Spectroscopy exchange processes

Jeener J, Meier B H, Bachmann P and Ernst R R 1979 Investigation of exchange processes by two-dimensional NMR spectroscopy J. Chem. Rhys. 71 4546-53... 

Energy differences between conformations of substituted cyclohexanes can be measured by several physical methods, as can the kinetics of the ring inversion processes. NMR spectroscopy has been especially valuable for both thermodynamic and kinetic studies. In NMR terminology, the transformation of an equatorial substituent to axial and vice versa is called a site exchange process. Depending on the rate of the process, the difference between the chemical shifts of the nucleus at the two sites, and the field strength... 

Occasionally, separation, e.g. by solvent extraction or by an ion exchange process, may be necessary to remove an interfering element such separations are most frequently necessary when dealing with flame emission spectroscopy. 

Note that the exchange process in benzene solution is significantly slower than in water, which greatly facilitates rate measurements by 170 NMR spectroscopy. 

Molecular Motions and Dynamic Structures. Molecular motions are of quite general occurrence in the solid state for molecules of high symmetry (22,23). If the motion does not introduce disorder into the crystal lattice (as, for example, the in-plane reorientation of benzene which occurs by 60° jumps between equivalent sites) it is not detected by diffraction measurements which will find a seemingly static lattice. Such molecular motions may be detected by wide-line proton NMR spectroscopy and quantified by relaxation-time measurements which yield activation barriers for the reorientation process. In addition, in some cases, the molecular reorientation may be coupled with a chemical exchange process as, for example, in the case of many fluxional organometallic molecules. ... 

When performing 2D-NMR experiments one must keep in mind that the second frequency dimension (Fx) is digitized by the number of tx increments. Therefore, it is important to consider the amount of spectral resolution that is needed to resolve the correlations of interest. In the first dimension (F2), the resolution is independent of time relative to F. The only requirement for F2 is that the necessary number of scans is obtained to allow appropriate signal averaging to obtain the desired S/N. These two parameters, the number of scans acquired per tx increment and the total number of tx increments, are what dictate the amount of time required to acquire the full 2D-data matrix. 2D-homo-nuclear spectroscopy can be summarized by three different interactions, namely scalar coupling, dipolar coupling and exchange processes. 

UV spectroscopy has been particularly informative about the oxepin-arene oxide ratio at room temperature (67AG(E)385) where NMR spectroscopy has been of little value (due to fast exchange processes). In order to use the UV method of analysis indane 8,9-oxide (the valence tautomer of oxepin 22) was assumed to be totally in the oxirane form while... 

A further field of application for the 2D NOESY and the 2D ROESY experiment.s are dynamic systems, where exchange processes may be recognized and may be analysed quantitatively (EXCSY-spectroscopy). 

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