Chirality and Electronic Structure of the Thiolate-Protected Au-38 Nanocluster

Structural, electronic, and optical properties of the thiolate-protected Au-38(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 Au-23@(Au(SR)(2))(3)(Au-2(SR)(3))(6) having a bi-icosahedral core and a chiral arrangement of the protecting gold thiolate Au,,(SR), units yields an excellent match between the computed (for R = C6H13) and measured (for R = C12H25) powder X-ray diffraction function. We interpret in detail the electronic structure of the Au-23 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 Au-38(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.