Chemical and magnetic equivalence

Chemical shift equivalence

If a set of nuclei exist in identical environments, they are expected to have the same chemical shift. Such nuclei are called chemical shift equivalent or chemically equivalent.

A pair of nuclei in a molecule are chemically equivalent if they are interchangeable through any symmetry operation of the molecule OR if they interchange by a rapid process (rapid with respect to the NMR timescale).

If a pair of nuclei can be interchanged by rotation about an axis of symmetry of the molecule then they are chemically equivalent and are called homotopic. e.g. the pair of protons in dichloromethane are chemically equivalent.

If a pair of nuclei can be interchanged by an improper rotational symmetry operation of the molecule then they are chemically equivalent and are called enantiotopic. e.g. pair of protons attached to the alpha-Carbon in glycine amino acid (they are not chemically equivalent if glycine is part of a polypeptide chain).

If a pair of geminal protons (CH2) cannot be interchanged through a symmetry operation of the molecule, then these protons are diastereotopic and are NOT chemically equivalent. e.g. the $\beta$-methylene protons of amino acids where the methylene group is attached to chiral C$\alpha$ atom.

Chemical shift equivalence by rapid interconversion of structures may occur due to rapid rotation about bonds or due the rapid chemical changes such as keto-enol tautomerism.

Magnetic equivalence (spin coupling equivalence)

If in a set of chemically equivalent nuclei, each member of the set has exactly the same interaction (J-coupling) to every other magnetically active nucleus in the molecule, then the nuclei are also magnetically equivalent. e.g. the pair of protons in dichloromethane are chemically as well as magnetically equivalent.

A set of nuclei that are magnetically equivalent will also be chemically equivalent, however, chemical equivalence does not guarantee magnetic equivalence. e.g., the two protons ortho to hydroxy group in tyrosine are chemically equivalent but they are not magnetically equivalent.

References: Spectroscopic identification of Organic Compounds. (1998) R.M.Silverstein and F.X.Webster. Wiley.

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