The introduction of Fourier transform methods has not only remarkably enhanced the sensitivity of high‐resolution NMR spectroscopy, thus allowing measurements to be made on less sensitive nuclei of the Periodic Table, but also has paved the way for the development of a large number of new experimental techniques. On the one hand, procedures already known have been improved and can now be performed more rapidly, and, on the other, completely new experimental approaches have become available. This situation resulted mainly from the introduction of programmable pulse transmitters and the separation of the experiment into preparation, evolution, and detection. In particular, the concept of two‐dimensional spectroscopy has opened up new possibilities important for the analysis of complicated spectra and is able to provide information previously not accessible. As elsewhere, optimum application of the techniques and correct interpretation of the results require sound understanding of the underlying physical principles. Since a rigorous mathematical treatment is complicated and does not necessarily improve the comprehensibility, this article attempts to give an illustrative presentation of the new pulse techniques within the framework of the Bloch vector model. After a short introduction covering the basic principles, one‐dimensional pulse techniques that can be applied using standard experimental equipment are dealt with. The main areas of application are signal assignment, sensitivity enhancement for measurements on less abundant nuclei, and selective excitation of individual resonances. Subsequently, the various techniques of two‐dimensional NMR spectroscopy are treated: these enable shift correlations for different types of nuclei to be made, the presentation of spin multiplets without overlap, and the analysis of geometrical relations as well as of chemical exchange phenomena.
The controversy about the 13C NMR assignment of the methyl groups in angelates and tiglates was settled by two-dimensional 'H-=C chemical shift correlations for angelic and tiglic acid.
Dedicated to Professor Emanuel Vogel on the occasion of his 60th birthdayModern methods of NMR spectroscopy, in particular the two-dimensional techniques, offer new chances for structure determinations in the field of organolithium compounds, where the combination of 'H-, I3C-, and '"'Li-NMR spectroscopy is a n especially useful feature. Chemical shift correlations which also include the lithium nuclei allow a complete assignment of the 'H-, I3C-, and 6Li-NMR spectra and thereby a better characterization of the various aggregates and complexes present in solution. Spatial proximities of 6Li and 'H can be detected by nuclear Overhauser experiments, and 6'7'Li-NMR exchange spectroscopy can provide new information with regard to the mechanisms and energetics of dynamic processes like aggregate interchange and complexation. After a short resume of the experimental aspects of the NMR spectroscopy of organolithium compounds and a discussion of the NMR parameters of these systems, new experimental techniques are presented. Areas of application of these newly conceived one-and two-dimensional NMR experiments are illustrated with selected examples. The results show that even more detailed information about the structure and reactivity of organolithium compounds, which are so important for organic synthesis, can be expected in the future.
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