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

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.