Spectres infrarouges et raman du complexe solide MgBr22Et2O: Changement de phase et isomérie de rotation des molécules d'éther coordonnées

Infrared and Raman spectra of solid magnesium bromide dietherate MgBr₂2Et₂O have been studied at different temperatures between 300 and 90 °K in the 4000 to 30 cm^? range. Infrared spectra of this compound at various pressures up to 10 kbar have also been investigated. The MgBr₂2Et₂O crystal has two phases and there is a reversible transition between them. This phase transition concerns primarily the conformational change of (C₂Hg₅)₂O molecules and much less the arrangement of the ligands around the central magnesium atom. At room temperature or low pressure the GG conformation appears to be predominant while at low temperature or high pressure there is only TG (or TT) conformation of ether molecules. The enthalpy difference and transition temperature between the phases have been determined calorimetrically. An assignment of the spectra of both phases is given. The analysis of intramolecular vibrations gives information about the relative contribution of conformational change and coordination effect to the frequency shifts of (C₂H₅)₂O vibrations. The assignment of magnesium-ligand vibrations on the other hand allows us to estimate the Mg-Br and Mg-O force constants.