Role of Au4 Units on the Electronic and Bonding Properties of Au28(SR)20 Nanoclusters from X-ray Spectroscopy

The emergence of FCC-ordered core structures in thiolate-protected gold nanoclusters has motivated researchers in this area to reconsider the possibility of nonicosahedral core frameworks. With the recent elucidation of a four-membered FCC-ordered core series (Au-20(SR)(16), Au-28(SR)(20), Au-36(SR)(24), and Au-44(SR)(28)), it is now of significant interest to understand how their electronic and bonding properties are influenced by the cluster composition and how they differ from, or may be linked to, their icosahedral counterparts. Of recent, attention has been focused on the presence of small Au-4 units, which comprise the majority of the known FCC-ordered thiolate-protected gold nanocluster core structures. Herein, the Au-28(SR)(20) nanocluster with a 20 Au atom FCC-ordered core is investigated with temperature-dependent X-ray absorption spectroscopy experiments and ab initio simulations to elucidate the bonding and electronic properties of the second member of the FCC-ordered core series. By comparing the results of Au-28(SR)(20) with its larger FCC-ordered relative, Au-36(SR)(24), and its icosahedral counterpart, Au-25(SR)(18), the significant role of Au-4 core units on the bonding properties of FCC-ordered gold nanoclusters is highlighted in this work.