NMR spin-echo method

Self-diffusion coefficients were measured with the NMR spin-echo method and mutual diffusion coefficients by digital image holography. As can be seen from Figure 4.4-3, the diffusion coefficients show the whole bandwidth of diffusion coeffi-... 

Self-difFusion coefficients were measured with the NMR spin-echo method and mutual diffusion coefficients by digital image holography. As can be seen from Figure 4.4-3, the diffusion coefficients show the whole bandwidth of diffusion coefficient values, from 10 m s on the methanol-rich side, down to 10 on the [BMIM][PFg]-rich side. The concentration dependence of the diffusion coefficients on the methanol-rich side is extreme, and shows that special care and attention should be paid in the dimensioning of chemical processes with ionic Hquids. 

NMR spin-echo method 165,167 nuclear Overhauser enhancement (NOE) 168 ff., 172 nucleophilic displacement reaction 184 ff. 

Figure 12.1 Clearance of small-molecule impurities from process buffers in a formulated protein product. Trace A the NMR spectrum of a control sample containing a mixture of three components (succinate, tetraethylammonium, and tetramethylammonium) in the final formulation buffer (sodium acetate). These three components were used in the recovery process for a biopharmaceutical product. Traces B and D the proton NMR spectra of the formulated protein product. No TEA or TMA were detected, but a small amount of succinate was observed in this sample. Traces C and E the proton NMR spectra of a formulated protein product spiked with 10 jag/ml of succinate, TEA, and TMA. Traces D and E were recorded with CPMG spin-echo method to reduce the protein signals. The reduction of NMR signals from the protein allows for better observation of the small-molecule signals.

Figure 12.3 Clearance of MES in a formulated protein product. Trace A the proton NMR spectrum of a formulated protein product spiked with 8 jig/ml of MES. Trace B the proton NMR spectrum of a formulated protein product. The arrows indicate the positions where MES signals would be detected if present. Trace C the difference of traces A and B (A-B). Trace D proton NMR spectrum of 8 ftg/ml of MES in the formulation buffer. The NMR spectra in traces A, B, and D were recorded with the CPMG spin-echo method to reduce protein signals. Only the region where MES signals appear is shown.

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. 

The former is a protein of 14.7 kDa involved in the multienzyme nucleotide excision repair (NER) pathway with a determined NMR solution structure . In this protein, the Zn + possesses rather a structural than a catalytic role. Zn NMR spectra were acquired using a rather sophisticated probe (for details, see Reference 87) and operating at temperatures 5-250 K. Data acquisition was performed with the application of spin-echo methods for enhanced sensitivity . Specifically, experiments were carried out at 25 K using a combination of CP (cross-polarization) and spikelet echo pulse sequences which provide a considerable increase in signal-to-noise ratio (of the order of 30) relative to a classical quadrupole echo pulse sequence. The proton field strength applied to the above measurements was 60 kHz with a matching field of 20 kHz for zinc and a contact time... 

In polymers, the field-gradient spin-echo methods of measuring self-diffusion have been useful in three more or less distinct areas, the diffusion of polymers in their own melt and in concentrated solutions, in dilute and semidilute solutions, and the diffusion of penetrants and diluents in polymer hosts. A fourth category, the diffusion of bulky or flexible molecules in polymer hosts, is useful for subject matter not closely associated with the first and third category. It should be noted that the work reviewed here represents only a small fraction of the diffusion studies in polymers, including those using other NMR methods. 

The self-diffusion coefficients in supercritical ethylene were measured using the pulsed NMR spectrometer described elsewhere (9,10), automated for the measurement of diffusion coefficients by the Hahn spin echo method (11). The measurements were made at the proton resonance frequency of 60 MHz using a 1 1.2 kG electromagnet. 

