Molecular motion and methyl group rotation barriers from 13C nmr relaxation times. Norbornane and adamantane derivatives in solution

1980 ◽  
Vol 58 (2) ◽  
pp. 102-109 ◽  
Author(s):  
Helmut Beierbeck ◽  
Robert Martino ◽  
John K. Saunders

The carbon-13 relaxation time data for several norbornane and adamantane derivatives are presented. It is shown that the most probable principal axis of the rotational diffusion tensor is an axis which contains the centre of mass of the molecule and the heterosubstituent. The influence of the C—Ĉ—H bond angle on the calculation of methyl group rotation barriers from relaxation time data and on the barrier is discussed.

1982 ◽  
Vol 60 (10) ◽  
pp. 1173-1177 ◽  
Author(s):  
Helmut Beierbeck ◽  
John W. Easton ◽  
John K. Saunders ◽  
Russell A. Bell

The 13C nmr relaxation time data for a number of compounds related to podocarpic acid are presented. The T1 values of these molecules were satisfactorily reproduced by application of Woessner's equations for an asymmetric top assuming that the preferred principal axis of the rotational diffusion tensor was either the preferred principal inertial axis or an axis containing the centre of mass and the heterosubstituent at C-4. The values for hindrance to rotation for the various methyl groups are given and their variations discussed.


1992 ◽  
Vol 47 (5) ◽  
pp. 689-701 ◽  
Author(s):  
Günter Burbach ◽  
Norbert Weiden ◽  
Alarich Weiss

Abstract The molecular dynamics of tetramethylstibonium hexafluorophosphate, [Sb(CH3)4]PF6, is investigated over a broad temperature range. NMR spin lattice relaxation times T1 and the NMR second moments of the 1H and 19F nuclei were determined in the range 8.6 ≦ T/K ≦ 332.3 for polycrystalline [Sb(CH3)4]PF6. The complex cation undergoes isotropic tumbling for T > 260 K and thermally activated methyl group rotation in the temperature range T < 196 K. The activation energies for the transition from methyl group rotation to cation reorientation, as derived from NMR wideline (18.1 kJ/mol) and relaxation (22.7 kJ/mol) measurements, match. At very low temperatures pseudo classical line narrowing is observed, indicating tunneling motions of the methyl groups. The existence of two crystallographically inequivalent methyl groups is found by X-ray structure analysis at room temperature. The space group is P63mc, Z = 2; a = 738.6 pm, c = 1089.3 pm. It is confirmed by two steps in the temperature dependence of the signal intensity of the quasielastic line in neutron fixed window measurements in the temperature range 2 < T/K <148. The low temperature spin lattice relaxation times can be explained qualitatively by contributions of two crystallographically inequivalent methyl groups. Apparent activation energies for the two crystallographically different methyl groups are estimated. The complex anion undergoes isotropic tumbling in the temperature range 95 < T/K < 330. Above 330 K additionally translational motion is activated. Below 95 K the rotational motion of PF-6 is freezing in via an uniaxial state in range 40 < T/K <80. Activation energies for both isotropical tumbling (10.5 kJ/mol) and uniaxial rotation (5.8 kJ/mol) have been derived from 19F NMR spin lattice relaxation


2017 ◽  
Vol 85-86 ◽  
pp. 1-11
Author(s):  
Peter A. Beckmann ◽  
Joseph M. Bohen ◽  
Jamie Ford ◽  
William P. Malachowski ◽  
Clelia W. Mallory ◽  
...  

2021 ◽  
Vol 37 (2) ◽  
pp. 219-231
Author(s):  
Jean Frederic Isingizwe Nturambirwe ◽  
Willem Jacobus Perold ◽  
Umezuruike Linus Opara

