Abstract:
During the initial reaction of iron with the vapour of aqueous hydrofluoric acid, an iron(II) fluoride tetrahydrate (FeF2 •4H2O) layer forms. The structure of this layer could possibly determine the mechanism of the reaction. In the literature different crystal structures have been assigned to FeF2 •4H2O Further anomalies exist in the spectroscopic information acquired with different techniques. This study was undertaken in order to determine the structure of FeF2 •4H2O uniquely by using different spectroscopic techniques and by studying the isomorphous series MF2 •4H2O (M = Fe, Co, Ni, Zn). Pure microcrystal line FeF2 •4H20 was prepared and the structure of the compound was indexed by means of X-ray diffraction spectrometry as orthorhombic (Pca21 ). Mossbauer effect, electronic spectroscopy and infrared (IR) data supported a distorted octahedral coordination of the iron atom. The fluorine atoms are located at trans-positions in the octahedron and different Fe-OH2 bond lengths exist. The distorted molecular structure of FeF2 •4H20 was investigated in greater detail by predicting the IR-active bands from group theoretical considerations, and by means of deuterium substitution and low temperature studies. Group theory indicates all sites to be asymmetric and predicts 133 IR active bands. All experimentally observed bands were assigned. The majority of these were associated with H20, HDO, and D20 vibrations. The number of D20 and HDO vibrations are indicative of two crystallographically distinct water molecules. The 0-H bond lengths (r(O-H)), the hydrogen bond lengths and hydrogen bond strengths were calculated and indicated relatively strong hydrogen bonds.
