Electronic structure studies of six-atom gold clusters

Combined theoretical and experimental studies of the hexagold phosphine-sta bilized complex [Au-6(PPh3)(6)][BF4](2) (1) and of related systems are repo rted. 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 expe rimental efforts involved the measurement of the visible-ultraviolet (UV) a bsorption 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 confi guration interaction (CI) methods. The CI wave functions and energies were obtained for a range of excited states and were used to simulate the absorp tion spectra of Au-6 and Au-6(2+) clusters. The theoretical CI absorption s pectra for Au-6 can be correlated with the visible-UV absorption spectra wh ile the theoretical spectra for Au-6(2+) cannot be correlated with the expe riments. 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 evide nce for this distribution of the ionized charge has been obtained from HF a nd 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 cl uster 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 delocalizati on of positive charge is responsible for facilitating the adhesion of the g old cluster as finely dispersed particles onto the metal oxide support. (C) 2001 American Institute of Physics.