Is C-60 buckminsterfullerene aromatic?
C-60 does not have "superaromatic'' or even aromatic character, but is a spherically pi antiaromatic and enormously strained species. This explains its very large and positive heat of formation (610 +/- 30 kcal mol(-1)). The pi electron character of C-60 was analyzed by dissected nucleus independent chemical shifts (NICS). The results were employed to examine the scope and limitations of Hirsch's 2(N + 1)(2) spherical aromaticity rule for several globular cages. C-20(2+) (18 pi electrons) and C-60(10+) (50 pi electrons) are spherically pi aromatic, but C-20 (20 pi electrons) and C-60 (60 pi electrons) are spherically pi antiaromatic, due to the high paratropicity of their half-filled pi subshells. Limitations for Hirsch's rule, for clusters with more than 50 pi electrons, are illustrated by e.g. the pi aromaticity of the 66 pi electron C-60(6-) and the lack of pi aromaticity of the 72 pi electron C48N12 and C-60(12-)