Publication Date:
2015-02-11
Description:
A 1 H and 19 F nuclear magnetic resonance study of [Mg(H 2 O) 6 ](BF 4 ) 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 M 2 H and M 2 F and of spin-lattice relaxation times T 1 H and T 1 F . The study revealed anisotropic reorientations of whole [Mg(H 2 O) 6 ] 2+ cations, reorientations by 180° jumps of H 2 O ligands, and aniso- and isotropic reorientations of BF 4 − 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 C 3 axis and that at T c2 with isotropic reorientation of the BF 4 − anions. The temperature dependence of the full width at half maximum value of the infrared band of ρ t (H 2 O) mode (at ∼596 cm −1 ) indicated that in phases I and II, all H 2 O ligands in [Mg(H 2 O) 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 ( τ R ≈ 10 −12 s) reorientational motions of H 2 O ligands. Below T c2 (in phase III), the reorientations of certain part of the H 2 O 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 H 2 O ligands at the observed phase transitions suggests a reduction of the symmetry of the octahedral [Mg(H 2 O) 6 ] 2+ complex cation.
Print ISSN:
0021-9606
Electronic ISSN:
1089-7690
Topics:
Chemistry and Pharmacology
,
Physics
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