Techniques in NMR spectroscopy

With the development of Fourier transform (FT) techniques in NMR spectroscopy (early 1970s), the first major advance in the NMR technology was made. A significant increase in the sensitivity, as compared to the conventional continuous wave method, resulted in the NMR spectroscopy of rare nuclei, particularly 13C NMR, which is essential for polymer studies. The 13C NMR analysis of swollen lightly crosslinked polymers was made possible. The relaxation measurements, based on the different pulse sequences, provided additional information on the network dynamics. 

MRI is based on nuclear magnetic resonance (NMR) spectroscopy which is widely used to deduce the structures of organic molecules. Given the public perception of the word nuclear (nuclear bombs, nuclear waste, etc) this has been dropped from the acronym used for the technique. In NMR spectroscopy a sample... 

Stable metal complexes can be favorably formed when a bidentate metal-binding site is available, such as a- and -diketone moieties which are the tautomeric forms of a- and /3-ketoenols. Some /S-diketonate complexes of paramagnetic lanthanides such as Pr(III), Eu(III) and Yb(III) have been extensively utilized as paramagnetic shift reagents for structural assignment of molecules with complicated stereochemistry prior to 2D techniques in NMR spectroscopy. Their syntheses and application are discussed in separate chapters in this volume. The examples below provide some dynamic and structural basis for better understanding of metal enolates in biomolecules and biochemical processes. 

M. R. Bendall, I. L. Cartwright, P. I. Clark, G. Lowe, and D. Nurse. Inhibition of papain by N-acyl-aminoacetaldehydes and N-acyl-aminopropanones. Evidence for hemithioacetal formation by a cross-saturation technique in NMR spectroscopy. Eur. J. Biochem. 79 201 (1977). 

The papers presented in this volume represent a fraction of the applications developed from new techniques in NMR spectroscopy over the past decade. This flood of new methods has generated new terms which, though mathematically well defined, are difficult to visualize physically. As a result, the advantages of the newer methods (and the information to be gleaned from them) are often lost to the scientist not intimately familiar with NMR spectroscopy. 

The low natural abundance of 15N nuclei can be overcome by isotopic enrichment. However, the enrichment is not a straight-forward procedure and it is relatively expensive. The spectra of 15N enriched samples were reported beginning from 1964, but the major advance in nitrogen NMR was made in the early 1970s with the development of Fourier Transform (FT) techniques in NMR spectroscopy. The application of FT NMR to nitrogen nuclei in solution made possible the observation of natural abundance nitrogen spectra within practical experimental time limits. 

Broadband decoupling A technique in NMR spectroscopy that removes the splitting of signals caused by coupling of and nuclei. Thus, all of the signals appear as singlets. 

Hyperpolarization is a term which collects a set of dilferent signal-enhancement techniques in NMR spectroscopy. They have in common that they create population differences (polarizations) of the nuclear spin, which are far larger than the thermal equilibrium value (Boltzmann polarization). Following a proposal by Overhauser in 1953, which was experimentally verified by Carver and Slichter Dynamic Nuclear Polarization (DNP) schemes were developed. These techniques employ an auxiliary reservoir of fast and effectively polarizable spins to generate spin order which is subsequently transferred to the spin system of interest. In case of DNP experiments the strong polarization of electron spins is transferred to the nuclear spins by means of microwave irradiation. In the early days DNP experiments were restricted to relatively low fields ( H-frequency below 60 MHz) due to the technical limitations of klystrons used as microwave sources. 

Although the emphasis in this chapter has been on tbe synthesis and mechanism of formation of simple enamines, brief mention will be made of the addition of amines to activated acetylenes to indicate the interest and activity in this area of substituted enamines. Since such additions tend to be stereospecific, inclusion in this section seems apropos. The addition of amines to acetylenes has been much studied 130), but the assigning of the stereochemistry about the newly formed double bond could not be done unequivocally until the techniques of NMR spectroscopy were well developed. In the research efforts described below, NMR spectroscopy was used to determine isomer content and to follow the progress of some of the reactions. 

In the one-dimensional NMR experiments discussed earlier, the FID was recorded immediately after the pulse, and the only time domain involved (ij) was the one in which the FID was obtained. If, however, the signal is not recorded immediately after the pulse but a certain time interval (time interval (the evolution period) the nuclei can be made to interact with each other in various ways, depending on the pulse sequences applied. Introduction of this second dimension in NMR spectroscopy, triggered byjeener s original experiment, has resulted in tremendous advances in NMR spectroscopy and in the development of a multitude of powerful NMR techniques for structure elucidation of complex organic molecules. 

Spectral width, dynamic range, resolution and sensitivity are expected to be pushed toward further limits. An emerging advancement in NMR spectroscopy is the DOSY technique (Section 5.4.1.1) which offers a separation capability as a function of the rates of steady state diffusion of molecules in solution. 

On-line coupling of separation techniques to NMR has recently been reviewed [459,651-653], and solvent suppression methods in NMR spectroscopy in particular [654],... 

Much of the research in NMR spectroscopy has been in the field of devising new and improved techniques for extracting ever more information from samples. Nowadays, the plethora of available techniques can be daunting for the relative newcomer to NMR. In the following sections, we shall endeavour to guide you through the veritable forest of acronyms by describing the most important and useful techniques and demonstrate how they can be used to solve real-world problems. 

In some very recent work by Karssenberg et al. [130], attempts have been made to improve the analytical ability of a technique like NMR spectroscopy to effectively predict the distribution of sequence lengths in polyethylene-alkene copolymers. They analyzed the entire [ C-NMR spectrum for homogeneous ethylene-propene copolymers. They used quantitative methods based on Markov statistics to obtain sequence length distributions as shown in Figure 22 [130]. The... 

When one looks at the hydrogens in the chair conformation of cyclohexane, one can see that they are of two types. Six of them are parallel to the central rotational axis of the molecule, so are termed axial. The other six are positioned around the outside of the molecule and are termed equatorial. One might imagine, therefore, that these two t)T>es of hydrogen would have some different characteristics, and be detectable by an appropriate spectral technique. Such a technique is NMR spectroscopy but, at room temperature, only one type of proton is detectable. At room temperature, all hydrogens of cyclohexane can be considered equivalent this is a consequence of conformational mobility, and the interconversion of two chair conformations. 

Major parf of analytical chemisfry is relafed to different spectroscopic techniques. In optical spectroscopy, ILs are already used as solvents for wide range of solutes to study their properties and behavior in conditions not available with organic solvents. ILs have their limits regarding the transparency, but knowing that many other limits are shifted far away. The use of ILs as solvents does not preclude the application of NMR techniques. After careful parameter adjustment, virtually all standard and advanced NMR techniques can be applied. The same can be said about mass spectrometry, which has a great potential to get a key method in almost all fields of IL research, including analytical applications with IL as necessary component to get good result. 

Up to here, we have considered only the interaction between a proton and the F19 nucleus bonded to it. There are also weaker interactions between a given proton and more-distant nuclei in the crystal. These interactions superimpose doublets with smaller splittings on the main proton doublet. The net effect is a doublet whose lines are greatly broadened. For this reason, NMR studies of solids are called broad-line NMR. Because of this broadening and the smallness of chemical-shift differences as compared to spin-spin splittings, different resonance lines due to chemically nonequivalent protons are not observed in NMR spectroscopy of solids (unless special techniques are used). 

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