Two-Dimensional Spectra and Beyond

Two-dimensional (2D) spectroscopy is used to obtain some kind of correlation between two nuclear spins 7 and J, for instance through scalar or dipolar connectivities, or to improve resolution in crowded regions of spectra. The parameters to obtain 2D spectra are nowadays well optimized for paramagnetic molecules, and useful information is obtained as long as the conditions dictated by the correlation time for the electron-nucleus interaction are not too severe. Sometimes care has to be taken to avoid that the fast return to thermal equilibrium of nuclei wipes out the effects of the intemuclear interactions that are sought through 2D spectroscopy. 

2D experiments are devised in the assumption that the various times involved in the cycle of Fig. 8.1 (with the exception of when present) are small with respect to the nuclear relaxation times. When the latter are short for any reason, e.g. in the case of paramagnetic molecules because of the presence of unpaired electrons, the system of spins may have reached the equilibrium, or almost reached the equilibrium, before the detection pulse. Under these circumstances no memory is left for the state of the spins during the preceding steps. As a consequence, cross peaks may be decreased in intensity until below detectability. It is necessary, therefore, to match all the time intervals with the nuclear relaxation times, in order to detect the maximum possible cross peak intensities. The ideal case is that t  

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We are presenting a series of one dimensional (ID) and two dimensional (2D) NMR spectra to exemplify the power of the technique. It is beyond the purpose of the present chapter to cover all types of NMR spectroscopy. Sometimes the same technique is known under several alternative acronyms, and in other cases different techniques provide the same information. We mention some of the most popular techniques listing alternative acronyms, but without detailing the full names or technical aspects. The interested reader should refer to already quoted books for more information [10,11,13]. A good compendium on basic ID and 2D types of NMR spectra is Nakanishi s book [26] whereas some 2D-, 3D- and 4D-spectra are well explained in Evans book [27]. However, as the techniques advance very fast, following more recent reviews and original publications is essential. 

Two methods are useful to measure slow motion spin label spectra saturation transfer (ST-EPR) and two-dimensional electron spin echo spectroscopy (2D ESE). In the ST-EPR experiment, " a cw spectrometer is operated at high microwave power, and this causes partial saturation of the nitroxide spectrum. As the field is scanned, molecular motion carries this saturation to nearby regions of the spectrum, yielding spectra that are very sensitive to the rotational correlation time. One of the great advantages of ST-EPR is that little instrumentation is required beyond the conventional cw EPR instrument. Most any laboratory equipped to perform EPR can also perform ST-EPR. 

It is beyond the scope of the present work to discuss the different options for the analytical methods. We only present one example here, showing what can be achieved with modern instrumental analysis. Fig. 4.15 shows a NMR-spectrum of a formaldehyde + water + methanol mixture taken with an online technique with a 400 MHz NMR spectrometer. Signals from a large number of different species can be resolved. Obviously, the band assignment is non-trivial for such complex mixtures and special techniques, such as two dimensional NMR, have to be applied. One of the most attractive features of NMR spectroscopy compared with other spectroscopic methods is that the quantitative evaluation of spectra such as that shown in Fig. 4.15 can be achieved without calibration, as the area below the peaks is directly proportional to the number of the different nudei in the solution if the NMR experiment is carried out properly. 

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