NMR 3

Protons found in chemically equivalent environments usually have the same chemical shift. Below are some examples. These use spectra predicted by a computer program, so the actual location of the shift is not very reliable.

One environment:

Two environments:

Three environments:

In H¹ NMR, the area under each peak is proportional to the number of protons generating that peak. NMR machines typically integrate (find the area under) the curves automatically, which is often represented as an additional line. The example of benzyl acetate is below:

Benzyl acetate NMR

You can tell from the molecular formula that there are three H environments (assume the aromatic protons are in resonance), populated in a 5:3:2 ratio. The integrals on the spectra reflect this.

Note that integrals give relative numbers, not absolute numbers. There needs to be more than one peak for the integral to be useful. Integrals also have quite a large error range, of about 10%. Forgetting this error range is a common mistake on spectroscopy exams - where students find the correct formula but believe it has the wrong number of hydrogens.

The shift of each peak also tells us about the environment of those protons, for example, aromatic protons are typically found between 7 - 8 ppm, and methyl groups between 0 - 2 ppm. You can find good charts for these in the "spectroscopy data" link at the top of the blog.

You should also remember that deshielding shifts peaks to the left. Electronegative atoms like oxygen will inductively deshield nearby atoms. In the molecule above, the CH₂ is next to an oxygen, while the CH₃ is two bonds away, hence the CH₂ is more deshielded. The even higher amount of deshieldling for the aromatic molecules are explained by magnetic anisotropy, described in a future post.