Phase transition, thermal dissociation and dynamics of NH3 ligands in [Cd(NH3)4](ReO4)2

High temperature phase transition in [Cd(NH3)4](ReO4)2 at Tc=368.5K (on heating) was reported for the first time. Thermal stability was investigated by thermal analysis methods. The titled compound decomposes in three main stages. The first two are connected with deamination process whereas in the last step Re2O7 evaporates. The activation energy for NH3 lost processes was estimated from TG measurements. The dynamics of NH3 ligands in the low temperature phase was probed by various complementary techniques. Temperature dependent band shape analysis of properly chosen infrared and Raman scattering vibrational bands was performed. It was found that activation energy for NH3 reorientational motion (below 300K) is rather small and is equal to ca. 4kJmol(-1). The quasielastic neutron scattering measurements revealed that NH3 groups perform fast stochastic reorientational motion even in the low temperatures. The neutron and X-ray powder diffraction data do not revealed any drastic changes in the crystal structure in the wide temperature range.

Vibrations and reorientations of NH3 molecules in [Mn(NH3)6](ClO4)2 studied by infrared spectroscopy and theoretical (DFT) calculations

The vibrational and reorientational motions of NH3 ligands and ClO4(-) anions were investigated by Fourier transform middle-infrared spectroscopy (FT-IR) in the high- and low-temperature phases of [Mn(NH3)6](ClO4)2. The temperature dependencies of full width at half maximum (FWHM) of the infrared bands at: 591 and 3385cm(-1), associated with: ρr(NH3) and νas(N-H) modes, respectively, indicate that there exist fast (correlation times τR≈10(-12)-10(-13)s) reorientational motions of NH3 ligands, with a mean values of activation energies: 7.8 and 4.5kJmol(-1), in the phase I and II, respectively. These reorientational motions of NH3 ligands are only slightly disturbed in the phase transition region and do not significantly contribute to the phase transition mechanism. Fourier transform far-infrared and middle-infrared spectra with decreasing of temperature indicated characteristic changes at the vicinity of PT at TC(c)=137.6K (on cooling), which suggested lowering of the crystal structure symmetry. Infrared spectra of [Mn(NH3)6](ClO4)2 were recorded and interpreted by comparison with respective theoretical spectra calculated using DFT method (B3LYP functional, LANL2DZ ECP basis set (on Mn atom) and 6-311+G(d,p) basis set (on H, N, Cl, O atoms) for the isolated equilibrium two models (Model 1 - separate isolated [Mn(NH3)6](2+) cation and ClO4(-) anion and Model 2 - [Mn(NH3)6(ClO4)2] complex system). Calculated optical spectra show a good agreement with the experimental infrared spectra (FT-FIR and FT-MIR) for the both models.