The effects of aqueous solvent and biological ligands on the structural and electronic properties of thiolate-protected Au25(SR)18(-) clusters have been studied by performing quantum mechanics/molecular mechanics (QM/MM) simulations. Analysis of bond distances and angles show that the solvated nanocluster experiences modest structural changes, which are reflected as flexibility of the Au core. The hydrophilic glutathione ligands shield the metallic core effectively and distort its symmetry via sterical hindrance effects. We show that the previously reported agreement between the calculated HOMO-LUMO gap of the cluster and the optical measurement is due to cancellation of errors, where the typical underestimation of the theoretical band gap compensates the effect of the missing solvent. The use of a hybrid functional results in a HOMO-LUMO gap value of 1.5 eV for the solvated nanocluster with glutathione ligands, in good agreement with optical measurements. Our results demonstrate that ligand/solvent effects should be considered for a proper comparison between theory and experiment.
Keywords: Car−Parrinello molecular dynamics; HOMO−LUMO gap; density functional theory; quantum mechanics/molecular mechanics; thiolate monolayer-protected gold clusters.