title: The relationship between reorientational molecular motions and phase transitions in [Mg(H2O)6](BF4)2, studied with the use of (1)H and (19)F NMR and FT-MIR.
abstract: A (1)H and (19)F nuclear magnetic resonance study of [Mg(H2O)6](BF4)2 has confirmed the existence of two phase transitions at Tc1 ≈ 257 K and Tc2 ≈ 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 Tc1 is associated with the reorientation of the cation as a whole unit around the C3 axis and that at Tc2 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 Tc1 is associated with a sudden change of speed of fast (τR ≈ 10(-12) s) reorientational motions of H2O ligands. Below Tc2 (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.
authors: Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland.
authors: Edward
authors: E
authors: Mikuli
authors: Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland.
authors: Joanna
authors: J
authors: Hetmańczyk
authors: Department of Forest Pathology, Faculty of Forestry, Agricultural University of Cracow, Aleja 29 listopada 46, 31-425 Kraków, Poland.
authors: Bartłomiej
authors: B
authors: Grad
authors: Faculty of Physics, A. Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland.
authors: Asja
authors: A
authors: Kozak
authors: Faculty of Physics, A. Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland.
authors: Jan W
authors: JW
authors: Wąsicki
authors: Faculty of Physics, A. Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland.
authors: Paweł
authors: P
authors: Bilski
authors: Faculty of Physics, A. Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland.
authors: Krystyna
authors: K
authors: Hołderna-Natkaniec
authors: Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznań, Poland.
authors: Wojciech
authors: W
authors: Medycki