13C Integration Can Be Very Helpful If Used Wisely

Although 13C spectra are commonly known as not being quantitative, peak integrations can be helpful in many situations. All protonated carbons have similar integration values, as you can see in this example:


Figure 1 is aromatic area, in which all protonated carbons have integration values of ca. 1.0. All unprotonated carbons have values of 0.4-0.5. The tallest peak is solvent.


Figure 2 is aliphatic area, in which each carbon (all are protonated) has an area of 1.1, slightly bigger than the protonated aromatic ones, but quite close.

Integrations can also help to solve puzzles of missing peaks. In this molecule, seven aromatic carbon peaks are expected, but only six show up on Figure 1. Why?


Let’s look at the 134.6ppm peak closely.  Integration of this is 1.4 – this means that this is not just one protonated carbon (area = 1), but contains two overlapping peaks – one protonated and one unprotonated, which would have a total area of ca. 1.4. This solves the mystery of the missing carbon.