NMR spectroscopy and relaxation

Davis JH, Bradley EK, MUjanich GP, Nadasdi L, Ramachandran J, Basus VJ. Solution structure of omega-conotoxin GVIA using 2-D NMR spectroscopy and relaxation matrix analysis. Biochemistry 1993 32(29) 7396 5. 

The adsorption of trialkoxysilane on Si3N4 for colloidal processing has been studied using Si NMR spectroscopy. The interaction of dodecyl phosphate with mesostructured Zr02 has been investigated using MAS NMR spectroscopy.and relaxation measurements have been made to study the surface structure and dynamics of thiophenol-capped CdS nanocrystals. 

Molecular Motions and Dynamic Structures. Molecular motions are of quite general occurrence in the solid state for molecules of high symmetry (22,23). If the motion does not introduce disorder into the crystal lattice (as, for example, the in-plane reorientation of benzene which occurs by 60° jumps between equivalent sites) it is not detected by diffraction measurements which will find a seemingly static lattice. Such molecular motions may be detected by wide-line proton NMR spectroscopy and quantified by relaxation-time measurements which yield activation barriers for the reorientation process. In addition, in some cases, the molecular reorientation may be coupled with a chemical exchange process as, for example, in the case of many fluxional organometallic molecules. ... 

Methods for measuring Ti and T2 are discussed in Chapter 5 of reference 21. Suffice it to say here that understanding the method for measuring T2 (the Carr-Purcell pulse sequence or spin-echo method) becomes important for discussing two-dimensional NMR spectra. When spin-spin coupling is present, a modulation of spin echoes is produced, and it is this fact that is important in 2-D NMR spectroscopy. Nuclear relaxation rates and mechanisms become important when discussing the effect of paramagnetic metal centers on NMR spectroscopy. 

For si and sll, Davidson et al. (1977a, 1981) performed NMR spectroscopy and dielectric relaxation measurements where applicable, in order to estimate the barriers to molecular reorientation for simple hydrates of natural gas components, except carbon dioxide. Substantial barriers to rotation should also affect such properties as hydrate heat capacity. 

The study of nucleic acid bases by NMR has been reported in a number of monographs (/), but very little data is available on the, 3C and, 5N NMR chemical shift tensors in these compounds. The low sensitivity of NMR spectroscopy and the long relaxation times exhibited by many of these compounds have posed the main impediments for these studies. The use of sample doping with free radical relaxation reagents, to reduce the relaxation times facilitating 2D multiple pulse experiment (2, 3), enables one to measure and analyze the principal values of the chemical shift tensors in natural abundance samples. In previous papers from this laboratory we have presented, 5N NMR chemical shift principal values for adenine, guanine, cytosine, thymine and uracil (4, 5). 

One of the mayor drawbacks is that only volatile and temperature-resistant compounds can be investigated. Gases are magnetized faster than liquids, because they have shorter spin-lattice relaxation times (T ), due to an effective spin rotation mechanism. Therefore, pulse repetition times in flow experiments can be in the range of 1 s and some dozen transients can be accumulated per separated peak. Nevertheless, the sample amounts used nowadays in capillary GC are far from the detection limit of NMR spectroscopy, and therefore the sensitivity is low or insufficient, due to the small number of gas molecules per volume at atmospheric pressure in the NMR flow cell. In addition, high-boiling components (> 100 °C) are not easy to handle in NMR flow probes and can condense on colder parts of the apparatus, thus reducing their sensitivity in NMR spectroscopy. 

NMR spectroscopy and solid-state and 13C NMR relaxation-time experiments. However, the sensitivity of solid-state 13C NMR is not as high as that of Raman and IR spectroscopy. For instance, solid-state 13C NMR of sulfur-vulcanised EPDM could only be performed when the ENB unsaturation of EPDM was fully isotopically enriched with 13C NMR [124]. 

Structures 13C, 2H and 2H NMR spectroscopy (T2 relaxation times, signal intensity ratio versus generation) rheology studies electron microscopy computer-assisted molecular simulations, comparison to CPK models electrospray mass spectroscopy fluorescence probe analysis. 

Wrackmeyer et prepared tetrakis(trimethylstannyl)germanium (131) and studied this compound by Ge and Sn NMR spectroscopy. Quadrupolar relaxation of the Ge nucleus is slow, which allows one to measure 7( Sn- Ge) = 37 Hz and to determine its positive sign by a 2D heteronuclear Ge- H shift con-elation. The absolute... 

Landry and Henrichs [63] applied dynamic mechanical spectroscopy and H NMR to investigate sub-T motion in polycarbonate(PC)/PMMA and PC/poly(cyclohexylene dimethylene terephthalate)(PCHDMT). Examination of H NMR spectra and relaxation times led them to conclude that local... 

Particle Microstructure (NMR Spectroscopy and Elemental Analysis). The 29Si CP NMR spectra were much easier to obtain than the direct excitation spectra, because of the long spin-lattice relaxation time of the silicon nuclei. Nevertheless, direct excitation was used to obtain the quantitative results of the siloxane structure, because not all Q4 nuclei were detected with CP (Figure 3 and Table III) (47). 

Much work has also been carried out on non-cereal plant proteins. Solid-state H and 2H NMR transverse relaxation measurements were used to investigate the effect of hydration on the plasticization of vicilin, legumin, and albumin fractions from peas.81 Their behaviour indicated that the plasticization of the globular legume proteins is considerably less than that found before for the linear barley protein C-hordein. The effects of microbial transglutaminase treatment on soy protein samples have been studied by H and solid-state 13C NMR spectroscopy, and its relation to the glass transition temperature was examined.82-83... 

Some information can be obtained on porous media from conventional NMR spectroscopy, and this is discussed in Section 2. Relaxation time measurements have been widely used to characterize porous solids, and this technique is discussed in Section 3. Pulsed field gradient (PFG) methods may be used to probe the local structure of the pore space and to characterize transport within it, and these are discussed in Section 4. Magnetic resonance imaging (MRI) techniques can also be used to characterize the pore space and to measure transport, and applications are discussed in Section 5. The bulk of this review will be concerned with mesoporous and macroporous materials, as it is for these systems that NMR is particularly useful in characterizing the pore space. However, some applications of NMR techniques to probe the pore space and transport within microporous materials will be mentioned in Section 6. Finally, some general conclusions are given in Section 7. 

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