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¹³C NMR

01¹³C NMR

Carbon environments

This section covers equivalent carbon atoms and how different carbon environments produce separate ¹³C NMR signals.

Equivalent carbon atoms

In a 13C NMR spectrum, carbon atoms with the same chemical surroundings are chemically equivalent. One signal is produced for each set of equivalent carbon atoms.

Carbon atoms in different chemical environments produce separate signals. The number of sample signals therefore gives the number of distinct carbon environments in the molecule.

Recognising equivalent carbons

Symmetry is important when counting carbon environments. Two or more carbon atoms may occupy identical environments and therefore contribute to the same signal.

Propanone, CHX3COCHX3\ce{CH3COCH3}, contains two carbon environments.

  • The two methyl carbon atoms are equivalent because the two CHX3\ce{CH3} groups occupy identical environments.
  • The carbonyl carbon is in a different environment.

Its 13C NMR spectrum therefore contains two sample signals.

The same approach can be applied to larger molecules: identify carbon atoms related by symmetry, group equivalent carbons together, then count the different sets.

Exam Tip: Start a 13C NMR question by counting different carbon environments. Do not assume that the number of signals must equal the total number of carbon atoms; symmetry can make several carbons equivalent.

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