ESI-MS Identification of Abundant Copper−Gold Clusters Exhibiting High Plasmonic Character

The protected noble-metal structures comprising 145 metal-atom sites and 60 ligands are among the frequently identified larger metal-cluster systems exploited in many avenues of research. Herein we report a comparative electrospray ionization-mass spectrometry (ESI-MS) investigation of the 60-fold thiolated Au144 and CuAu144 clusters, in various positive charge-states, in conjunction with a density-functional theoretical (DFT) analysis based upon the icosahedral Pd145-structure-type applicable to these systems. Samples rich in the hexanethiolate-protected CuAu144 clusters are obtained via a single-phase reduction process. The predicted electronic structure of the vacancy-centered Au144(SR)60 system provided a simple rationale for the limiting [4+] charge-state observed of Au144, whereas the maximal [3+] charge detected on the CuAu144(SR)60 cluster can be explained if the 145th atom occupies the central site. Occupancy of the center-site stabilizes the superatomic 3S-orbital, and thereby shifts the shell-c...