Accidentally equivalent 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. 

Bonded to a group that withdraws part of the electron density from around the nucleus. The absorptions of deshielded nuclei are moved downfield, resulting in larger chemical shifts, (p. 568) Nuclei that occupy diastereomeric positions. The replacement test for diastereotopic atoms gives diastereomers. Diastereotopic nuclei can be distinguished by NMR, and they can split each other unless they are accidentally equivalent, (p. 592)... 

Nuclei are said to be chemically equivalent when they have the same chemical shift, usually as a result of molecular symmetry (e.g., the 2 and 6 protons or the 3 and 5 protons in phenol) but occasionally as a result of an accidental coincidence of shielding effects. Nuclei in a set are magnetically equivalent when they all possess the same chemical shift and all nuclei in the set are coupled equally with any other single nucleus in the molecule. Thus, in the tetrahedral molecule difluoromethane (I) H and H(, are magnetically equivalent because by symmetry they must have... 

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