Heptagons in C₆₈: Impact on Stabilities, Growth, and Exohedral Derivatization of Fullerenes

Density functional investigations have been performed on C₆₈ fullerene set and related exohedral derivatives. All of the heptagon-containing isomers are systematically studied, and a hepta-C₆₈ (namely, by C₆₈_id146:<i>C</i>₁) is predicted to be the most thermodynamically favorable framework, which accords with the experimental results. To clarify the growth mechanism from C₆₈ to C₇₀, a novel loop–loop connection between C₆₈ and C₇₀ has been proposed, and an unreported C₆₈_id9:<i>C</i>_<i>s</i> is proven to be the most favorable parent molecule to afford the well-known ^#8149C₇₀:<i>D</i>_5<i>h</i> with a barrier of 2.16 eV. In comparison, 6.01 eV is required if C₆₈_id146:<i>C</i>₁ is selected as the starting point. Besides the hollow cages, the negative formation energies and the large HOMO–LUMO gaps of C₆₈_id146X₆ (X: H, F, Cl) imply that hydrogenation, fluorination, and chlorination are all feasible ways to stabilize the carbon cage. Moreover, the regioselectivity and sequence of chlorination sites are discussed to elucidate the selective synthesis process related to experiments