Separated local field experiment

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. 

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...

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.)...

For measuring heteronuclear dipolar couplings in oriented systems, a new separated local field experiment has been presented. The method is based on the dipolar assisted polarization transfer (DAPT) pulse sequence proposed recently for transfer of polarization between two spins I and DAPT utilizes the evolution of magnetization of the I and S spins under two... 

In separated local field experiments, the sites are separated in the Vj dimension of a 2D experiment according to their chemical shifts and correlated with their individual dipole-dipole couplings (their local fields) in the V2 dimension. The original C-detected local-field spectra are more complicated than the more recently introduced proton-detected local-field spectra [86]. When using C detection, each C... 

Separated-local-field experiments are commonly used to measure the heteronuclear dipole-dipole interactions (called local-field in the molecule) in solids [2, 4, 30], These are two-dimensional pulse sequences as depicted in Figure 3-4 that consists of preparation, evolution and detection periods. A 90° pulse on the I nuclear spin (abundant nuclei like protons) channel followed by the cross-polarization sequence is the typical preparation period of the pulse sequence. This preparation step generates transverse magnetization of S nuclei (less sensitive nuclei like 13C) that evolves during the evolution period (tl) of the sequence under a suitably dressed Hamiltonian. There are a number of pulse sequences that differ in the... 

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.

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-... 

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... 

As specific examples, the detected local field experiment has recently successfully been applied to the characterisation of liquid crystals [155, 156] and membranes [157]. As illustrated by Fig. 9.33, this approach has even been shown to yield sufficient resolution in 3D versions to allow the direct measurement of internuclear dipolar couplings between nuclei separated by up to five bonds in liquid crystals, thereby providing very strong conformational constraints [158]. 

A novel MAS NMR approach has been presented for the determination of motionally averaged heteronuclear dipolar couplings in unoriented materials. The technique is based on the proton-detected local field protocol and achieves dipolar recoupling by R-type RF irradiation. The experiment, which is called R-PDLF spectroscopy, is demonstrated on solid and liquid-crystalline systems. For mobile systems, it is shown that the R-PDLF scheme provides better dipolar resolution as compared to techniques combining conventional separated local field spectroscopy with R-type recoupling. 

Lobo and Ramanathan have combined adiabatic and Hartmann-Hahn cross-polarization for sensitivity enhancement in solid-state separated local field 2D NMR experiments of partially ordered systems The magnetization in double- and zero-quantum reservoirs of an ensemble of spin-1/2 nuclei has been examined and their role in determining the sensitivity of a class of separated local field NMR experiments based on Hartmann-Hahn cross-polarization has been described. Lobo and Ramanathan report that for the liquid crystal system studied, a large dilute spin-polarization, obtained initially by the use of adiabatic cross-polarization, can enhance the sensitivity of the above experiment. The signal enhancement factors, however, are found to vary and depend on the local dynamics. The experimental results have been utilized to obtain the local order-parameters of the system. 

Two-dimensional separated-local-field NMR experiments have been applied to magnetically aligned bicelles to measure dipolar cou-... 

One way to determine heteronuclear dipolar couplings with MAS is to perform separated local field spectroscopy (SEE) experiments, a 2D NMR protocol which resolves for... 

It has been suggested that the relative sign and the magnitude of the scalar and the residual dipolar couplings in homonuclear spin 1/2 systems can be determined based on the sign and the amplitude of the transfer fimctions in a TOCSY experiment. The efficiency of different mixing sequences and different transfer functions has been examined both theoretically and experimentally. The efficiencies of various separated local field (SLF) experiments... 

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