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 (C6FnH(6−n), n = 1−6) have similar ring-LMO-NICSπzz 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-NICSπzzand ECRE values, unless extremely electronegative heteroatoms (e.g., Y = O+) are involved.