Multidimensional NMR techniques

One-dimensional111 and 13C NMR experiments usually provide sufficient information for the assignment and identification of additives. Multidimensional NMR techniques and other multipulse techniques (e.g. distortionless enhancement of polarisation transfer, DEPT) can be used, mainly to analyse complicated structures [186]. 

An astonishing number of powerful 2D and higher multidimensional NMR techniques have been developed. The common thread to all of them is to show correlations between certain nuclear properties chemical... 

So-called multidimensional NMR techniques can provide important information about macromolecular conformation. In these cases, the sequence of a protein is aheady known, and establishing covalent connectivity between atoms is not the goal. Rather, one seeks through-space information that can reveal the solution conformation of a protein or other macromolecule. Two-or three-dimensional techniques use pulses of radiation at different nuclear frequencies, and the response of the spin system is then recorded as a free-induction decay (FID). Techniques like COSY and NOESY allow one to deduce the structure of proteins with molecular weights less than 20,000-25,000. 

There is a bewildering variety of multidimensional NMR techniques available to the modem NMR spectroscopist interested in stmcture elucidation, and used in a concerted fashion these provide efficient and economical tools for determining chemical structures. There is a trade-off implicit in the use of multidimensional experiments, between the volume of information that they provide and the minimum time taken to perform them in most cases the extended experimental duration is more than compensated for by the wealth of structural information revealed. For a small proportion of stmctural problems, however, such experiments may provide far more information than is actually required, and where only a few critical parameters need to be measured they may represent a far from efficient use of spectrometer time. 

Undesired homonuclear spin interactions can be also suppressed using suitable multiple-pulse sequences while still exploiting the information content provided by interactions that are not affected. Using a combination of MAS and pulse decoupling it is even possible to reintroduce parts of an interaction that would be averaged out by one of the manipulation techniques alone ( recoupling ) [11]. This high flexibility of solid-state NMR enables one to fully exploit the rich information content provided by the spin interactions. It becomes particularly powerful if such experiments are combined to multidimensional NMR techniques as discussed in Section 14.3. 

The ID NMR techniques described above already allow a detailed investigation of polymer dynamics but they are mostly not selective in the sense that they do not provide information on the averaging of particular couplings. For the interpretation of some of the above results therefore a representative spin-pair along the chain was assumed, thus neglecting local site-specific motions as well as the geometry of the bonds. This is where multidimensional NMR techniques come into play. 

Another way of dealing with the resolution problem for powder lineshapes is to use multidimensional NMR techniques to separate powder pattern lineshapes (or magic-angle spinning sideband patterns) according to isotropic chemical shift, as mentioned previously. 

Multidimensional NMR techniques, such as 2D-INADEQUATE, can allow the direct tracing out of carbon... 

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