Calculations by the quantum chemical method DFT using U-B3LYP, 6-31G (d, p) have showen that the electronic ground state (EGS) of ideal hexagon-shape carbon nanoclusters (CNC) C54 – C294 is not singlet. Their equilibrium spatial structure is such that the 2pz-orbitals of the atoms of the boundary cyclic chain (BCC) form a conjugate system weakly bound to the π-system of the inner part of the cluster, which allows it to be considered relatively independent. The distributions of the molecular electrostatic potential for their EGS have significant anisotropy, determining their electron and proton donor properties. The spectrum of one-electron energy levels in them is such that the molecular orbitals (MO) distributed along the BCC bonds remain vacant, although the corresponding energies are lower than the energies of some occupied frontier MOs. For small CNC of high symmetry, the usual dependence of the band gap on the number of atoms is not realized, due to the presence in them of twice-coordinated carbon (C(2)) atoms.
The properties of CNCs and similar in structure and gross composition of polyaromatic molecules (PAM) differ significantly. This is especially true for the high-spin states of CNCs and the presence of two loosely coupled systems.
For defect-containing clusters obtained from C96 CNC of ideal hexagonal shape by removing one (C96-1(1)) or two non-adjacent (C96-2(1)), or one (C96-1(2)) or two (C96-2(2)) non-adjacent pairs of carbon atoms, it is found that, despite the even number of electrons in them, the GES is not singlet. A similar conclusion is valid for systems that are obtained by removing from PAM C96H24 of one or two non-adjacent carbon atoms.
The EGS of C95N isomers obtained from C96 CNC by replacing one carbon atom with a nitrogen atom (CNC-N) is in some cases, dependent on the position of the N atom, not a doublet. The most stable of the C95N clusters are those in which the Nitrogen atom occupies a pyridine position at one of the zigzag edges. The maximum of the core level line N1s in the calculated density spectra of single-electron states of CNC C95N, C94N2 and C93N3 is characterized by a positive chemical shift relative to the position of this line in the reference nitrogen-containing compound – methylamine. The magnitude of the chemical shift of the core level N1s is the smallest for the pyridine arrangement of the nitrogen atom and increases as the distance of the N atom embedded in the graphene matrix from the zigzag edge is removed.
The reaction of dissociative adsorption of molecular hydrogen on nitrogen- and boron-containing graphene-like matrices is exothermic, and for models of pure carbon graphene-like matrix, the calculated data indicate a low probability of chemical adsorption of the H2 molecule on normal conditions.
Regardless of the size of the polyaromatic molecule, for a pure carbon analogue, the energy value of the lower vacant molecular orbital has the lowest absolute value and the highest for boron-containing, indicating the highest electron acceptor capacity and therefore oxidative capacity relative to the hydrogen molecule as a reducing agent.
The catalytic activity of nitrogen-doped graphene in the oxygen reduction reaction (ORR) can be explained by a combination of several factors, namely the reduction of the band gap in nitrogen-containing clusters compared to the original, purely carbon, cluster and the activation of carbon atoms by the neighbor nitrogen atoms.
In the proposed consideration of ORR, there was used one of the possible structures that are formed by doping the original pure carbon graphene matrix. At other mutual arrangements of nitrogen atoms other types of activation of carbon atoms which are neighbors of the N atoms embedded in a matrix are possible dependent on the microstructure of the active sites formed during the chemical production of nitrogen-doped samples of graphene.
Keywords: graphene, CNC, DFT method, vacancies, defectcontaining clusters, spin states, doping with N and B atoms, ORR.