Subsequently, Uhrinova et al.29 reconsidered the problem using both proton-and carbon-detected experiments. For example, couplings of anomeric carbons were measured from the 13C satellites in proton NMR spectra. The critical factor in these methods is the suppression of signals from protons bound to, 2C atoms. In the pulse-sequence proposed, these protons were selectively inverted by a BIRD (Bilinear Rotation Decoupling) pulse,30 and the spin-echo method introduced by Bendall et al.31 was used. 

The ultrafast infrared vibrational echo experiment and vibrational echo spectroscopy are powerful new techniques for the study of molecules and vibrational dynamics in condensed matter systems. In 1950, the advent of the NMR spin echo (1) was the first step on a road that has led to the incredibly diverse applications of NMR in many fields of science and medicine. Although vibrational spectroscopy has existed far longer than NMR, the experiments described here are the first ultrafast IR vibrational analogs of pulsed NMR methods. In the future, it is anticipated that the vibrational echo will be extended to an increasingly diverse range of problems and that the technique will be expanded to new pulse sequences, including multidimensional coherent vibrational spectroscopies such as the vibrational echo spectroscopy technique describe above. 

The spin echo was discovered in 1950 by Erwin Hahn24 and is sometimes called a Hahn echo. The real significance of the spin-echo method lies not in its use to measure T2, but in the demonstration that an apparently irreversible dephasing of nuclear spins and decay of the FID (even to zero) can be reversed. As we see in Chapter 7, application of this concept is extremely important in obtaining narrow NMR lines in solids. We shall also encounter numerous examples in NMR of liquids where a spin echo is employed. 

Spin-echo methods are less widely used, but certainly rival the ENDOR method, and the sensitivity can be much greater. The method has been most widely developed in studies of transient species produced in flash photolysis or pulse radiolysis experiments. At this stage, the great advantages found in NMR spectroscopy have not been fully realized in ESR spectroscopy, but time will tell. 

Chapek et a/.349,350 pointed out that, among the two types of water of hydration, a significant major proportion of the water is weakly bound, weakly oriented, or osmotic. Its heat of adsorption is approximately 2 kcal/ mol. Later studies351352 with the NMR spin-echo technique and dielectric methods illustrated that barely 3% or less of the water enters discrete cavities (micropores that are comparable in size to the size of water molecules) in the first stage of sorption. Subsequent sorption proceeds in these micropores with the formation of clusters of water molecules. 

Meiboom S, Gill D. Modified spin-echo method for measuring nuclear relaxation times. Rev. Scient. Inst. 1958 29 688-691. Millet O, et al. The static magnetic field dependence of chemical exchange linebroadening defines the NMR chemical shift time scale. J. Am. Chem. Soc. 2000 122 2867-2877. 

The broad La NMR spectrum of the single La site in polycrystalline LaaOa (Figure 10.22A) has been determined by the frequency-swept spin-echo method (Bastow 1994), yielding an isotropic chemical shift of 424.8 ppm and Xq = 58.52 MHz. The lanthanum site in this compound has axial symmetry, making t = 0. 

It has recently become more widely appreciated that the presence of rotational diffusional anisotropy in proteins and other macromolecules can have a significant affect on the interpretation of NMR relaxation data in terms of molecular motion. Andrec et al. used a Bayesian statistical method for the detection and quantification of rotational diffusion anisotropy from NMR relaxation data. Sturz and Dolle examined the reorientational motion of toluene in neat liquid by using relaxation measurements. The relaxation rates were analyzed by rotational diffusion models. Chen et al measured self-diffusion coefficients for fluid hydrogen and fluid deuterium at pressures up to 200 MPa and in the temperature range 171-372 K by the spin echo method. The diffusion coefficients D were described by the rough sphere (RHS) model invoking the rotation translational coupling parameter A = 1. 

Fig. 19.8. Dipolar-decoupled MAS C NMR spectra of the rubbery component in annealed A-PVA films with a water content of 18%, which were measured by the modified C spin-echo method (Ref. [33]).

Fig. 17. NMR 3D and 2D images of (a) Goonyella coal (b) Witbank coal obtained by SPRITE method and (c) Goonyella coal obtained by spin echo method.

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