The relationship between reorientational molecular motions and phase transitions in [Mg(H2O)6](BF4)2, studied with the use of 1H and 19F 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.

Structural Dynamics of An ELM-11 Framework Transformation Accompanied with Double-Step CO2 Gate sorption: An NMR Spin Relaxation Study

“…− and/or 4,4 -bipyridine, then the apparent activation energy (0.5 kJ mol −1 ) is much smaller than the reported E a values for the isotropic rotation of BF 4 − (10-26 kJ mol −1 ) [40,[42][43][44] and/or the torsional flipping of 4,4 -bipyridine (~10 kJ mol −1 ) [45,46]. ELM-11 contains paramagnetic Cu 2+ (S = 1/2) ions and four kinds of NMR-active nucleus: 1 H (I = 1/2), 19 F (S = 1/2), 10 B (S = 3), and 11 B (S = 3/2).…”

“…The van Vleck formula can be used to calculate NMR second moments in rigid lattices of solid-state materials with well-known molecular and crystal structures [39,40]. A theoretical description of the NMR second moment is given in Appendix A.…”

“…Using the above-mentioned formula, we calculated the 1 H and 19 F second moments of the rigid lattices of the three crystal structures of ELM-11. Second-moment reductions were also calculated by taking into account the anisotropy parameter [40,41] The reduction in the 1 H second moment, ∆M H 2 , which is the sum of ∆M HH 2, intra , ∆M HH 2, inter , and ∆M HF 2, inter , is about (0.4-2) × 10 −8 T 2 , which indicates a low contribution to the total magnetic dipolar relaxation rate. On the other hand, the reduction in the 19 F second moment, ∆M F 2 , which is the sum of ∆M FF 2, intra , ∆M FF 2, inter , ∆M FH 2, inter , and ∆M FB 2, intra , ranged between 21 × 10 −8 and 24 × 10 −8 T 2 .…”

1H and 19F spin-lattice relaxation and CH3 or CF3 reorientation in molecular solids containing both H and F atoms

“…The complicated λ 1 and λ 2 versus T -1 over determines these parameters and we feel it is most instructive to fit the linear lnλ 1 E NMR and τ ∞ characterize the reorientation of the CF 3 group while ε < 1 suggests that there is a very small distribution of E NMR values, probably because of surface effects in very small crystallites 4 or because of crystal imperfections in general. Since E NMR is in the exponential of τ = τ ∞ exp(E NMR /kT), the uncertainty in E NMR leads to a very large uncertainty in τ ∞ , about ± 50%.…”

“…Here we report solid state nuclear magnetic resonance (NMR) 1 H and 19 F relaxation experiments in, and an X-ray diffraction study of the molecular and crystal structure of, 3-trifluoromethoxycinnamic acid (1) (Fig. 1).…”