFT were used to characterize the onset of diradical character via relative energies of open-shell and closed-shell singlet states. The perfect pairing (PP) active space approximation of coupled cluster theory was used to establish diradical character on the basis of promotion of electrons from occupied to unoccupied molecular orbitals. The role of zigzag and armchair edges in the formation of open-shell singlet states is elucidated. It is found that elongation of the zigzag edge results in an increase of diradical character whereas elongation of the arm chair edge leads to a decrease of diradical character. Analysis of orbitals from PP calculations suggests that diradical states are formally Mobius aromatic multiconfigurational systems. As a prelude to exploring the relative energies and thermodynamic stabilities of graphene-edge oxidation, the homolytic O-H bond dissociation energy (BDE) of phenol was determined from diffusion Monte Carlo (DMC) calculations using single determinant trial wave functions. The phenol O-H BDE using DMC with restricted Hartree-Fock orbitals and restricted B3LYP Kohn-Sham orbitals are in good agreement with previous theoretical and experimental findings.