Ab initio study of thiolate-protected Au102 nanocluster

A total structural determination of the Au-102(p-MBA)(44) nanocluster has been recently achieved via successful crystallization of the thiolated-protected gold nanocluster (Jadzinsky et al. Science 2007, 318, 430). The embedded Au-102 cluster may be viewed as a multilayered structure described as Au-54(penta-star)@Au-38(ten wings)@Au-10(two pentagon caps), where the inner Au-54 "penta-star" consists of five twinned Au-20 tetrahedral subunits. To gain more insight into high stability of the Au-102(p-MBA)(44) nanocluster, we have performed ab initio calculations to study electronic properties of a homologue Au-102(SCH3)(44) nanocluster, an Au-102(SCH3)(42) nanocluster (with two SCH3 groups less), and an "effectively isoelectronic" Au-104(SCH3)(46) nanocluster with a more symmetric embedded Au-104 structure. Electronic structure calculations suggest that the Au-102(SCH3)(44) nanocluster possesses a reasonably large gap (similar to 0.54 eV) between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO gap), which is comparable to the measured HOMO-LUMO gap (similar to 0.65 eV) of the bare Au-58 cluster. Likewise, the Au-104(SCH3)(46) nanocluster has a HOMO-LUMO gap of similar to 0.51 eV, comparable to that of Au-102(SCH3)(44) nanocluster. In contrast, the Au-102(SCH3)(42) nanocluster has a zero HOMO-LUMO gap. These results confirm that high stability of the Au-102(p-MBA)(44) nanocluster may be attributed in part to the electronic shell closing of effective 58 (= 102 - 44) valence electrons, as in the case of Au-25(SCH2CH2Ph)(18) - cluster whose high stability may be attributed to the electronic shell closing of effective 8 (= 26 - 18) valence electrons.