On the Importance of Clar Structures of Polybenzenoid Hydrocarbons as Revealed by the π-Contribution to the Electron Localization Function

The degree of pi-electron (de) localization and aromaticity of a series of polybenzenoid hydrocarbons (PBHs) has been analyzed through the pi-contribution to the electron localization function (ELF pi), calculated at the B3LYP/ 6-311G(d, p) hybrid density functional theory level. The extent of. -electron delocalization in the various hexagons of a PBH was determined through analysis of the bifurcation values of the ELF. basins (BV(ELF.)), the spans in the bifurcation values in each hexagon (BV(ELF pi)), and the ring-closure bifurcation values of the ELF pi (RCBV(ELF pi)). These computed results were compared to the qualitative description of local aromaticities of the different hexagons in terms of Clar structures with pi-sextets. Benzene, [18] annulene, and thirty two PBHs were analyzed at their equilibrium geometries, and benzene and triphenylene were also analyzed at bond length distorted structures. In general, the description of PBHs in terms of Clar valence structures is supported by the ELF pi properties, although there are exceptions. For PBHs at their equilibrium geometries there is a clear sigmoidal relationship between the CC bond lengths and the amount of pi-electron (de) localization at these bonds, however, this relationship is lost for bond distorted geometries. In the latter cases, we specifically examined benzene in D3h symmetric 1,3,5-cyclohexatriene structures and triphenylene in eight different structures. From the distorted benzenes and triphenylenes it becomes clear that there is a distinct tendency for the pi-electron network to retain delocalization (aromaticity). The ELF. analysis thus reveals an antidistortive rather than a distortive behavior of the pi-electrons in these investigated compounds.