Nuclear Magnetic Resonance (NMR)
Description of method:
NMR (nuclear magnetic resonance) spectroscopy is a non-destructive method for determining the structure of organic and organometallic as well as to a lesser extent inorganic samples. NMR studies can be performed on liquids and solids.
To this end, the sample is placed in a strong magnetic field. In the case of magnetically active nuclei, i.e. atomic nuclei such as 1H, 13C, 31P, 11B, 29Si, 19F, which have nuclear spin and thus magnetic moment, this leads to a cancellation of the equal-energy states. The atomic nuclei can thus take on various energy levels. The occupation differences, and thus the differences in energy of the energy states affected, are so small that transitions between the energy levels can already be induced by exciting the sample with low-energy electromagnetic radiation (radiofrequencies in the MHz range). The difference between the energy levels and thus the resonant frequency depend on the immediate chemical environment of the atomic nuclei measured and thus enable direct conclusions to be drawn about the chemical structure of the molecules studied.
Structural determination is the main field of application for NMR spectroscopy. As a rule, high-resolution spectra of samples in solution are recorded and, in addition to one-dimensional methods, multidimensional techniques (e.g. COSY, NOESY, HSQC, HMBC) are used for structural determination.
The techniques for studying samples in solution are different to those used for solids. This is due to the fact that molecules can move freely in solution and anisotropic interactions are largely averaged out.
Molecules cannot move freely in solids. Resonances therefore also result from the different orientations of the molecules to the external magnetic field. In addition, anisotropic interactions of the magnetically active nuclei among themselves must also be taken into consideration. Suitable measuring techniques avoiding or minimizing this problem are, for example, CP or MAS.