DOI:10.1021/jp902983r

The Effect of Perfluorination on the Aromaticity of Benzene and Heterocyclic Six-Membered Rings

Despite having six highly electronegative F’s, perfluorobenzene C₆F₆ is as aromatic as benzene. Ab initio block-localized wave function (BLW) computations reveal that both C₆F₆and benzene have essentially the same extra cyclic resonance energies (ECREs). Localized molecular orbital (LMO)-nucleus-independent chemical shifts (NICS) grids demonstrates that the F’s induce only local paratropic contributions that are not related to aromaticity. Thus, all of the fluorinated benzenes (C₆F_nH_(6−n), n = 1−6) have similar ring-LMO-NICS_πzz values. However, 1,3-difluorobenzene 2b and 1,3,5-trifluorobenzene 3c are slightly less aromatic than their isomers due to a greater degree of ring charge alternation. Isoelectronic C₅H₅Y heterocycles (Y = BH⁻, N, NH⁺) are as aromatic as benzene, based on their ring-LMO-NICS_πzzand ECRE values, unless extremely electronegative heteroatoms (e.g., Y = O⁺) are involved.