How to Stabilize a Heptagon-Containing C₈₀ Cage by Endohedral Derivation

Nonclassical fullerene is a new member of the fullerene family. In the present work, a systematic investigation on LaSc_3-N@C₈₀ (x = 0-3) covering both classical and nonclassical C₈₀ cages was performed utilizing density functional theory combined with statistical mechanics. At absolute zero, LaSc₂N@Hept(6)-C(2)-C₈₀ with a heptagon-containing nonclassical carbon is the second most stable isomer, whereas at the temperature range of endohedral metallofullerene (EMF) formation, due to the large vibrational frequencies, LaSc₂N@Hept(6)-C(2)-C₈₀ is the third most abundant isomer, and its mole fraction is very low, accounting for the low experimental yield of LaSc₂N@Hept(6)-C(2)-C₈₀; La₂ScN@Hept(6)-C(2)-C₈₀, and La₃N@Hept(6)-C(2)-C₈₀ are the overwhelming isomers of the corresponding series, but compared with the cases of Sc₃N@C₈₀ and LaSc₂N@C₈₀, La₂ScN and La₃N clusters suffer much larger constraints from the C₈₀ cages, perhaps preventing the synthesis of La₂ScN@C₈₀ and La₃N@C₈₀ species. Because of the large mole fractions and large electron donation and back-donation of La₂ScN@Hept(6)-C(2)-C₈₀ and La₃N@Hept(6)-C(2)-C₈₀, it can be inferred that La₂ScN and La₃N clusters may be used to stabilize some other larger nonclassical fullerene cages. This work will provide useful insights into the origins of stabilization of nonclassical fullerene cages by endohedral derivation and guidelines for synthesis EMF with nonclassical cages.