Aromaticity of [60]fullerene derivatives (C60Xn, X = H, F; n = 18, 36) constrained to have planar hexagonal rings

Abstract

Density functional calculations accurately reproduce the known bond lengths for C₆₀F₁₈, shown recently (single crystal X-ray structure) to possess a fully aromatic hexagonal ring, and they correctly predict the planarity of this ring. Isostructural C₆₀H₁₈ is also calculated to have an aromatic benzenoid hexagonal ring, and to be marginally less planar. The four benzenoid hexagonal rings of T symmetry C₆₀H₃₆ and C₆₀F₃₆, and the three benzenoid rings of C₃ symmetry C₆₀H₃₆ and C₆₀F₃₆ are predicted to be aromatic, with both the T symmetry and the fluorinated species again having the slightly shorter bond lengths and the greater planarity; by these measures T-C₆₀F₃₆ is as aromatic as C₆₀F₁₈. In another C_3v isomer of C₆₀H₁₈, which had been predicted (AM1) to be thermodynamically more stable than the known isomer, density functional calculations show the central benzenoid ring to have slightly greater bond alternation, greater bond lengths, to be less planar and the molecule to be overall less stable than the known isomer; this is attributed to the greater strain in the three adjacent pentagonal rings. Sites for ether formation are conjectured based on the recent finding that oxygen inserts into long FC–CF bonds of fluorofullerenes.