We can categorize multi-dimensional experiments in two main classes, through-bond and through-space. Through bond effects are the result of scalar coupling, which is the same effect that gives rise to multiplet structure in 1-D NMR spectra. The coupling of 1H transitions with other 1H nuclei on neighboring carbon atoms is a useful method to establish connectivity in assignments of molecular spectra. These effects are complemented by through-space dipolar coupling that is observed between any two 1H nuclei that within ~5 Angstroms of each other. These two effects complement each other nicely and help to provide unambiguous assignements of molecular structure. For small molecules, the 2-D experiments resulting from each of these effects suffice to provide unique assignments. The 2-D through-bond experiment is known as the COSY (Correlation Spectroscopy) experiment and the 2-D through-space experiment is known as NOESY. First we consider the COSY in the following segment.
The Nuclear Overhauser Effect (NOE) is one of the most powerful tools in all of spectroscopy. The NOE is a dipolar coupling between any two nuclei, whether they are bonded or not. It permits transfer of magnetization over distances of up to 5 Angstroms. The 1-D NOE experiment shows that if we selective irradiate one spin and saturate its transition, there is an effect on the intensity of other spins in the spectrum. The observed changes in intensity have been shown to arise only from spins that are within a cut-off distance of about 5 Anstroms. IN theory the effect is distance dependnent, but it is sufficiently difficult to use this information that most scientists regard the NOE as a binary marker. The observation of a NOE means that two 1H spins are within the cutoff distance. The 2-D NOESY experiment permits one to examine all of the NOE signals in a molecules in a map. The map has off-diagonal peaks whereever two nuclei have a NOE effect. One use the list of such NOEs as constraints in macromolecular structure determination. These methods are discussed in the following segment.
