Which fits better?. Centered and non-Centered coordination modes of small endohedral Fullerenes, insights from Ti@C28 and Zr@C28

Abstract

The endohedral metallic fullerenes (EMF), exhibit the interplay between stabilizing interaction and steric hindrance effects owing to the internal size provided by the fullerene cage. The small C28 fullerene, offers the smallest cavity realized to encapsulate metal ions. The characterized energy minima structure for Ti@C28 exhibits a non-centered disposition of the Ti atom in a spherical cage, in contrast to Zr@C28, where Zr atom is located at the very center. Here, we explore variations of the 3d-Ti-π-C28 bonding interaction from a centered conformation towards the non-centered disposition in comparison to Zr@C28. Our results show that in such structures, a variation of both electrostatic and orbital character of the metal-cage interaction can be observed. Interestingly, the orbital term is largely favored underlying the more effective bonding in the latter conformation, playing a crucial role in the determination of the coordination mode. In addition, molecular dynamics simulations were employed to determine the endohedral atom variation at 300°K, denoting that Zr@C28 cage exhibits more structural variation in comparison to Ti@C28, owing to the movement of a more fitted metal center induces larger variations to the cage distance.