Question

How many NMR signals does a compound exhibit?

Answer 1

A compound exhibits as many signals as there are magnetic environments within the molecule.

Just because a molecule has `x` H atoms doesn't mean that there will be `x` resonances. Symmetry will reduce the number of resonances.

Let's look at 2,2-dimethyl-1-propanol.

We have 12 H atoms, but some of these have identical environments.

The 9 H atoms in the three methyl groups have the same connections. They exchange their environments constantly by rapid rotation about C-C σ bonds.

They experience the same deshielding, so they resonate at the same frequency. We say that they are chemically equivalent.

The two H atoms in the CH₂ group are also equivalent.

And the H atom in the O-H group has its own environment.

We should expect to see three signals from this compound with area ratios of 9:2:1.

Here's an actual spectrum of the compound.