LOCAL AROMATIC CHARACTER OF C-60 AND C-70 AND THEIR DERIVATIVES

The ambiguity of the aromatic character of C-60 and C-70 may result fr om two reasons: (i) aromaticity is a multidimensional phenomenon and ( ii) the local aromatic character of the molecular fragment in question may vary considerably depending on its topological and chemical envir onment. Application of the aromaticity index HOMA, based on experiment al bond lengths, supported (ii): pentagons in fullerenes and their der ivatives exhibit very weak or even antiaromatic character, with HOMA v alues ranging from -0.26 to 0.25. In contrast to that, hexagons exhibi t a much larger variation in aromatic character, with HOMA values rang ing from 0.1 (anion of C-60) to 0.76 in C-70. 6S(8) Pentagons are less aromatic than hexagons, except in the case of C-60(-), and C-60 is in significantly less aromatic than C-70; both conclusions are in line wi th magnetic susceptibility studies, the latter also remains in agreeme nt with enthalpy of formation measurements.