Separated local field

Dynamic parameters for heterogeneous systems have been explored in the liquid, liquid like, solid like, and solid states, based on analyses of the longitudinal or transverse relaxation times, chemical exchange based on line-shape analysis and separated local field (SLF), time domain 1H NMR, etc., as summarized in Figure 3. It is therefore possible to utilize these most appropriate dynamic parameters, to explore the dynamic features of our concern, depending upon the systems we study. 

In the separated local field technique, dipolar I-S interactions are separated from chemical shifts of nucelus S. As dipolar interactions are highly sensitive to internuclear distances, the obvious use of the method is for the determination of molecular structure in the solid state. An example is provided by the work of Hester et al. (411) and Rybaczewski et al. (414) on... 

Ramamoorthy and co-workers126 have recently reported proton-detected local field (PDLF) sequence to define the structure and dynamics of biological membranes. As was shown for this kind of laboratory-frame separated-local-field experiment, simple modification in the 2D PDLF sequence enables the measurement of remote 13C-31P and 1H-31P couplings in magnetically aligned bicelles. 

FIGURE 10.8 Pulse sequence for the separated local field experiment. After cross polarization, spin S precesses under the influence of dipolar interactions during , but is decoupled from I during (2. 

DIPSHIFT Separated local fields to determine dipolar/J-coupling interaction at sites. e.g. Webb Zilm (1989), Jakobsen et al. (1982)... 

Correlation of tensor information - separated local field experiments... 

Most separated local field experiments concentrate on the CSA. In speetra of halfinteger nuclei with / > 1/2, the CSA is often much less important than the second-order quadrupole interaction. Static or MAS spectra display a typical quadrupole powder pattern from which the quadrupole coupling constant and the asymmetry parameter can be determined. These quadrupole parameters can then be used to determine the principal components of the electric field gradient tensor, which are related to the local environment of the quadrupole nucleus. Knowledge of the local geometry is extremely valuable, and correlation of the quadrupole and dipole information of OH groups provides useful insights into structure. 

Figure 3.26. A quadrupole-dipole separated local field experiment for the OH group in Mg(OH)x(OCH3)2-x with A. the experimental data compared with B. the simulation of the intensity assuming the tensors are collinear from van Eck and Smith (1998) with permission...

Figure 15 presents the pulse sequence for a general separated local field (SLF) experiment.133-136 The basic principle of the SLF technique is that a spinning-sideband pattern, from which the heteronuclear dipolar coupling can be extracted, is obtained in the indirect dimension for each resolved resonance in the direct dimension, i.e., the dipolar interaction is separated from the chemical shift interaction. In the original SLF papers, a homonuclear decoupling method is applied in t, but recently McElheny et al. 

Figure 15. Pulse sequence for a conventional separated local field (SLF) heteronuclear experiment.

Fig. 1. A two-dimensional pulse sequence for separated-local-field spectroscopy. The pulse sequence correlates the chemical shift of a low-sensitive nuclei S (e.g., or with the dipolar coupling between S and the abundant nuclei I (e.g., or F). During the delays before (r—q) and after (t) the 7t-pulse, / spins are decoupled from S spins by continuous rf irradiation on S spins. The details of this pulse sequence are discussed in the text.

ID, 2D and 3D experiments, all H, and Sn chemical shifts for four postulated Sn species could be resolved unambiguously. Variable temperature and variable stress NMR spectroscopic and imaging studies are reported for a sample of high impact polystyrene. NMR microscopy and solid state stray field imaging studies indicated that imposed stress affects the molecular chain dynamics only of the polybutadiene region and not of the polystyrene(PS) matrix.The principal values of the chemical shift tensors and the separated-local field pattern are reported for the all-... 

Fig. 4.4. Spectral factor/ fj(0) in the spin-diffusion rate constant calculated from the experimental separated-local-field spectrum of amorphous polystyrene [21, 30] for (a) a static sample, and (b) for a spectrum obtained under slow-MAS conditions. (Adapted from Ref. [21], with permission).

Several new assignment techniques for C solid-state NMR spectra have recently been proposed Spectral Editing by Multiple-Quantum Traps (SEMUT) [15], separated local field MAS [17-19], Windowless /sotropic Mixing for Spectral Editing (WIMSE) [20], Off-/ esonance Proton /rradiation (ORPI) [21,22], and cross-polarization with fast MAS ]23]. Their detailed performance has yet to be assessed, but it is already clear that the latter two methods are simple and eminently worthwhile. 

Fig. 25. Restriction plots calculated from the experimental spectral data associated with one molecule in the unit cell of NAV (A) N overtone shift, (B) N/ H dipolar splitting, (C) H chemical shift, (D) H/ N dipolar splittings. (E) the intersection of all four types of experimental data, (F) the intersection of the data obtained with a N separated local field experiment alone, (G) the intersection of the data with the N overtone shift and H chemical shift alone, and (H) the intersection of the data obtained with N overtone shift and H/ N dipolar splitting alone. (Reproduced ftrom McNamara et with permission.)...

An interesting three-dimensional separated-local-field/dilute-spin-exchange solid state NMR experiment has been reported where homonuclear spin-exchange cross-peaks provide correlation in the un u>z plane. Resonances are resolved along the u> axis by their heteronuclear dipolar couplings. The experiment was demonstrated experimentally with the heteronuclear and N- N homonuclear dipole-dipole interactions in... 

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