On the structure of Au₁₁(SR)₉ and Au₁₃(SR)₁₁ clusters

Thiolated gold clusters are constituted by building blocks (Au₄, Au₆, Au₁₂ and so on) and protected by staple motifs (-S-Au-S-Au-S-…). In this study, we propose the structure of Au₁₁(SR)₉ and Au₁₃(SR)₁₁ clusters that are in the synthesis route of the ubiquitous Au₁₅(SR)₁₃ cluster. Our DFT-D calculations support one triangular Au₃ unit as the smallest one comprising the structure of the Au₁₁(SR)₉ cluster, while it competes with the Au₄ unit found in the Au₁₃(SR)₁₁ cluster. The ligand effects on the electronic, optical and chiroptical properties were studied by considering H, CH₃, phenyl and adamantyl as protecting ligands. In the case of the Au₁₁(SR)₉ cluster, its Au₃ inner core is protected by one dimer motif and one [Au₆(SR)₆] cyclomer when H and CH₃ were considered as ligands, and the preference for Au₃ over Au₄ inner core was calculated to be 0.042 (H), 0.190 (CH₃), and 0.117 eV (adamantyl). In contrast, the preference for one Au₄ core increased when using phenyl ligands (0.23 eV energy difference) and dimer and pentamer motifs. Moreover, the Au₁₃(SR)₁₁ cluster (R = CH₃) has one Au₄ inner core and is protected by the combination of cyclomer, monomer and dimer motifs, and the isomer containing one Au₃ inner core and protected by one tetramer and one [Au₆(SR)₆] cyclomer is 0.170 eV less stable. This implies that the Au₃ unit is important in these small sizes and that the energetic preference depends on the used ligand types. Moreover, we discuss the IR/Raman, optical absorption (UV-vis), and circular dichroism (CD) spectra of our predicted new structures.