Although 3D NMR experiments cannot be regarded as "basic experiments", we provide in this book four examples as a possible introduction to this exciting field of NMR spectroscopy, also as starting examples for the more demanding experiments in the field of structural biology described in Chapter 15.

3D experiments are typically constructed from two 2D sequences; thus one has a /, period from the first 2D experiment, a f, period from the second 2D experiment, and the acquisition time, which is often designed as the 4; period. The detection period of the first 2D sequence is replaced by the evolution part of the second 2D sequence, so that the first pulse of the second sequence is usually missing.

The remarks given in the introduction of Chapter 10 with regard to frequency discrimination and phase-sensitive detection in F; and F> similarly apply to the 3D case, except that now three time domains have to be considered.

Window functions, Fourier transformation, and phase correction are applied with respect to these three domains to finally yield a cuboid with three frequency axes, which are dependent on the particular experiment. They can be fully heteronuclear (e.g. ¹H, ¹³C, ¹⁵N) or mixed homo- and heteronuclear (e.g. ¹H, ¹H, ¹³C). Up until now all reported 3D experiments have been proton-detected, which, for sensitivity reasons, has been the only viable option. Phasing of the indirect dimensions in 3D can be tedi- ous, and is best performed by calculation as described in the introductory chapter for 2D (see Ch. 10).

The processing of 3D NMR spectra involves very large data files and is therefore limited to laboratories that are equipped with reasonably fast workstations and suffi-