Chemical Equivalence—A Brief Overview

AH of the protons found in chemically identical environments within a molecule are chemically equivalent, and they often exhibit the same chemical shift. Thus, all the protons in tetramethylsilane (TMS), or aU the protons in benzene, cyclopentane, or acetone—which are molecules that have protons whieh are equivalent by symmetry considerations— have resonance at a single value of 8 (but a different value from that of each of the other molecules in the same group). Each such compound gives rise to a single absorption peak in its NMR spectrum. The protons are said to be chemically equivalent. On the other hand, a moleeule that has sets of protons that are chemically distinct from one another may give rise to a different absorption peak from each set, in which case the sets of protons are chemically nonequivalent. The following examples should help to clarify these relationships. 

Molecules giving rise to one NMR absorption peak—all protons chemically equivalent 

Molecules giving rise to three NMR absorption peaks—three different sets of chemically equivalent protons 

You can see that an NMR spectrum furnishes a valuable type of information on the basis of the number of different peaks observed that is, the number of peaks corresponds to the number of chemically distinct types of protons in the molecule. Often, protons that are chemically equivalent are also magnetically equivalent. Note, however, that in some instances, protons that are chemically equivalent are not magnetically equivalent. We will explore this circumstance in Chapter 5, which examines chemical and magnetic equivalence in more detail. 

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