Normal Modes of [FeIIICl4]- (tetrahedral - Td symmetry)

    The [FeCl4]- complex is one of the substances our group has studied, in order to figure its iron-isotope fractionation behavior. When chemists and geochemists want to separate iron from other metals in solution, they often take advantage of the ability of [FeCl4]- to stick to an anion-exchange resin while other metals and complexes stay in solution.  In addition, this complex is one of the simplest and most symmetric iron-bearing species that has been studied spectroscopically, and it is a useful analogue for other chlorine-metal complexes.  Rubredoxin, one of the simplest iron proteins found in living cells, has an iron-sulfur complex at its core which is electronically very similar to [FeCl4]-, suggesting that the complex and protein have roughly similar iron-isotope fractionation behaviors.  The vibrational frequencies of this [FeCl4]- have been measured in a variety of solutions and salts. Here we report frequencies measured by Raman spectroscopy in aqueous solution (Murata K., and Irish D.E. 1988, Spectrochim. Acta 44A:739-743). The characteristic motions of the vibrational modes were calculated ab initioat the HF/6-31G* level. Ab initio calculated frequencies are shown in parentheses.

o1 (A1) - Symmetric Stretch
330 cm-1 (333.4 cm-1)

 o3 (F2) - Asymmetric Stretch
384 cm-1 (382.6 cm-1)

o2 (E) - Symmetric Bend
105 cm-1 (104.4 cm-1)

 o4 (F2) - Asymmetric Bend
127 cm-1 (148.8 cm-1)

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