The effect of perfluorination on the aromaticity of benzene and heterocyclic six-membered rings.

Despite having six highly electronegative F's, perfluorobenzene C(6)F(6) is as aromatic as benzene. Ab initio block-localized wave function (BLW) computations reveal that both C(6)F(6) and 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 (C(6)F(n)H((6-n)), n = 1-6) have similar ring-LMO-NICS(pi zz) 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 C(5)H(5)Y heterocycles (Y = BH(-), N, NH(+)) are as aromatic as benzene, based on their ring-LMO-NICS(pi zz) and ECRE values, unless extremely electronegative heteroatoms (e.g., Y = O(+)) are involved.