Fast Pulsing 2D NMR Methods

In typical NMR experiments 90 percent of the time is used for doing nothing. It is necessary to wait until sufficient polarization is recovered for the next scan. Many tricks have been developed during decades of NMR spectroscopy. The first systematic study involved the use of β-pulses, i.e. pulses with flip angles significantly smaller than 90°, in conventional 1D experiments to obtain the maximum signal-to-noise ratio per experiment time, leading to the well-known Ernst-angle. The principle of retaining a certain amount of reservoir polarization for the following scan can be expanded to many different types of 2D experiments, covering basic correlation experiments, triple resonance experiments, as well as specialized experiments like diffusion-ordering. The polarization reservoir can also be enhanced by efficient relaxation pathways or coherent magnetization transfer. This can lead to (i) 2D experiments with the highest sensitivity for a given experiment time starting from thermal equilibrium polarization, (ii) very short, uncompromised 2D experiments of a few minutes to few seconds duration, and (iii) accessibility to 2D experiments with an extremely well resolved indirect dimension. In this chapter, a systematic derivation of the different principles is given together with an incomprehensive overview of established experiments.