RAMAN-SPECTRA OF THE VAPOR MOLECULES FE2CL6, FECL3, AND FEALCL6

Raman spectra of iron(III) chloride vapors (Fe2Cl6(g) and FeCl3(g)) and of the vapor complex FeAlCl6(g) have been measured in the temperature range 500-900 K. Experimental difficulties arising from decomposition of the vapors by the laser lines used were solved by designing and using a rotating, fused silica cell, inside an optical furnace. The Fe2Cl6(g) dimer spectra were interpreted in terms of D2h molecular symmetry. The main features of the spectra are four resonance enhanced polarized bands at 422, 305, 150 and 78 cm-1 as well as four depolarized bands. All the observed Raman active dimer fundamentals were assigned and, in combination with literature data, were used for normal coordinate analysis calculations. The resonance-Raman spectra of the FeCl3(g) monomer were interpreted in terms of a D3h molecular symmetry. Three bands at 450,370 and 111 cm-1 were assigned to the v4,v1 and v3 Raman active fudamentals, respectively. A relatively strong polarized band at 231 cm-1 was assigned to the first overtone of the v2,IR-active fundamental of FeCl3(g). Raman spectra of the complex FeAlCl6(g) were measured in equilibrium with a mixture of Al2Cl6(g) and iron (III) chlorides vapors. Two relatively strong polarized bands at 413 and 88 cm-1 as well as four weak polarized and six depolarized bands, seen in the spectra of the vapor mixture, were attributed to the vapor complex. The spectra were interpreted in terms of a C2v symmetry for the FeAlCl6(g) molecule. Relative intensity measurements, carried out for both the iron(III) chloride vapors and the vapor mixture containing the complex, indicate that by increasing the laser frequency from red to blue, the spectroscopic temperature increases and the Fe2Cl6(g) and/or the FeAlCl6(g) molecules dissociate to form FeCl3(g). The vibrational frequencies, observed and assigned in this work were combined with literature data and were used to calculate the thermodynamic functions of Fe2Cl6(g), Al2Cl6(g) and FeAlCl6(g) molecules.