Multiple-quantum spectroscopy

T.H. Mareci, Principles of Multiple-Quantum Spectroscopy, in Pulse Methods in ID and 2D Liquid-Phase NMR, ed. W.S. Brey (Academic Press, New York, 1988) Ch. 4. 

As mentioned earlier, the proton NMR spectra of small molecules oriented in nematic liquid crystals become rapidly complex with the increase of number of interacting nuclei. In order to obtain information on orientation and structure, it is therefore essential to develop strategies to interpret such spectra with the help of special techniques. Techniques such as Automatic Analysis and the use of Multiple Quantum Spectroscopy have been employed for these purposes during the period under report. A method based on the Automatic Analysis of Multiple Quantum proton spectra of flexible molecules dissolved in liquid crystalline phases has been suggested for extracting starting parameters for the... 

In the last few years the combination of high spinning speed with 2D multiple quantum spectroscopy has opened up new possibilities in the area of hydrogen bond investigation by 1H solid state NMR. The field has been exhaustively reviewed by Schnell and Spiess [77]. In Fig. 3.2.26 is reported, for example, the information available by ID and 2D double quantum proton spectra in the case of malonic acid. Each signal can be unequivocally assigned to the respective proton, and the chemical shift of the carboxylic protons can be correlated to the 0-0 distance. Furthermore the intermolecular correlation between CH2 and COOH protons can be used as a constraint for establishing the conformation and array structure in the solid state. 

Another very interesting example of the use of H multiple-quantum spectroscopy under rapid MAS for the purpose of investigating rapid molecular motion concerns the motion of ethanol adsorbed on a silica surface.24 The DQ sideband patterns obtained from DQ SQ correlation experiments using different DQ excitation times are shown in Fig. 17, along with their simulation according to the motion of the OCH2 group of the adsorbed ethanol. 

Multiple quantum spectroscopy offers complementary information to COSY to elucidate scalar coupling networks [452]. The multiple quantum transitions are observed indirectly during fi. The two-quantum (2Q) experiment is commonly used to circumvent problems in COSY due to diagonal peaks, self-cancellation of... 

A. Lessage, D. SakeUariou, S. Steuernagel, L. Emsley, Carbon—proton chemical shift correlation in sohd-state NMR by through-bond multiple-quantum spectroscopy, J. Am. Chem. Soc. 120 (1998) 13194-13201. 

Key Words Geopolymers, Aluminosilicates, Solid-state NMR, Al MAS NMR, Al 3Q/ MAS NMR, Multiple-quantum spectroscopy. Principle component analysis... 

To conclude this section on 2D NMR of liquid crystals, some studies of more exotic liquid crystalline systems are pointed out. Polymer dispersed nematic liquid crystals have attracted much attention because of their applications as optical display panels. Deuteron 2D quadrupole echo experiments have been reported [9.28] in the isotropic and nematic phases of / -deuterated 5CB dispersed in polymers. A similar technique was used [9.29] to study two model bilayer membranes. Both studies allow determination of the lineshape F(u ) due to quadrupolar interactions and the homogeneous linewidth L(u ) of the individual lines [9.28]. The 2D quadrupole echo experiment has also been used [9.30] to separate chemical shift and quadrupolar splitting information of a perdeuterated solute dissolved in a lyotropic liquid crystal. The method was compared with the multiple-quantum spectroscopy that is based on the observation of double-quantum coherence whose evolution depends on the chemical shift but not on the quadrupolar splitting. The multiple-quantum method was found to give a substantial chemical shift resolution. The pulse sequences for these methods and their treatment using density matrix formalism were summarized [9.30] for a spin 1=1 system with non-zero chemical shift. Finally, 2D deuteron exchange NMR was used [9.31] to study ring inversion of solutes in liquid crystalline solvents. 

Tolman, J. R., Chung, J. and Prestegard, J. H. Pure-phase heteronuclear multiple-quantum spectroscopy using field gradient selection. J. Magn. Reson. 98 462—467, 1992. 

A number of techniques have been developed for the simplification of LCNMR spectra of complex spin systems, which augment the time-honoured procedures of deuteria-tion (selective, partial, or random) and double resonance methods. The most powerful of these include variable angle sample spinning [32], multiple quantum spectroscopy [33-35] and various other 2D or multiple pulse [36-41] techniques, and zero field NMR [42,43]. 

The fourth class of experiments is multiple quantum spectroscopy such as 2D-INADEQUATE. In these experiments, homonuclear shifts (such as those of are plotted along the fi dimension. If two or more nuclei are /-coupled to each other, then they can be made to share a common multiple quantum precession frequency during the period. In the 2D-INADEQUATE experiment if and Cg are coupled to each other, then the signals from each of these components will precess at a common double quantum frequency (/ + in the dimension. 

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. 

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