What Is Magnetic Equivalence?
To describe a spin system it is necessary to state which nucleus is coupled with which. When two equivalent nuclei have identical relations with the same partners, they are “magnetically equivalent”. Only then can you define them as a group and not individually.
Two nuclei are magnetically equivalent when they have:
- The same chemical shift
- The same coupling constants...
- With the same partners!
- At left, H4 and H6 are magnetically equivalent, because they are both coupled with H5 and with no other.
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At right, H2 and H6 are related by symmetry, therefore they have the same chemical shift and the same coupling constants.
Their partners, however, are different: H2 is coupled with H3, while H6 is not (or not with the same intensity).
In conclusion: H2 and H6 are NOT magnetically equivalent and must be declared separately.
This table, which refers to ortho-dichlorobenzene, provides an illuminating demonstration of what the concept of magnetic equivalence is all about. Nuclei A and D have identical chemical shifts and identical coupling constants. One may therefore be tempted to put them in the same row. When it is time to specify the first J, the problem becomes apparent. JDB=1.524, while JAB=8.08, so whatever he writes it is wrong in at least one case.
Groups of magnetically equivalent nuclei are frequently encountered. A common example is: the three hydrogens of a methyl group. The two members of a methylenic pair, on the contrary, are rarely magnetically equivalent, and often they do not even share the same chemical shift!