Abstract
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 Au6(Ph2P(CH2)3PPh2)4(NO3)2, Au8(PPh3)8(NO3)2, Au9(PPh3)8(NO3)3, and Pd(PPh3)Au6(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–Ph3 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 (Au6) 165.1 and 166.4 cm−1 (Au8), 170.1 and 185.2 cm−1 (Au9), 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 Au7 cluster in the gas phase (Science, 2008, 321, 674–676). Au–P stretching vibrations only occur for Au6 near 420 cm−1, although they appear near 180 cm−1 for Au6Pd and involve gold core vibrations.
Highlights
IntroductionInfra-red spectroscopy is commonly used to observe the vibrational frequencies of key functional groups to monitor changes in chemical bonding during chemical synthesis
Over the past decade there has been great interest shown in the size-dependent chemical properties of metal clusters deposited onto active surfaces, with a focus on catalysis
Less common are studies that involve the preparation of ligand-stabilised metal clusters via chemical synthesis, immobilisation of these onto a support, and the subsequent activation of the cluster cores by removal of some, or all, of the ligands
Summary
Infra-red spectroscopy is commonly used to observe the vibrational frequencies of key functional groups to monitor changes in chemical bonding during chemical synthesis. The experimentally obtained spectra are compared with DFT-computed vibrational frequency calculations of the full cluster, including all ligands This allows us to identify key features in the IR spectrum that can be assigned to speci c vibrational motions of the metal core, as well as vibrations between the metal atoms and ligands (i.e. Au–P bonds). Geometry optimisation and harmonic vibrational frequency calculations of the gold cluster compounds, including all ligands, were undertaken using the M06 density functional[19] in the Gaussian 09 suite of programs.[20] Starting geometries were taken from crystal structures obtained from X-ray diffraction patterns of the synthesised clusters (reported earlier by others or ourselves) which were deposited in the Cambridge Crystallographic Database.[21] All counter ions were removed, with the appropriate number of electrons removed from the calculations to balance the charge. Full geometric information for each optimised structure is provided in the accompanying ESI le.†
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