Chirality and electronic structure of the thiolate-protected Au ₃₈ nanocluster

Structural, electronic, and optical properties of the thiolate-protected Au₃₈(SR)₂₄ cluster are studied by density-functional theory computations (R = CH₃ and R = C₆H₁₃) and by powder X-ray crystallography (R = C₁₂H₂₅). A low-energy structure which can be written as Au₂₃at(Au(SR) ₂)₃(Au₂(SR)₃)₆ having a biicosahedral core and a chiral arrangement of the protecting gold-thiolate Au_x(SR)_y units yields an excellent match between the computed (for R = C₆H₁₃) and measured (for R = C ₁₂H₂₅) powder X-ray diffraction function. We interpret in detail the electronic structure of the Au₂₃ core by using a particle-in-a-cylinder model. Although the alkane thiolate ligands are achiral, the chiral structure of the ligand layer yields strong circular dichroism (CD) in the excitations below 2.2 eV for Au₃₆(SCH₃) ₂₄. Our calculated CD spectrum is in quantitative agreement with the previously measured low-energy CD signal of glutathione-protected Au ₃₈(SG)₂₄. Our study demonstrates a new mechanism for the strong chiral response of thiolate-protected gold clusters with achiral metal cores and ligands.