Is the largest aqueous gold cluster a superatom complex? Electronic structure & optical response of the structurally...

The new water-soluble gold cluster Au₁₄₆(p-MBA)₅₇, the structure of which has been recently determined at sub-atomic resolution by Vergara et al., is the largest aqueous gold cluster ever structurally determined and likewise the smallest cluster with a stacking fault. The core presents a twinned truncated octahedron, while additional peripheral gold atoms follow a C₂ rotational symmetry. According to the usual counting rules of the superatom complex (SAC) model, the compound attains a number of 92 SAC electrons if the overall net charge is 3- (three additional electrons). As this is the number of electrons required for a major shell closing, the question arises of whether Au₁₄₆(p-MBA)₅₇ should be regarded as a superatom complex. Starting from the experimental coordinates we have analyzed the structure using density-functional theory. The optimized (relaxed) structure retains all the connectivity of the experimental coordinates, while removing much of its irregularities in interatomic distances, thereby enhancing the C₂-symmetry feature. On analyzing the angular-momentum-projected states, we show that, despite a small gap, the electronic structure does not exhibit SAC model character. In addition, optical absorption spectra are found to be relatively smooth compared to the example of the Au₁₄₄(SR)₆₀ cluster. The Au₁₄₆(SR)₅₇ does not derive its stability from SAC character; it cannot be considered as a superatom complex.