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

Abstract

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.