Towards graphite: Magnetic properties of large polybenzenoid hydrocarbons

The geometries of four different series of D-6h-symmetric polybenzenoid hydrocarbons (PBH) up to and including C222H42 have been optimized at the B3LYP/6-31 G(d) level of theory. Excluding C48H24 and C138H42, which have D-3d minima due to 1,5 (HH)-H-... repulsions between adjacent perimeter rings, optimized geometries are planar D-6h minima. Nucleus Independent Chemical Shifts (NICS), at the same level, indicate the presence of individual aromatic rings, which correspond to Clar's qualitative sextets rule (Clar, E. The Aromatic Sextet, Wiley: London, 1972). NICS and the Clar valence electron topologies agree perfectly in the molecule plane; however, the NICS values computed in parallel planes further away from the molecular surface converge, indicating the presence of a uniform magnetic shielding field. For each series, PBH total NICS values (i.e., the sum of NICS values for all rings in the PBH) correlate linearly with the number of carbon atoms, indicating constant magnetic field development within a series. The C-C lengths depend on their proximity to the more olefinic rich molecular perimeters. However, the large PBH (C48H24) internal C-C distances converge to similar to1.426 Angstrom. In agreement with Clar's rule, HF/6-31 G(d)//B3LYP/6-31 G(d) vertical ionization potentials and B3LYP/6-31 G(d) HOMO-LUMO gaps are largest within the "fully benzenoid" series, where all carbon atoms are members of a single sextet. The largest members of the four series studied are predicted to exhibit semiconducting properties.