Ab initio study of thiolate-protected Au₁₀₂ nanocluster

A total structural determination of the Au₁₀₂ (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 Au₁₀₂ cluster may be viewed as a multilayered structure described as Au₅₄ (penta-star)ΑU₃₈ (ten wings)Αu₁₀(two pentagon caps), where the inner Au₅₄ "penta-star" consists of five twinned Au₂₀ tetrahedral subunits. To gain more insight into high stability of the Au₁₀₂ (p-MBA)₄₄ nanocluster, we have performed ab initio calculations to study electronic properties of a homoloque Au₁₀₂ (SCH₃)₄₄ nanocluster, an Au₁₀₂ (SCH₃)₄₂ nanocluster (with two SCH₃ groups less), and an "effectively isoelectronic" Au₁₀₄ (SCH₃)₄₆ nanocluster with a more symmetric embedded Au₁₀₄ structure. Electronic structure calculations suggest that the Au₁₀₂ (SCH₃)₄₄ nanocluster possesses a reasonably large gap (∼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 (∼0.65 eV) of the bare Au₅₈ cluster. Likewise, the Au₁₀₄ (SCH₃)₄₆. nanocluster has a HOMO-LUMO gap of ∼0.51 eV, comparable to that of Au₁₀₂ (SCH₃)₄₄ nanocluster. In contrast, the Au₁₀₂ (SCH₃)₄₂ nanocluster has a zero HOMO-LUMO gap. These results confirm that high stability of the Au₁₀₂ (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 AU₂₅ (SCH₂ CH₂Ph)₁₈^- cluster whose high stability may be attributed to the electronic shell closing of effective 8 (= 26-18) valence electrons.