Uranium Stabilization of C ₂₈ : A Tetravalent Fullerene

TLDR

Ab initio calculations predict a tetrahedral cage for C(28), yet the cluster remains electronically open shell. The study aims to create stable closed‑shell derivatives of C(28) by functionalizing its four tetrahedral vertices or encapsulating a tetravalent atom inside the cage. The authors produced an endothedral fullerene, U@C(28), using laser vaporization and carbon‑arc techniques. Laser vaporization experiments show that C(28) is the smallest fullerene formed in abundance, behaves as a hollow superatom with an effective valence of four, and that encapsulating uranium yields a stable U@C(28) derivative.

Abstract

Laser vaporization experiments with graphite in a supersonic cluster beam apparatus indicate that the smallest fullerene to form in substantial abundance is C(28). Although ab initio quantum chemical calculations predict that this cluster will favor a tetrahedral cage structure, it is electronically open shell. Further calculations reveal that C(28) in this structure should behave as a sort of hollow superatom with an effective valence of 4. This tetravalence should be exhibited toward chemical bonding both on the outside and on the inside of the cage. Thus, stable closed-shell derivatives of C(28) with large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps should be attainable either by reacting at the four tetrahedral vertices on the outside of the C(28) cage to make, for example, C(28)H(4), or by trapping a tetravalent atom inside the cage to make endothedral fullerenes such as Ti@C(28). An example of this second, inside route to C(28) stabilization is reported here: the laser and carbon-arc production of U@C(28).