Higher-dimensional exchange spectra

Three-dimensional and higher dimensional exchange spectra can be recorded by simple extensions of the basic two-dimensional pulse sequence20 (Fig. 22). The resulting multidimensional frequency spectra then correlate the molecular orientation at three (or more for higher-dimensional spectra) points in time. 

Four-dimensional exchange NMR was introduced some time ago for examining the correlations between different molecular motions.53 The 

Static exchange experiments can give immense amounts of information on dynamical processes, but they suffer from an inherent lack of resolution. Clearly, this is not important if there is only one chemical site for the observed nucleus, but it is critical if there is more than one site. Various experiments have been suggested in the review period to deal with this problem. 

It can be seen from the following discussion that this can lead to a three-dimensional spectrum in which two-dimensional anisotropic exchange spectra for the different chemical sites are separated according to the isotropic chemical shifts associated with the different sites. Consider first a two-dimensional VACSY experiment (Fig. 26), in which a simple FID is collected under sample spinning at different spinning angles. The signal acquired has the form 

Equation (14) has the form of the signal from a three-dimensional experiment with the time variables defined by Eq. (15). The three-dimensional time domain dataset implied by these variables can be collected by recording 

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