Studies of Successive Phase Transitions and Molecular Motions in [Mg(H₂O)₆][SiF₆] by ^1,2H and ¹⁹F NMR

Abstract

The successive phase transitions of [Mg(H?0)6][SiF6] were studied by measuring 2 H NMR spectra. The quadrupole coupling constant e 2 Qq/h and asymmetry parameter 77 changed drastically at each transition temperature. 12 H and 19 F NMR T\ were measured for this compound to study the relation between the molecular motions and the successive phase transitions. The activation energy E a and the pre-exponential factor TO for the reorientation of [SiFö] 2~ were estimated as 28 kJmol" 1 and 6.0x 10~1 4 s, and those of the 180° flip of H 2 0 as 33 kJmol -1 and 4.0x 10" 14 s. These two motions occur rapidly even in phase V. For the reorientation of [Mg(H20)6] 2+ , E a = 62 kJmol -1 and TO = 1.1 x 10 -16 s were obtained from the simulation of : H NMR spectra. The jump rate of this motion is of the order of 10 4 -10 6 s _I in phase II. These results suggest that the successive phase transitions are closely related to the motion of [Mg(H 2 0) 6 ] 2+ .

Dynamical structure of paramagnetic [M(H2O)6][SiF6] (M = Fe2+,Ni2+) crystal studied by means of2H nuclear magnetic resonance

“…A number of studies on the phase transition and the disorder structure of [M(H 2 O) 6 ][SiF 6 ] (M = Mg, Mn, Fe, Co, Ni, Zn) crystal have been performed [1][2][3][4][5][6][7][8][9][10][11][12]. These compounds have similar crystal structures and form a rhombohedrally distorted CsCl-type structure.…”

Study of Structural Phase Transitions in [Mg(H2O)6][SiF6] by Means of Single Crystal 2H NMR

“…Three 2(∆ ) which can be attributed to three unequal water molecules in [Mg(H 2 O) 6 ] 2+ were observed for each rotation. It is found that the electric field gradient (EFG) tensors for two deuterons in a water molecule are averaged by the fast 180 flip of H 2 O resulting in the same tensors [9,12] and that inversion symmetry exists for [Mg(H 2 O) 6 ] 2+ at this temperature [2]. The fitting calculation for the rotation around the axis was performed with the equation [13] 2(∆ ) = 3 4 + ( ) cos (2 ) (1) + 2 sin (2 ) where is the angle between the axis and the external magnetic field 0 .…”

“…The temperature dependence of , obtained by the spectral simulation in phase II, is shown in Figure 4. Assuming an Arrhenius relation, is given by [9]. We believe that the former is better than the latter, because in this study the spectra were analyzed by separating the effect on the spectral shape of the motion of [Mg(H 2 O) 6 ] 2+ and that of the modulated structure.…”

Molecular Dynamics in Paramagnetic Materials as Studied by Magic-Angle Spinning 2H NMR Spectra

“…The line shape of a broadline 2 H NMR spectrum is sensitive to the molecular motion in the dynamic range of 10 3 s -1 e k e 10 7 s -1 . [2][3][4][5][6][7][8][9][10] Since the ordinary quadrupole-echo sequence refocuses only dephasing due to the quadrupole interaction, the distortion of the line shape is caused by the paramagnetic shift in the measurements of broadline 2 H NMR spectra of paramagnetic compounds. Two-or four-pulse sequences which remove the distortion of a spectrum because of the paramagnetic shift have been proposed.…”

“…The line shape of a 2 H NMR spectrum depends on the anisotropic interactions, such as nuclear quadrupole interaction, chemical shift, and dipolar interaction, and is affected by the molecular motion. The line shape of a broadline 2 H NMR spectrum is sensitive to the molecular motion in the dynamic range of 10 3 s -1 ≤ k ≤ 10 7 s -1 . − Since the ordinary quadrupole-echo sequence refocuses only dephasing due to the quadrupole interaction, the distortion of the line shape is caused by the paramagnetic shift in the measurements of broadline 2 H NMR spectra of paramagnetic compounds. Two- or four-pulse sequences which remove the distortion of a spectrum because of the paramagnetic shift have been proposed. , The simulation method for the 2 H NMR broadline spectrum of paramagnetic compounds obtained by these pulse sequences has been developed for the study of molecular dynamics in paramagnetic compounds. ,− …”