Multiple-pulse sequence spectroscopy

The influence of the homonuclear magnetic dipole-dipole interaction on can be reduced either by an increase of the sample spinning frequency, Vjot, (Eq. (20)) or by the application of a multiple-pulse sequence causing an additional averaging of this interaction (combined rotation and multiple-pulse spectroscopy, CRAMPS 19-21 ). With today s instruments, sample spinning frequencies of up to 40 kHz can be reached using MAS NMR rotors with an outer diameter of 2.0 mm. 

The acronym COSY stands for Correlated SpectroscopV and this technique is widely used to determine all of the coupling interactions in a single experiment. This proves to be more efficient than the decoupling experiment in which each signal is irradiated in turn to determine its coupling partners. COSY involves a multiple pulse sequence (which we do not need to know anything about in order to use the technique) and is an example of two-dimensional (2D) spectroscopy. 

The major breakthroughs, however, have come from the use of high magnetic fields and further from the use of different multiple pulse sequences to manipulate the nuclear spins in order to generate more and more information time domain NMR spectroscopy, that is used to probe molecular dynamics in solutions. The latter made it also possible to "edit" sub-spectra and to develop different two-dimensional (2D) techniques, where correlation between different NMR parameters can be made in the experiment (e.g. SH versus 813c, see later). Solid state NMR spectroscopy is used to determine the molecular structure of solids. 

Combined Rotation and Multiple Pulse Spectroscopy (CRAMPS) is a technique in which the dipolar interaction is averaged through a multiple-pulse sequence [54, 55]. The simultaneous spinning around the magic angle, as in MAS NMR, averages the chemical shift anisotropy. Under appropriate conditions, CRAMP spectra can be of greater resolution than MAS NMR spectra. While CRAMPS is not exclusively a surface-sensitive technique, the majority of catalytic applications have focused on the study of adsorbed species, and the information on surface structure that can be extracted from their spectra. 

In this chapter multiple-pulse sequences for homonudear Hartmann-Hahn transfer are discussed. After a summary of broadband Hartmann-Hahn mixing sequences for total correlation spectroscopy (TOCSY), variants of these experiments that are compensated for crossrelaxation (clean TOCSY) are reviewed. Then, selective and semiselective homonudear Hartmann-Hahn sequences for tailored correlation spectroscopy (TACSY) are discussed. In contrast to TOCSY experiments, where Hartmann-Hahn transfer is allowed between all spins that are part of a coupling network, coherence transfer in TACSY experiments is restricted to selected subsets of spins. Finally, exclusive TACSY (E.TACSY) mixing sequences that not only restrict coherence transfer to a subset of spins, but also leave the polarization state of a second subset of spins untouched, are reviewed. 

Recently, in this laboratory, we have applied time-dependent quantum mechanics-wavepacket dynamics to several bona fide time-domain spectroscopies. Specifically, we have formulated time-dependent theories of coherent-pulse-seque nee-induced control of photochemical reaction, picosecond CARS spectroscopy, and photon echoes. These processes all involve multiple pulse sequences in which the pulses are short or comparable in time scale to the... 

More generally, note that the application of almost any multiple pulse sequence, where at least two pulses are separated by a time comparable to the reciprocal of the coupling constants present, will lead to exchanges of intensity between multiplets. These exchanges are the physical method by which coupled spins are correlated in 2D NMR methods such as correlation spectroscopy (COSY) [21]. 

Another method to remove the anisotropy of spin interactions and to obtain high resolution spectra of solids is by multiple-pulse sequences. The combination of multiple-pulse sequences and MAS (CRAMPS combined rotational and multiple-pulse spectroscopy) is used to obtain highly resolved proton spectra. 

In the first section the basic NMR theory is developed with particular reference to deuteron spin-labels in anisotropic media. The model takes into account both intermolecular rotational diffusion in an ordering potential as well as intramolecular isomerization. It also considers various double- and multiple-pulse sequences, recently employed in Fourier transform NMR spectroscopy. ... 

The arsenal of techniques used for solutions is being applied to solids or liquid crystal solutions as well. For example, multiple quantum spectroscopy and two-dimensional NMR have been combined with the techniques described above. Density matrix theory and the individual spin operator formalism have been used to understand the mechanisms of multiple pulse sequences and the design of new ones. Since FT spectrometers offer many of the techniques mentioned in this Section 6 as standard features, the chemist can utilize an array of multiple resonance multiple pulse techniques without benefit of theory, using Refs. 41-42 as guides. 

Our province is chemical and biochemical rather than physical or technical only passing reference is made to metallic solids or unstable species, or to practical NMR spectroscopy. Our aim is depth as well as breadth, to explain the fundamental processes, whether of nuclear magnetic shielding, spin-spin coupling, relaxation, or the multiple pulse sequences that have allowed the development of high-resolution studies of solids, multidimensional NMR spectroscopy, techniques for sensitivity enhancement, and so on. 

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