NMR in non-viscous liquids

Flere then, the molecular species can be deemed to be tumbling rapidly enough so that we need consider only the time average value of the shift matrix a, that is its isotropic component. Usually the systems are so dilute that the molecules can be considered not to see each other. 

Following Abragam,1 let us consider nuclides all of the same isotopic variety and let us demand in NMR that they have identical chemical shifts, presumably over a very appreciably wide temperature range. These nuclei will be called isochronous, since they will all have the same Larmor frequency. 

The chemical shift (in Hz) of any particular nuclide is proportional to its yn value. Thus, different isotopes, such as H and 2H, cannot be isochronous. Also, it is worth noting that replacing one isotopic species by another at some distance from a given nuclide can affect the location of the latter s NMR line.27 

In other later works (e.g. Ref. 5), the terms isochronous and anisochronous are often replaced by the usage chemically equivalent and chemically nonequivalent. The above-cited conditions on the / interactions have led to the nomenclature magnetically equivalent and magnetically non-equivalent. Note that isochronous nuclei are not necessarily chemically equivalent. 

Nuclei are chemically equivalent if they can be interchanged by a non-trivial symmetry operation. Nuclei interchangeable by a pure rotation (C , n=2, 3,4.) are called homotopic. Nuclei related only by a centre (i) of inversion or by a plane ( t) of symmetry are said to be enantiotopic (also prochiral ). 

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