Vibrational spectroscopy studies of solid-solid phase transitions in [Mg(OS(CH3)2)6](ClO4)2 and in its deuterated analogue

On the basis of Fourier transform Raman and infrared absorption (FT-RS and FT-IR) spectra of [Mg(OS(CH3)2)6](ClO4)2 and [Mg(OS(CD3)2)6](ClO4)2 coordination compounds the improved assignments of the observed bands related to vibrational modes were proposed. Phase transitions in these compounds were investigated by differential scanning calorimetry (DSC) and also by analysis of temperature dependences of far- and middle-infrared spectra (FT-FIR and FT-MIR). The DSC studies showed that deuteration of the ligands leads to a decrease of the mean value of the phase transition temperature Tc1 ≈ 407 K for non-deuterated compound (by about 5 K) and has a negligible effect on the mean value of the phase transition temperature Tc2 ≈ 318 K. The characteristic changes observed in the FT-IR spectra of title compounds in the regions of the phase transitions suggest that the phase transitions denoted as: KI ↔ K01 ↔ K02 are associated with the crystal structure changes. Moreover, the characteristic broadening of some IR bands connected with vibrational modes within ClO4− anions and DMSO ligands with increasing temperature indicate that in the high and intermediate temperature phases the CH3(CD3) groups (from DMSO) and ClO4− anions perform fast reorientational motions. In turn, in the low temperature phase these reorientational motions become suddenly much slower. The temperature dependences of the integral intensity of the IR bands associated with νs(CH3)/νs(CD3) and ν4(F2) modes indicate that hydrogen bond-like or dipole-dipole interactions between methyl groups and ClO4− anions can play an important role in the mechanism of the KI ↔ K01 ↔ K02 phase transitions.