Inelastic X‑ray Scattering of a Transition-Metal Complex (FeCl4 –): Vibrational Spectroscopy for All Normal Modes

The tetraethylammonium salt of the transition-metal complex FeCl4 – has been examined using inelastic X-ray scattering (IXS) with 1.5 meV resolution (12 cm–1) at 21.747 keV. This sample serves as a feasibility test for more elaborate transition-metal complexes. The IXS spectra were compared with previously recorded IR, Raman, and nuclear resonant vibrational spectroscopy (NRVS) spectra, revealing the same normal modes but with less strict selection rules. Calculations with a previously derived Urey-Bradley force field were used to simulate the expected Q and orientation dependence of the IXS intensities. The relative merits of IXS, compared to other photon-based vibrational spectroscopies such as NRVS, Raman, and IR, are discussed.