Pu@C24: A New Example Satisfying the 32-Electron Principle

A transition in point group symmetry from C-2 to D-6d in the classical cage isomer of C-24 cluster is observed after encapsulation of a plutonium atom within it. This encapsulation leads to highly stable Pu@C-24 cluster, which is well supported by the structural, energetic, and thermodynamic aspects. This study has been carried out with first five low-lying isomers of C24 fullerene, among which one is classical fullerene consisting of only five- and six-membered rings (MR) and remaining four isomers are nonclassical fifflerene consisting of five and six MR along with varying number of four MR. The structural and stability aspects are investigated extensively for the Pu@C24 cluster. For the purpose of comparison we have also considered some other lanthanides/actinides with 8 valence electrons (Cm2+, Sm, and Gd2+) and investigated the encapsulation process into different C24 isomers. It has been found that the Pu@C24 cluster is the most stable one among all the metallofullerenes considered here. For Pu@C24, it has been observed that HOMO-LUMO gap is changed from 1.83 to 3.26 eV after encapsulation of a Pu atom into the bare classical C24 cage. It is interesting to note that the bare C24 nonclassical (C2 symmetry) cluster is the most stable one; however, after encapsulation, the classical isomer becomes the most stable with significant energy differences. The detail vibrational and electronic spectral studies have also been carried out for the classical Pu@C24 metallofullerene. High HOMO-LUMO gaps and positive binding energy values for the Pu@C24 clusters and negatives values of reaction enthalpy and free energy corresponding to the encapsulation process indicate toward possible formation of Pu@C24 and subsequent experimental detection. High stability of the Pu@C24 cluster can be rationalized through the fulfilment of 32 valence electron count corresponding to the fully occupied spdf shells for the central metal atom.