Dephasing, dipolar

The homonuclear dipolar recoupling technique of radio frequency-driven recoupling (RFDR) involves a hard n pulse per rotor period and its recoupling mechanism is based on the modulation of chemical shift difference [35-37]. When the delta-pulse approximation is relaxed so that the pulse width of the 7t pulse is about one-third of the rotor period, the so-called finite-pulse RFDR (fpRFDR) could selectively reintroduce the homonuclear dipole-dipole interaction under fast MAS conditions [38], Because the recoupling mechanism of fpRFDR does not require the presence of chemical shift difference, it can be applied to study samples with a singly labeled site. For the study of amyloid fibrils, the technique of fpRFDR is usually applied in a constant-time framework (see below). 

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The novelty of (9) is that the net homonuclear dipolar dephasing can be controlled by a systematic variation of the number of the Ho, Hi, and Hi blocks. This technique has the acronym of PITHIRDS-CT [55] and has the virtues that the effect of T2 is identical for all data points and that the rf field of all the pulses is only 1.67 times the spinning frequency. The only experimental concern is that very stable spinning or active rotor synchronization may be required for the implementation of PITHIRDS-CT. 

DQ coherence between C (,-) and C m and then let the DQ coherence evolve under the influence of the heteronuclear I3C-15N dipole-dipole interaction [181, 182]. The virtue of this design is that it can be easily combined with other resolution enhancement technique such as INADEQUATE [183]. Alternatively, the magnetization of C (,) dephased under the 13C-15N dipolar coupling can be transferred to C (j) for another period of 13C-15N dipolar dephasing [183]. This idea can be combined with the NCOCA experiment so that the superior resolution provided by the C (,-)-N(j+i) correlation could be exploited. The overall efficiency, however, is relatively low due to the use of two polarization-transfer steps, viz. 15N —> 13C and 13C —> 13C [183]. In comparison with the techniques, the advan-... 

Fig. 8 Schematic diagrams for the following pulse sequences (A) single pulse excitation/magic-angle spinning, (B) total suppression of sidebands, and (C) delayed decoupling, or dipolar dephasing.

Fyfe, C.A., Mueller, K.T., Grondey, H., and Wong-Moon, K.C. (1992) Dipolar dephasing between quadrupolar and spin-1/2 nuclei. REDOR andTEDOR NMR experiments on VPI-5. Chem. Phys. Lett., 199, 198-204. 

NMR. NMR analyses were performed on selected samples using dipolar dephasing and off magic angle spinning methods developed at the University of Utah (18-22). This work was performed at the University of Utah under the direction of Professor Ronald Pugmire. The technique allows quantitative analysis of functional groups in the solid coals and chars. 

Currently, there are no accurate methods available for quantifying the aliphatic bridges in the coal macromolecule. Quantitative nature of the application of infrared (IR) spectroscopy is limited to certain general types of functional groups or bond types. Nuclear magnetic resonance spectroscopy, despite the success of dipolar dephasing techniques to decipher the extent of substitution on carbon atoms, is still inadequate to distinguish distinct structural entities . 

Since in the case of an isolated pair of spin nuclei the dipolar dephasing, and hence the REDOR evolution curve, is exclusively governed by the dipolar Hamiltonian, the data analysis proves to be straightforward employing a universal REDOR curve, in which the normalised difference intensity AS/Sq is plotted as a function of the dimensionless product NTRxd.2 ... 

A significant advantage of the methodology of NMR spectroscopy is that it allows application of pulse sequences for the discrimination of nuclei in specific local structures, if these nuclei are characterized by a coupling with other nuclei. Examples are dipolar-dephasing techniques such as those used in TRAPDOR experiments (Section II.C). CP experiments can be applied for the discrimination of nuclei in various structures, for example, for the identification and investigation of strongly adsorbed species with low mobility. 

In chemical shift calculations for acylium ions, it was not necessary to model the ionic lattice to obtain accurate values. These ions have tetravalent carbons with no formally empty orbitals, as verified by natural bond orbital calculations (89). Shift calculations for simple carbenium ions with formally empty orbitals may require treatment of the medium. We prepared the isopropyl cation by the adsorption of 2-bromopropane-2-13C onto frozen SbF5 at 223 K and obtained a 13C CP/MAS spectrum at 83 K (53). Analysis of the spinning sidebands yielded experimental values of = 497 ppm, 822 = 385 ppm, and (%3 = 77 ppm. The isotropic 13C shift, 320 ppm, is within 1 ppm of the value in magic acid solution (17). Other NMR evidence includes dipolar dephasing experiments and observation at higher temperature of a scalar doublet ( c-h = 165 Hz) for the cation center. 

Fig. 47. 15 MHz 13C MAS NMR spectrum of cured furfuryl alcohol resin a. The spectrum of the same resin obtained with dipolar dephasing (100 ps interrupted decoupling) b (reprinted from Ref.2341 with permission)...

FIGURE 20. A H-29Si dipolar-dephasing 29Si CPMAS NMR experiment. Reprinted with permission from Reference 135. Copyright 1992 American Chemical Society... 

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