Isochronous nuclei

Each ifi nucleus is shielded or screened by the electrons that surround it. Consequently each nucleus feels the influence of the main magnetic field to a different extent, depending on the efficiency with which it is screened. Each nucleus with a different chemical environment has a slightly different shielding and hence a different chemical shift in the H NMR spectrum. Conversely, the number of different signals in the iff NMR spectrum reflects the number of chemically distinct environments for iff in the molecule. Unless two iff environments are precisely identical (by symmetry) their chemical shifts must be different. When two nuclei have identical molecular environments and hence the same chemical shift, they are termed chemically equivalent or isochronous nuclei. Non-equivalent nuclei that fortuitously have chemical shifts that are so close that their signals are indistinguishable are termed accidentally equivalent nuclei. 

The concept of chemical shift equivalence is central to NMR spectrometry. Chemical-shift equivalent (isochronous) nuclei comprise a set within a spin system (Pople notation, Section 3.5). 

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. 

Isochronous nuclei have the same nmr chemical shift by symmetry. 

Conformations in rapid equilibrium Sets of isochronous nuclei Time-average... 

Name of family Members Sets of isochronous nuclei carbon hydrogen ... 

Why do equivalent or isochronous nuclei not split each other, even when they couple ... 

In Section 4-2, the terms isochrony, equivalence, and topicity were introduced to describe nuclei that are of interest in NMR spectroscopy. Isochronous nuclei, or groups, were seen to be chemically (symmetry) equivalent. Magnetic equivalence was, however, found to be a more strict requirement than chemical equivalence, as it is determined by the coupling con-stant(s) of each nucleus in a group of chemically equivalent nuclei. Finally, topicity was seen to be dependent on the nature of symmetry operations that interchange chemically equivalent nuclei or groups. 

Magnetically equivalent nuclei must be chemically equivalent and hence must have the same chemical shift that is, they must be isochronous. Nonequivalence resulting from unequal couplings to a single partner nucleus sometimes is referred to as magnetic nonequivalence by the coupling-constant criterion. The term is intended to provide a distinction from chemically nonequivalent nuclei, which have different chemical shifts and illustrate magnetic nonequivalence by the chemical-shift criterion. 

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