Structural analysis of metal clusters using exafs

Three gold phosphine clusters, Au55(PPhs)12Cl6, Au55PPh2(C6H4SO3Na)12Cl6 and Au11{PPh2(p-ClC6H4)}7I3 (which we shall denote Au55, Au55* and Au11 for convenience), and the large Palladium cluster approximated to Pd561phen36O200 (denoted Pd561 here) have been studied using EXAFS.

For the gold clusters, ligand shells have been successfully modelled, and values for Au-P, Au-I and Au-Cl distances compare favourably to chemically similar compounds. Average near-neighbour bond lengths are contracted from the bulk by approximately 0.13A (+-0.004A) for Au55 and 0.22A (+-0.01A) for Au11. Debye-Waller factors for samples at room temperature and at 80K are compared for Au55. Results indicate clearly that static structural disorder predominates over thermally induced vibrational disorder.

Results for the large palladium cluster were limited to metal atoms only. Bond distances show that, within the limits of experimental error, no contraction/dilation occurs. Values for near neighbour distances suggest a close packed structure. Debye-Waller factors indicate a remarkably rigid structure.

Analysis is pursued further by use of the Reverse Monte-Carlo (RMC) modelling method, which gives a three dimensional particle configuration of the system consistent with the EXAFS data. Results indicate that this modelling method has distinct weaknesses when attempting to simulate this type of system.