Electronic, vibrational and optical properties of two-electron atoms and ions trapped in small fullerene-like cages

The problem of atoms and molecules caged inside fullerenes has attracted renewed interests since a new endohedral species has been experimentally realized (Bloodworth et al 2019 Angew. Chem., Int. Ed. 58 5038). In this sense, detailed theoretical studies on the spectroscopic properties of atoms and ions spatially confined in fullerene-like structures are convenient. Here we perform density functional theory (DFT) and time-dependent DFT (TDDFT) calculations to investigate the electronic, vibrational and optical properties of two-electron atomic systems, X, caged in C-20 and C20H20 endohedral complexes; i.e. X@C-20 and X@C20H20 (X = He, Li+, and Be++). Among these endohedral complexes, only the encapsulated Be++ ion gives rise to strongly bound complexes, whereas the encapsulated Li+ ion depends on the confining environment, and the encapsulated He atom seems to be highly repulsive in both types of cages. Our calculated excitation energies indicate that the lowest-lying singlet states strongly depend on both the nature of the endohedral atom/ion and the type of the carbon cage. Although He@C20H20 and He@C-20 are obtained as repulsive complexes, they produce a small effect in the absorption spectra of the complexes. However, the presence of Li+ or Be++ in the endohedral complexes dramatically changes the electronic absorption profile of these cages. Overall, this study shows that the confinement of a Be++ ion in a very restricted space is energetically favorable, being its quantum states controllable by the confining environment.