Magnetic Equivalence of Nuclei

Two protons attached to a carbon a to a chiral center therefore often appear at different chemical shifts and exhibit coupling with one another. Such protons are said to be diastereotopic. Moreover, when two protons are 

Nonequivalence can also arise by restricted rotation imposed by steric or electronic factors. In such cases the affected protons will appear as separate peaks at lower temperatures, but as the temperature is raised the lines move closer together, and finally merge into a single peak. The methyl groups of dimethylformamide are nonequivalent at room temperature because of restriction of rotation about the N—CO bond due to the partial double 

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