Intermediates for Larger Endohedral Metallofullerenes: Theoretical Characterization of M@C44 Species

Endohedral metallofullerenes M@C44 containing several different endohedral species have been considered as intermediates in the path to form larger species. Such compounds containing interstitial atoms of groups 3 and 4, recently detected in experiments, are studied theoretically for the first time. Calculations carried out at a dispersion-corrected density functional theory level agree admirably well with experimental data for C44 and its endohedral compounds. The most suitable C44 isomer to form endohedral compounds is the D2 (89) isomer. The binding energy between the endohedral atom and the cage is a good indicator of the abundance found in synthesis. The properties of the endohedral compounds of C44-D2 (89) can be compared directly with those of the tri- and tetraanions of empty C44. In addition, the electron-richest regions in all of them are the four triple sequentially fused pentagon units. The centroids of the central pentagons of each such unit are approximately disposed in a seesaw structure ar...