Electronic structure studies of six-atom gold clusters

Combined theoretical and experimental studies of the hexagold phosphine-stabilized complex [Au-6(PPh3)(6)][BF4](2) (1) and of related systems are reported. The goal of these studies is to gain a better understanding of how 1 interacts with the TiO2(110) substrate to yield finely dispersed supported Au particles that are effective for practical catalytic reactions. The experimental efforts involved the measurement of the visible-ultraviolet (UV) absorption spectra of 1 and Au(PPh3)Cl in solution. The theoretical efforts involved the determination of the electronic structure of molecular models of 1 based on density functional theory (DFT), Hartree-Fock (HF), and configuration interaction (CI) methods. The CI wave functions and energies were obtained for a range of excited states and were used to simulate the absorption spectra of Au-6 and Au-6(2+) clusters. The theoretical CI absorption spectra for Au-6 can be correlated with the visible-UV absorption spectra while the theoretical spectra for Au-6(2+) cannot be correlated with the experiments. This suggests, even though the [Au-6(PPh3)(6)] unit of 1 carries a +2 charge, that the Au-6 portion is essentially neutral. More direct evidence for this distribution of the ionized charge has been obtained from HF and DFT calculations of the double ionization energies of models of 1. It is found that the energy required to remove two electrons from a bare Au-6 cluster is much larger than that from an Au-6 cluster with phosphine ligands present; this is again consistent with the +2 charge in 1 being delocalized onto the triphenylphosphine ligands. It is possible that this delocalization of positive charge is responsible for facilitating the adhesion of the gold cluster as finely dispersed particles onto the metal oxide support. (C) 2001 American Institute of Physics.