HighlightsMeasurements of relaxation times in intact banana at micro-Tesla field was achieved.Bulk spin-spin relaxation time highly correlated with best descriptors of banana ripening.A basis for quasi-continuous distribution of spin-spin relaxation in banana was given.Abstract. Achieving fast, low-cost, and non-destructive internal quality testing techniques in the horticultural industry is a challenge. Developing techniques such as ultra-low field nuclear magnetic resonance (NMR) is a promising solution. Banana is a fast ripening fruit, which undergoes many changes in quality characteristics during ripening, and was chosen as a fit choice for extensive fruit quality study by NMR. A commercial NMR system using a superconducting quantum interference device (SQUID) as a sensor and operating at 100µT was used to measure changes that occurred in banana fruit during ripening. The longitudinal and transverse relaxation times (T1 and T2, respectively), were measured on fruit samples progressively drawn from a larger batch under storage. Physico-chemical attributes such as total soluble solids (TSS), titratable acidity (TA), pH, and color parameters were measured and used as reference measurements. Statistical analysis using cross-correlation, linear regression, analysis of variance (ANOVA), and principal components analysis (PCA) were performed to probe the relationships between various quality attributes. T1 showed high correlations with total soluble solids (R = 0.84), sugar:acid ratio (R = 0.84) and color parameters (R from 0.49 to 0.88). T2, on the other hand, was most highly correlated to pH (R = 0.76) but also had a statistically significant but negative correlation with Ri (-0.58 at p &lt;0.05). PCA results separated the first day from the remaining days of the ripening process and the overall variation was mostly explained by color attributes (a* and h), T1, TSS, and TSS/TA. During seven days of ripening in storage, the trend of change in the peel color of banana was best described by L*, a*, h and total color difference (TCD). The index of ripening, Ri, defined based on the apparent change in peel color was highly correlated to TSS, TSS/TA, L*, a*, h, TCD, and T1. The strong similarity between the evolution of T1 and the most commonly approved characteristics of banana ripening suggest that T1 has great potential for characterizing the ripening process of banana. However, an investigation of the full metabolic profile of banana during ripening would provide an understanding of the link between NMR relaxation and ripening characteristics. A distribution of T1 relaxation time of intact banana fruit at the micro-Tesla field was successfully generated using Laplace inversion. A suitable framework of T1-domain based studies on banana ripening also applicable to other fruit was discussed; it would provide a comprehensive understanding of structural changes and water mobility that occur in ripening banana. The SQUID-detected ultra-low field NMR used here shows promise as a tool for probing the quality of intact banana fruit. Keywords: Banana quality, Laplace inversion, Relaxometry, SQUID-NMR.


Geophysics ◽  
2011 ◽  
Vol 76 (4) ◽  
pp. G73-G83 ◽  
Author(s):  
Elliot Grunewald ◽  
Rosemary Knight

Nuclear magnetic resonance (NMR) relaxation-time measurements can provide critical information about the physiochemical properties of water-saturated media and are used often to characterize geologic materials. In unconsolidated sediments, the link between measured relaxation times and pore-scale properties can be complicated when diffusing water molecules couple the relaxation response of heterogeneous regions within a well-connected pore space. Controlled laboratory experiments have allowed us to investigate what factors control the extent of diffusional coupling in unconsolidated sediments and what information is conveyed by the relaxation-time distribution under varied conditions. A range of sediment samples exhibiting heterogeneity in the form of a bimodal mineralogy of quartz and hematite were mixed with varied mineral concentration and grain size. NMR relaxation measurements and geometric analysis of these mixtures demonstrate the importance of two critical length scales controlling the relaxation response: the diffusion length ℓD, describing the distance a water molecule diffuses during the NMR measurement, and the separation length ℓS, describing the scale at which heterogeneity occurs. For the condition of ℓS > ℓD, which prevails for samples with low hematite concentrations and coarser grain size, coupling is weak and the bimodal relaxation-time distribution independently reflects the relaxation properties of the two mineral constituents in the heterogeneous mixtures. For the condition of ℓS < ℓD, which prevails at higher hematite concentrations and finer grain size, the relaxation-time distribution no longer reflects the presence of a bimodal mineralogy but instead conveys a more complex averaging of the heterogeneous relaxation environments. This study has shown the potential extent and influence of diffusional coupling in unconsolidated heterogeneous sediments, and can serve to inform the interpretation of NMR measurements in near-surface environments where unconsolidated sediments are commonly encountered.


2007 ◽  
Vol 80 (6-7) ◽  
pp. 473-488 ◽  
Author(s):  
M. Prager ◽  
A. Desmedt ◽  
J. Allgaier ◽  
M. Russina ◽  
E. Jansen ◽  
...  

1974 ◽  
Vol 47 (4) ◽  
pp. 347-348 ◽  
Author(s):  
A.E. Zweers ◽  
H.B. Brom ◽  
W.J. Huiskamp

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