The relationship between reorientational molecular motions and phase transitions in $[Mg(H_2O)_6](BF_4)_2$, studied with the use of $^{1}H$ and $^{19}F$ NMR and FT-MIR

A 1H and 19F nuclear magnetic resonance study of [Mg(H2O)6](BF4)2 has confirmed the existence of two phase transitions at T c1 ≈ 257 K and T c2 ≈ 142 K, detected earlier by the DSC method. These transitions were reflected by changes in the temperature dependences of both proton and fluorine of second moments M2 H and M2 F and of spin-lattice relaxation times T1 H and T1 F. The study revealed anisotropic reorientations of whole [Mg(H2O)6]2+ cations, reorientations by 180° jumps of H2O ligands, and aniso- and isotropic reorientations of BF4 − anions. The activation parameters for these motions were obtained. It was found that the phase transition at T c1 is associated with the reorientation of the cation as a whole unit around the C3 axis and that at T c2 with isotropic reorientation of the BF4 − anions. The temperature dependence of the full width at half maximum value of the infrared band of ρt (H2O) mode (at ∼596 cm−1) indicated that in phases I and II, all H2O ligands in [Mg(H2O)6]2+ perform fast reorientational motions (180° jumps) with a mean value of activation energy equal to ca 10 kJ mole−1, what is fully consistent with NMR results. The phase transition at T c1 is associated with a sudden change of speed of fast (τR ≈ 10−12 s) reorientational motions of H2O ligands. Below T c2 (in phase III), the reorientations of certain part of the H2O ligands significantly slow down, while others continue their fast reorientation with an activation energy of ca 2 kJ mole−1. This fast reorientation cannot be evidenced in NMR relaxation experiments. Splitting of certain IR bands connected with H2O ligands at the observed phase transitions suggests a reduction of the symmetry of the octahedral [Mg(H2O)6]2+ complex cation.