Multiple-pulse techniques

C 1-NMR spectroscopy is the method of choice for determining the molecular structure of polymers in solution [230]. Polyolefin 13C NMR is mainly quantitative ID 1-NMR multiple pulse techniques are used for spectral interpretation. The resolution obtained in 13C NMR spectra of LDPE is an order of magnitude larger than in the corresponding 1H-NMR spectra... 

Some readers may wonder at the inclusion of NMR under the rubric of modem instrumentation. After all, NMR spectroscopy has been a part of the curriculum in Organic Chemistry for years, and it is a rare student who cannot use ll NMR,, 3C NMR, and a host of multiple-pulse techniques to identify even structures of moderate complexity. However, NMR as a technique goes far beyond structure determination, and a number of these facets have been included in recent physical chemistry experiments. 

Efforts to understand the state of hydrogen in metals and metal hydrides have involved the use of NMR for many years. This study combines the conventional solid state NMR techniques with more recently developed high-resolution, solid state NMR techniques (5,6). Conventional NMR techniques furnish information on dipolar interactions and thus can furnish static geometrical information on hydrogen positions and information on proton motion within such solids. The newer multiple pulse techniques suppress proton-proton dipolar interaction and allow information on other, smaller interactions to be obtained. This chapter reports what the authors believe is the first observation of the powder pattern of the chemical shift tensor of a proton that is directly bonded to a heavy metal. 

Th4Hi5. Three kinds of information are obtained from the samples of Th4H15 information on rigid lattice structure from free induction decays, on proton motion from relaxation time measurements, and on internal fields from peak locations that were found using the multiple pulse techniques. Figure 1... 

In general, multiple pulse techniques sufficiently average the dipolar interactions, compress the chemical shift scale, but they do not affect heteronuclear dipolar interactions and the chemical shift anisotropy. A combination of both multiple pulse techniques and magic angle spinning, so-called CRAMPS (Combined Rotational And Multiple Pulse Spectroscopy) is found to yield satisfactory results in the solid state H NMR of solids 186). The limitations of all these techniques, from the analytical point of view, arises from the relatively small chemical shift range (about 10 ppm) as compared with some other frequently studied nuclei. However, high resolution H NMR of solids is useful in studies of molecular dynamics. 

New techniques for data analysis and improvements in instrumentation have now made it possible to carry out stmctural and conformational studies of biopolymers including proteins, polysaccharides, and nucleic acids. NMR, which may be done on noncrystalline materials in solution, provides a technique complementary to X-ray diffraction, which requires crystals for analysis. One-dimensional NMR, as described to this point, can offer structural data for smaller molecules. But proteins and other biopolymers with large numbers of protons will yield a very crowded spectrum with many overlapping lines. In multidimensional NMR (2-D, 3-D, 4-D), peaks are spread out through two or more axes to improve resolution. The techniques of correlation spectroscopy (COSY), nuclear Overhausser effect spectroscopy (NOESY), and transverse relaxation-optimized spectroscopy (TROSY) depend on the observation that nonequivalent protons interact with each other. By using multiple-pulse techniques, it is possible to perturb one nucleus and observe the effect on the spin states of other nuclei. The availability of powerful computers and Fourier transform (FT) calculations makes it possible to elucidate structures of proteins up to 40,000 daltons in molecular mass and there is future promise for studies on proteins over 100,000... 

The main features of the coherent multiple pulse technique are that it is self-calibrating in frequency that the signal strength is the same as that obtained with a single mode laser of the same average power and that frequency doubling efficiency can be good. 

Line narrowing by multiple-puhe techniques (cf Section 3,3.4) is a promising approach to solid-state imaging with good spatial re.solution. However, high demands on experimental set-up and equipment hamper the use of the technique. Moreover, for line narrowing by multiple-pulse techniques the data points of the free-induction decay are acquired between pulses (cf. Fig. 3.3.10), and the efficiency depends on the quality of the rf pulses [Hael, Mehl. Radio-frequency pulse.s with short rise and fall times require a low quality factor of the rf coil which, at the same time, reduces the sensitivity and therefore limits the. spatial resolution. 

Line narrowing with multiple-pulse techniques can also be implemented for imaging with pulsed Bj gradients [Choi]. This approach provides fast switching times, but gradients are low and additional rf coils have to be used. 

A cured epoxy synthesised from a mixture of the diglycidyl ether of bisphenol A (DGEBA) and 1,3-phenylenediamine was studied by NMR spectroscopy including multiple pulse techniques and spin-lattice relaxation in the rotating frame. Tip. The study [28] focused on the water distribution based upon possible variation in the cross-link density measured by spin diffusion. From the analysis involving a combination of Tip and multiple... 

Questions of crystallinity and motion at the molecular level have been further investigated by English and Vega [12-14] using simplified spectra obtained from the multiple-pulse technique (REVS) at various temperatures. They did not use MAS but deconvoluted the bands (e.g., at 259 K—see Fig. 

Vega and English [13] obtained F spectra and relaxation times for static samples of a copolymer of tetrafluoroethylene and hexafluoropropylene (TFE-co-HFP 85 mol% HFP) by the multiple-pulse technique (MREV8) at various temperatures (see Fig. 18.19). They observed CF2 group lineshapes in crystalline and amorphous regions, and also CF3 and CF lineshapes. After subtracting the crystalline contribution to obtain the amorphous lineshape, the latter contribution was analysed as a function of temperature to obtain information about the type of molecular motion present. The )8 and y relax-... 

The basic principle behind the multiple-pulse NMR techniques to achieve line narrowing (i.e., eliminate the H- H dipolar interaction) is to manipulate the H spin system with r.f. pulses rather than by motion of the whole system, as is done with MAS. This manipulation is performed by using a series of well-timed r.f. pulses such that the average Hamiltonian over the entire period of the pulse sequence does not include the homonuclear dipolar interaction, but still maintains a scaled-down chemical shift e ct. Because of the strict requirements on r.f. pulse widths, shapes, phasing and timing, the multiple-pulse techniques represent some of the most difficult solid-state NMR techniques to implement on a routine basis. The most popular multiple-pulse techniques are currently the eight-pulse MREV-8 and the 24-pulse BR-24 sequence. ... 

Bmte-force fast MAS is not the only means by which line narrowing can be achieved in solid-state NMR. A particularly ingenious alternative approach, first presented over 30 years ago by Waugh and co-workers, involves the removal of the dipolar broadening by specific multiple-pulse techniques, where radiofrequency pulses achieve rotations in spin space [51, 52]. These rotations can complement the effect of the physical rotation of the sample combined rotation and multiple-pulse spectroscopy (CRAMPS) [53—55] yields well-resolved H spectra [56]. 

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