Cyclosulfurization of C60 and C70 fullerenes: a DFT study

We have applied DFT calculations to investigate the cyclosulfurization of the hexagon-hexagon (h-h) C-C bonds of C-60 and C-70 fullerenes. Pentathiepin rings formed on the fullerene surfaces show chair-like structure and a high energy barrier (37 kcal mol(-1)) during ring inversion. The carbon atoms relax outwardly from the surface of the fullerene in the attached sites. This leads to the flattened polar caps and polygonal cross section at the equatorial belt of C-70 when all the h-h C-C bonds at the polar caps are functionalized. Reaction energies E-r are obtained to be negative, which indicate the exothermic character of the cyclosulfurization processes. Generally, the cyclosulfurization reaction of h-h bonds of C-60 and h-h bonds at the polar caps of C-70 is more energetically favorable, due to the higher curvature of carbon sites facilitating the formation of pentathiepin rings on the fullerene surface. For the successive cyclosulfurization of fullerenes, the E-r per addend show no considerable change in the C-60 derivatives while because of steric repulsion between addends the E-r per addend increase in the C-70 derivatives with two pentathiepin rings. The cyclosulfurization leads to an increase in the electrophilicity of the functionalized fullerenes. Moreover, the electrophilicity of the C-70 derivatives in which pentathiepin rings are bound to the equatorial h-h bonds is always greater than those at the polar caps.