Normal-Mode Analysis of FeCl4- and Fe2S2Cl42- via Vibrational Mössbauer, Resonance Raman, and FT-IR Spectroscopies

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Abstract
[NEt4][FeCl4], [P(C6H5)4][FeCl4], and [NEt4]2[Fe2S2Cl4] have been examined using 57Fe nuclear resonance vibrational spectroscopy (NRVS). These complexes serve as simple models for Fe−S clusters in metalloproteins. The 57Fe partial vibrational density of states (PVDOS) spectra were interpreted by computation of the normal modes assuming Urey−Bradley force fields, using additional information from infrared and Raman spectra. Previously published force constants were used as initial values; the new constraints from NRVS frequencies and amplitudes were then used to refine the force field parameters in a nonlinear least-squares analysis. The normal-mode calculations were able to quantitatively reproduce both the frequencies and the amplitudes of the intramolecular-mode 57Fe PVDOS. The optimized force constants for bending, stretching, and nonbonded interactions agree well with previously reported values. In addition, the NRVS technique also allowed clear observation of anion−cation lattice modes below 100 cm-1 that are nontrivial to observe by conventional spectroscopies. These features were successfully reproduced, either by assuming whole-body motions of point-mass anions and cations or by simulations using all of the atoms in the unit cell. The advantages of a combined NRVS, Raman, and IR approach to characterization of Fe−S complexes are discussed.

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