Analytical Instrument Documents

The NMR signal of a spin reflects its local magnetic environment. If a spin due to chemical exchange samples two magnetically different states then its NMR signal would reflect both states. Its appearance on a NMR spectrum would be determined by the dynamics of the exchange event. In the case of chemical exchange that is slow on the NMR time scale, it is possible to observe two distinct signals for the same spin, one signal for each state under exchange. Presence of chemical exchange is often demonstrated with the exchange experiment (2D NOESY experiment). A exchange peak (cross peak) of the same sign can be observed between the two autopeaks (diagonal peaks) that represent the two states under exchange. Because the same results can be interpreted in a different way based on NOE, further evidence is desirable.

Data obtained from two-dimensional NMR experiments is incredibly useful for structure elucidation of complex molecules, especially when their one-dimensional spectra feature overlapping peaks. However, some experiments require significant amounts of time in order to yield data with adequate resolution or signal to noise for unambiguous interpretation. Any means of reducing the total acquisition time is useful. In this Note, we’ll explore a technique known as Non Uniform Sampling (NUS), demonstrate how it can be used to speed up data collection, and highlight how it can be employed in Delta™ on JEOL Spectrometers.

A very powerful and useful general NMR technique is to use selective excitation to focus directly on a resonance frequency or region to allow acquisition of very specific information to efficiently answer a specific question. Many experiments have been developed that are in essence 1-dimensional analogues of 2-dimensional experiments. In particular NOESY-1D1 and TOCSY-1D have gained wide spread use and acceptance in the NMR community. The NOESY-1D and TOCSY-1D experiments can provide specific answers to questions in a fraction of the time needed for a full 2D result even with NUS techniques. In this Applications Note, we will explore the StepNoesy1D2 experiment, which can yield information typically obtained by performing time consuming 3D experiments.