NMR Relaxation Studies of Ferroelectric Colemanite

Proton spin–lattice relaxation of the Zeeman reservoir in the laboratory and rotating frames and of the dipolar reservoir have been studied in ferroelectric colemanite. These studies indicate the presence of two relaxation mechanisms which can be related to motions of the water molecules. The first, referred to as the flip mode and identified with 180° flips of the water molecules about their H—O—H bisectrices, is characterized by a correlation time and U-shaped relaxation minimum expected for a normal thermally activated process. The second, referred to as the jump mode and associated with a structural configuration change, produces a cusp-shaped dip in the relaxation over a very small temperature range in the vicinity of the phase transition. Above the transition temperature, the jump mode frequency is higher than the Larmor frequency and the relaxation behavior is that expected for two thermally activated processes in the white spectral region. However, in the vicinity of the phase transition, the water molecules are tightly coupled to the structure as a whole and the jump mode frequency is determined by the structural stability of the crystal. In this region its behavior is no longer that of a thermally activated process for an isolated molecular group and its frequency approaches zero critically near the phase transition.

Download Full-text

Proton Spin Lattice Relaxation in the Dimethylammonium Group

Zeitschrift für Naturforschung A ◽

10.1515/zna-1993-5-624 ◽

1993 ◽

Vol 48 (5-6) ◽

pp. 713-719

Author(s):

K. Venu ◽

V. S. S. Sastry

Keyword(s):

Experimental Data ◽

Lattice Relaxation ◽

Lattice Relaxation Time ◽

Proton Spin ◽

Spin Lattice Relaxation ◽

Methyl Groups ◽

Dipolar Interactions ◽

Spin Temperature ◽

Spin Lattice ◽

Molecular Group

Abstract A model for the spin lattice relaxation time of the protons of dimethylammonium in the Redfield limit and common spin temperature approximation is developed. The three fold reorientations of the methyl groups, the rotation of the whole molecular group around its two fold symmetric axis and possible correlations among these motions are considered. The effect of these processes on the dipolar interactions among the protons within the same molecular group is taken into account. The resulting relaxation rate is powder averaged and used to explain the experimental data in literature on [NH2(CH3)2]3Sb2Br9 . The analysis shows that dynamically inequivalent groups exist in this compound and that the effect of proposed correlation among the different motions on the final results is negligible.

Download Full-text

Proton Spin-lattice Relaxation in Amorphous States of Organic Molecular Solids

Zeitschrift für Naturforschung A ◽

10.1515/zna-1997-1101 ◽

1997 ◽

Vol 52 (11) ◽

pp. 757-764 ◽

Cited By ~ 2

Author(s):

M. T. Myaing ◽

L. Šekarić ◽

P. A. Beckmann

Keyword(s):

Time Scale ◽

Amorphous State ◽

Larmor Frequency ◽

Lattice Relaxation ◽

Proton Spin ◽

Spin Lattice Relaxation ◽

Ethyl Benzene ◽

Methyl Groups ◽

Spin Lattice ◽

Amorphous States

Abstract We have measured the temperature dependence of the proton spin-lattice relaxation rate R at 8.50 and 22.5 MHz in solid 1,3,5-tri-ethyl-benzene and solid 1,2,4-tri-ethyl-benzene. Analysis of the data strongly suggests that we are studying amorphous states in these slowly solidified organic solids (that are liquids at room temperature). The ethyl groups are static on the Larmor frequency time-scale. There are no simple-model interpretations of the data, but a reasonable model for the dominantly-occurring amorphous state data observed with 1,3,5-tri-ethyl-benzene suggests that two of the three methyl groups are reorienting and the third is static on the proton Larmor frequency time scale. The same approach for the two amorphous states observed in 1,2,4-tri-ethyl-benzene suggests that all three methyl groups are reorienting in one state and that three of the six methyl groups in each pair of molecules are turned off in a second state. We discuss that, whereas specific dynamical statements are model dependent, the proton spin relaxation technique does make some general qualitative statements about the mesostructure of the solid.

Download Full-text

Phase transitions in solid cycloheptene

Canadian Journal of Chemistry ◽

10.1139/v90-093 ◽

1990 ◽

Vol 68 (4) ◽

pp. 604-611 ◽

Cited By ~ 5

Author(s):

Julian Haines ◽

D. F. R. Gilson

Keyword(s):

