Vibrational spectroscopy and normal coordinate analysis of μ halo hexahalodialuminates ions Al2X7- (X = Cl, Br, I) in some salts and in Friedel-Crafts solutions

Article abstract of DOI:10.1016/0584-8539(82)80189-X

Infrared and Raman spectra are reported for (Me4N)Al2Cl7, (Et4N)Al2Cl7, KAl2Br7 and CsAl2I7 in the solid state at ordinary temperature.An assignment for practically all the vibrational modes is proposed for the three ions Al2Cl7^-, Al2Br7^- and Al2I7^-.To elucidate the species occurring in the Friedel-Crafts liquids , the spectra of these solutions were also recorded.The quasi identity of the halogenide part of the spectra with the previous one leads to the unique formulation ArH2⁺Al2X7^- for these solutions, in these cases the Al2X7^- ion appears at a higher local symmetry of the AlX3 groups than in the solid state.An approximate valence force field calculation confirms the assignments.It also shows the transferability of force constants for a X-AlX3 model.Spectroscopic particularities are explained in terms of different coupling between bridged and terminal stretching modes.