C74 endohedral metallofullerenes violating the isolated pentagon rule: a density functional theory study

Abstract

Precise studies on M₂@C₇₄ (M = Sc, La) series by means of DFT methods have disclosed that certain non-IPR isomers are more stable than the IPR structure. M₂@C₂(13295)-C₇₄ and M₂@C₂(13333)-C₇₄, both of which have two pentagon adjacencies (PA), present excellent thermodynamic stability with very small energy differences. Statistical mechanics calculations on the M₂@C₇₄ series reveal that M₂@C₂(13295)-C₇₄ and M₂@C₂(13333)-C₇₄ are quite favoured by entropy effects below 3000 K. Sc₂@C₇₄ and La₂@C₇₄ series are found to have similar electronic transfer but different electronic structures due to the distinct properties of scandium and lanthanum elements according to Natural Bond Orbital (NBO) analysis in conjunction with orbital interaction diagrams. Investigations of bonding energies reflect quite different influences of the two types of metal atoms to C₇₄ metallo-fullerenes. Further examinations on C₇₄ metallo-fullerenes uncover significant stabilization effects of metal atoms acting on PA fragments. Geometrical structures of certain non-IPR cages (from C₇₂ to C₇₆), which exhibit splendid stabilities when encapsulating metallo-clusters, are found to be related by Stone–Wales transformation and C₂ addition. Furthermore, IR spectra and ¹³C NMR spectra of M₂@C₂(13295)-C₇₄ and M₂@C₂(13333)-C₇₄ have been simulated to assist further experimental characterization.