General Principles, Inverse Techniques, Gradients

Two-dimensional NMR A strange concept, when we consider that all the spectra we have previously dealt with were of course plotted in two dimensions, the two axes (dimensions) being a frequency axis (horizontal, expressed in ppm rather than in Hz for reasons we have already discussed) and an intensity axis. To understand the basic idea of two-dimensional NMR (2D NMR) we should first remind ourselves that while the spectrum we see and use is plotted as a 

The two axes (dimensions) in our 2D spectra are thus both frequency axes. We shall see as we continue that we can adjust our experiment so as to choose different types of frequency information. An early experiment, known as the J-resolved experiment, was designed in such a way that one axis was the (proton or carbon) chemical shift axis and the other the one-bond proton-carbon coupling constant. However, this experiment is not generally very useful for structural determination, so that we shall not discuss it here. 

The important experiments for our purposes are the correlation experiments, where both axes are chemical shift axes. Certainly the most useful of these is the proton-proton correlation experiment, initially known as COSY (for Correlated SpectroscopY) and now, to make things more precise, as H,H COSY. This experiment is important, as it provides direct information on which proton nuclei couple with which. 

There is also the rather famous experiment known as 2D INADEQUATE (Incredible Natural Abundance DoublE QUAntum Transfer Experiment) which allows us to correlate carbon-13 with carbon-13. Potentially this experiment is very useful, since it allows us to see directly which carbon atoms are directly bonded. However, you will remember that the natural abundance of carbon-13 is only 1.1%, so a carbon-13/carbon-13 correlation requires us to detect only about 0.01% of the carbon nuclei present. Thus the experiment is very insensitive and requires large amounts of both sample and measuring 

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