Endohedrally Doped Golden Fullerenes [X@Au32] (X = Li+, Na+, K+, Rb+, Cs+)

The structure, stability, energy partition analysis, and charge redistribution of endohedral complexes formed between Au 32 cluster with alkali metal cations, such as Li +, Na +, K +, Rb +, and Cs +, have been investigated using the density functional theory method. All alkali metal cations can be stably encapsulated in both the I h and the C s symmetry structural forms [ I h -(X@Au 32 ) + and C s -(X@Au 32 ) + ]. Smaller cations are found to be more stable in the C s symmetry structure. A higher charge-transfer characteristic is noted when the Au 32 cage is encapsulated with smaller-sized alkali ions than larger-sized ions in both the structures. Vibrational frequency calculation data show imaginary frequency values for Li + and Na + ions in the I h -(X@Au 32 ) system, which indicates the free movements of these ions in the cage, whereas all dopant ions show positive IR frequency values for C s -(X@Au 32 ), suggesting the existence of weak bonding between dopants and the cage. The analysis of structural, energetic, and charge-transfer data indicates that the K +, Rb +, and Cs + dopant ions occupy the central position of the Au 32 cage and retain the I h symmetric structure of the Au 32 cluster, whereas the smaller-sized Li + and Na + occupy the off-center site and are more stable in C s -(X@Au 32 ). Morokuma-type energy decomposition analysis data indicate a significantly higher Pauli repulsion energy with Li + and Na + dopants in the C s -(X@Au 32 ) structure than in the I h -(X@Au 32 ) structure; however, for other dopant ions, this interaction is almost the same for both I h and C s structures. A similar trend is reflected in the variation in the orbital interaction energy term, which is considerably higher for Li + and Na + ions in the C s structure and is almost the same for other ions in the C s and I h structures. From the charge-transfer data, a halogen-like behavior of Au 32 is noted when the cluster is doped with the Cs + ion, which is rather unusual. Thus, encapsulation of larger cations, such as K +, Rb +, and Cs +, into the Au 32 fullerene may be one of the possible ways of detecting the elusive I h structure of Au 32 by mass spectrometry.