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

Abstract

The structure, stability, energy partition analysis, and charge redistribution of endohedral complexes formed between Au₃₂ 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>I</i>_<i>h</i> and the <i>C_s</i> symmetry structural forms [<i>I</i>_<i>h</i>-(X@Au₃₂)⁺ and <i>C</i>_<i>s</i>-(X@Au₃₂)⁺]. Smaller cations are found to be more stable in the <i>C_s</i> symmetry structure. A higher charge-transfer characteristic is noted when the Au₃₂ 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>I</i>_<i>h</i>-(X@Au₃₂) system, which indicates the free movements of these ions in the cage, whereas all dopant ions show positive IR frequency values for <i>C</i>_<i>s</i>-(X@Au₃₂), 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 Au₃₂ cage and retain the <i>I</i>_<i>h</i> symmetric structure of the Au₃₂ cluster, whereas the smaller-sized Li⁺ and Na⁺ occupy the off-center site and are more stable in <i>C</i>_<i>s</i>-(X@Au₃₂). Morokuma-type energy decomposition analysis data indicate a significantly higher Pauli repulsion energy with Li⁺ and Na⁺ dopants in the <i>C</i>_<i>s</i>-(X@Au₃₂) structure than in the <i>I</i>_<i>h</i>-(X@Au₃₂) structure; however, for other dopant ions, this interaction is almost the same for both <i>I</i>_<i>h</i> and <i>C</i>_<i>s</i> 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 <i>C</i>_<i>s</i> structure and is almost the same for other ions in the <i>C</i>_<i>s</i> and <i>I</i>_<i>h</i> structures. From the charge-transfer data, a halogen-like behavior of Au₃₂ 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 Au₃₂ fullerene may be one of the possible ways of detecting the elusive <i>I</i>_<i>h</i> structure of Au₃₂ by mass spectrometry.