Enhancement in the Stability of 36-Atom Fullerene through Encapsulation of a Uranium Atom

With an objective to rationalize the experimentally observed intense U@C-36 peak in the mass spectrum of U@C-2n metallofullerene, structural, stability, and spectroscopic aspects of the uranium doped C-36 fullerene have been studied in a unified and systematic way using density functional theory (DFT) and its time-dependent variant. Relativistic effects have been taken into account within the framework of zeroth-order regular approximation using scalar and spin-orbit-based approaches. Among all of the 15 possible classical isomers reported for the C-36 fullerene cage system, singlet D-2d and triplet D-6h structures are found to be isoenergetic and most stable. Encapsulation of uranium atom into various C-36 cages leads to 15 distinct isomers with considerable energy differences. It has also been shown that this encapsulation process results in significant gain in thermodynamic stability. The most stable U@C-36 isomer is found to be associated with C-6v symmetry and closed-shell electronic configuration, derived from the open-shell D-6h structure of C-36. The next stable isomer is associated with C-s symmetry and obtained from the corresponding singlet D-2d structure of the C-36 cage. Distinct changes have also been found in the calculated vibrational and UV-visible spectra of the U@C-36 cluster as compared to the corresponding bare C-36 cage. All of the calculated quantities reported here suggest that the stability of the U@C-36 duster is high enough for possible formation of cluster-assembled material leading to synthesis of this metallofullerene experimentally.