Phase Transition ◽

High Temperature ◽

Low Temperature ◽

Lattice Relaxation ◽

Proton Spin ◽

Spin Lattice Relaxation ◽

Temperature Phase ◽

Scanning Calorimetry ◽

Spin Lattice ◽

Low Temperature Phase

The phase transition behaviour of cycloheptene has been investigated by differential scanning calorimetry, proton spin-lattice relaxation, and vibrational spectroscopy (infrared and Raman). Two solid–solid phase transitions were observed, at 154 and 210 K, with transition enthalpies and entropies of 5.28 and 0.71 kJ mol−1 and 34.3 and 3.4 JK−1, respectively. Cycloheptene melted at 217 K with an entropy of melting of 4.5 JK−1 mol−1. The bands in the vibrational spectra of the two high temperature phases were broad and featureless, characteristic of highly disordered phases. The presence of other conformers, in addition to the chair form, was indicated from bands in the spectra. The ring inversion mode was highly phase dependent and exhibited soft mode type behaviour prior to the transition from the low temperature phase. The low frequency Raman spectra (external modes) of these phases indicated that the molecules are undergoing isotropic reorientation. In the low temperature phase, the vibrational bands were narrow; the splitting of the fundamentals into two components and the presence of nine external modes are consistent with unit cell symmetry of either C2 or Cs with two molecules per primitive unit cell. A glassy state can be produced from the intermediate phase and the vibrational spectra were very similar to those of the high temperature phases, indicating that static disorder was present. The barriers to reorientation, as obtained from proton spin-lattice relaxation measurements, are 9.0 kJ mol−1 in both the high temperature phases, and 15.4 kJ mol−1 in the low temperature, ordered phase. Keywords: cycloheptene, phase transition, differential scanning calorimetry, NMR, vibrational spectroscopy.

Download Full-text

Proton spin-lattice relaxation study of the phase transition in TlH2AsO4

Solid State Communications ◽

10.1016/0038-1098(78)91023-2 ◽

1978 ◽

Vol 27 (10) ◽

pp. 999-1001 ◽

Cited By ~ 18

Author(s):

R. Blinc ◽

M. Rožmarin ◽

F. Milia ◽

M. Melisaropoulou

Keyword(s):

Phase Transition ◽

Lattice Relaxation ◽

Proton Spin ◽

Spin Lattice Relaxation ◽

Spin Lattice ◽

Relaxation Study

Download Full-text

Investigation of the 1H Spin-Lattice Relaxation in Polycrystalline NH4ReO4 Induced by Mechanical Sample Rotation

Zeitschrift für Naturforschung A ◽

10.1515/zna-1976-1239 ◽

1976 ◽

Vol 31 (12) ◽

pp. 1707-1710 ◽

Cited By ~ 3

Author(s):

H. Rager

Keyword(s):

Relaxation Times ◽

Larmor Frequency ◽

Rotation Frequency ◽

Lattice Relaxation ◽

Lattice Relaxation Time ◽

Proton Spin ◽

Spin Lattice Relaxation ◽

Spin Lattice ◽

Sample Rotation ◽

Resonance Frequencies

Abstract Measurements of the proton spin-lattice relaxation time, T1 , in polycrystalline NH4ReO4 have shown that mechanical sample rotation in the static magnetic field reduces T1 for proton magnetic resonance frequencies within the range 30 - 42 MHz. The rotation induced proton spin-lattice relaxation times, T1′, were measured at different Larmor frequencies, sample rotation frequencies, and temperatures. At room temperature T1′ increases with increasing proton Larmor frequency. The dependence of T1′ on sample rotation frequency and temperature is small. The "normal" proton T1 times were determined as a function of temperature at 30 and 42 MHz. From these measurements an average activation energy of 2.4 kcal/mole was obtained for the NH4+ reorientation. The "normal" spin-lattice relaxation behavior was observed to be non-exponential at 30 MHz above 200 K and at 42 MHz above 260 K and is nearly independent on temperature above 300 K.

Download Full-text

Dielectric Behaviour of the Ferroelectric AgNa(NO2)2 and Spin Lattice Relaxation of 23Na

Zeitschrift für Naturforschung A ◽

10.1515/zna-1974-0710 ◽

1974 ◽

Vol 29 (7) ◽

pp. 1055-1059 ◽

Cited By ~ 4

Author(s):

J. Grossmann ◽

D. Müller ◽

J. Petersson

Keyword(s):

Pulse Sequences ◽

Lattice Relaxation ◽

Lattice Relaxation Time ◽

Spin Lattice Relaxation ◽

Spin Lattice ◽

Thermally Activated ◽

Small Influence ◽

Nuclear Spin Lattice Relaxation ◽

Diffusional Motion ◽

Thermally Activated Process

The characteristic ferroelectric dispersion of the dielectric constant of AgNa(NO2)2 is measured near the ferroelectric phase transition. In accordance with other works an extremely slow relaxation is observed which can be related to a thermally activated motion of the electrical dipoles. The nuclear spin lattice relaxation time T1 of 23Na in AgNa(NO2)2 is measured by 90° - t-90° pulse sequences. In the entire temperature range the temperature dependence of T1 is governed by a thermally activated process which is ascribed to a diffusional motion. There is only a small influence of the dielectric instability on T1.

Download Full-text