Far-infrared absorption spectra of synthetically-prepared, ligated metal clusters with Au6, Au8, Au9 and Au6Pd metal cores

The far infra-red absorption spectra of a series of chemically synthesised, atomically precise phosphine-stabilised gold cluster compounds have been recorded using synchrotron light for the first time. Far-IR spectra of the Au-6(Ph2P(CH2)(3)PPh2)(4)(NO3)(2), Au-8(PPh3)(8)(NO3)(2), Au-9(PPh3)(8)(NO3)(3), and Pd(PPh3) Au-6(PPh3)(6)(NO3)(2) clusters reveal a complex series of peaks between 80 and 475 cm(-1), for which all significant peaks can be unambiguously assigned by comparison with Density Functional Theory (DFT) geometry optimisations and frequency calculation. Strong absorptions in all spectra near 420 cm(-1) are assigned to the P-Ph-3 stretching vibrations. Distinct peaks within the spectrum of each specific cluster are assigned to the cluster core vibrations: 80.4 and 84.1 cm(-1) (Au-6) 165.1 and 166.4 cm(-1) (Au-8), 170.1 and 185.2 cm(-1) (Au-9), and 158.9, 195.2, and 206.7 cm(-1) (Au6Pd). The positions of these peaks are similar to those observed to occur for the neutral Au-7 cluster in the gas phase (Science, 2008, 321, 674-676). Au-P stretching vibrations only occur for Au-6 near 420 cm(-1), although they appear near 180 cm(-1) for Au6Pd and involve gold core vibrations.