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 C 28 fullerene, offers the smallest cavity realized to encapsulate metal ions. The characterized energy minima structure for Ti@C 28 exhibits a non-centered disposition of the Ti atom in a spherical cage, in contrast to Zr@C 28, where Zr atom is located at the very center. Here, we explore variations of the 3d-Ti-π-C 28 bonding interaction from a centered conformation towards the non-centered disposition in comparison to Zr@C 28. 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@C 28 cage exhibits more structural variation in comparison to Ti@C 28, owing to the movement of a more fitted metal center induces larger variations to the cage distance.