Effect of Aliphatic versus Aromatic Ligands on the Structure and Optical Absorption of Au20(SR)16

Despite the recent determination of the crystal structure of the Au-20(TBBT)(16) (TBBT = SPh-t-Bu) nanocluster, it is not certain if Au-20(SR)(16) adopts the same structure when R is an aliphatic ligand. To this end, we perform a theoretical investigation using density functional theory (DFT) and time-dependent DFT (TDDFT). We optimize a methylthiolate version of the new crystal structure geometry of Au-20(TBBT)(16) and compare the stability and optical properties with the three lowest energy isomers of Au-20(SCH3)(16) predicted previously. Furthermore, TDDFT calculations are performed for the Au-20(TBBT)(16) structure determined via X-ray crystallography and the related Au-20(SPh)(16) nanocluster. All the calculated optical absorption spectra are compared against the available experimental spectrum of Au-20(PET)(16) (PET = SCH2CH2Ph) and Au-20(TBBT)(16). We observe that the new geometry of the Au-20(SCH3)(16) nanocluster with an Au-7 core is more stable than the previous isomers.