The relationship between reorientational molecular motions and phase transitions in [Mg(H{sub 2}O){sub 6}](BF{sub 4}){sub 2}, studied with the use of {sup 1}H and {sup 19}F NMR and FT-MIR
- Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków (Poland)
- Department of Forest Pathology, Faculty of Forestry, Agricultural University of Cracow, Aleja 29 listopada 46, 31-425 Kraków (Poland)
- Faculty of Physics, A. Mickiewicz University, Umultowska 85, 61-614 Poznań (Poland)
- Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznań (Poland)
A {sup 1}H and {sup 19}F nuclear magnetic resonance study of [Mg(H{sub 2}O){sub 6}](BF{sub 4}){sub 2} has confirmed the existence of two phase transitions at T{sub c1} ≈ 257 K and T{sub 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 M{sub 2}{sup H} and M{sub 2}{sup F} and of spin-lattice relaxation times T{sub 1}{sup H} and T{sub 1}{sup F}. The study revealed anisotropic reorientations of whole [Mg(H{sub 2}O){sub 6}]{sup 2+} cations, reorientations by 180° jumps of H{sub 2}O ligands, and aniso- and isotropic reorientations of BF{sub 4}{sup −} anions. The activation parameters for these motions were obtained. It was found that the phase transition at T{sub c1} is associated with the reorientation of the cation as a whole unit around the C{sub 3} axis and that at T{sub c2} with isotropic reorientation of the BF{sub 4}{sup −} anions. The temperature dependence of the full width at half maximum value of the infrared band of ρ{sub t}(H{sub 2}O) mode (at ∼596 cm{sup −1}) indicated that in phases I and II, all H{sub 2}O ligands in [Mg(H{sub 2}O){sub 6}]{sup 2+} perform fast reorientational motions (180° jumps) with a mean value of activation energy equal to ca 10 kJ mole{sup −1}, what is fully consistent with NMR results. The phase transition at T{sub c1} is associated with a sudden change of speed of fast (τ{sub R} ≈ 10{sup −12} s) reorientational motions of H{sub 2}O ligands. Below T{sub c2} (in phase III), the reorientations of certain part of the H{sub 2}O ligands significantly slow down, while others continue their fast reorientation with an activation energy of ca 2 kJ mole{sup −1}. This fast reorientation cannot be evidenced in NMR relaxation experiments. Splitting of certain IR bands connected with H{sub 2}O ligands at the observed phase transitions suggests a reduction of the symmetry of the octahedral [Mg(H{sub 2}O){sub 6}]{sup 2+} complex cation.
- OSTI ID:
- 22416123
- Journal Information:
- Journal of Chemical Physics, Vol. 142, Issue 6; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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