6.9.5 Proton NMR

6.9.5 Proton NMR: Interpret proton NMR spectra using the number of peaks, chemical shifts, integration ratios and n+1 splitting patterns to deduce the structure of an organic compound.

Proton (¹H) NMR gives four separate pieces of information about hydrogen atoms in a molecule.

Number of peaks

Each peak or group of peaks represents one environment of hydrogen atoms, so equivalent hydrogens give a single signal.

Chemical shift

The position, in ppm relative to TMS at 0, depends on the atoms and groups close to the hydrogen.

Integration

The relative area under each signal is proportional to the number of hydrogens in that environment, giving a ratio.

Splitting

The signal splits into n + 1 lines when there are n hydrogens on the adjacent carbon atom or atoms.

Number of adjacent H	Splitting pattern	Relative line heights
0	Singlet	1
1	Doublet	1 : 1
2	Triplet	1 : 2 : 1
3	Quartet	1 : 3 : 3 : 1

Approximate ranges help assign each signal to a type of hydrogen.

Chemical Shift Ranges

Alkyl hydrogens

Hydrogens on saturated carbons that are not near a functional group appear at about 0.9–1.5 ppm.

Next to C=O or a ring

Hydrogens on a carbon attached to a carbonyl group appear at about 2.0–2.9 ppm.

Next to oxygen

Hydrogens on a carbon joined to an oxygen atom, as in an ester or alcohol, appear at about 3.3–4.3 ppm.

Alkene and aromatic

Alkene hydrogens are at about 4.5–6.0 ppm and aromatic hydrogens at about 6.5–8.0 ppm.

Aldehydes and acids

Aldehyde hydrogens appear at about 9–10 ppm and carboxylic acid hydrogens at about 10–12 ppm.

Run the spectrum of the sample in a deuterated solvent such as CDCl₃, which gives no interfering proton signals.

Add a few drops of D₂O to the sample, shake and run the spectrum again.

Hydrogens in O–H or N–H bonds exchange with deuterium, and deuterium does not appear in a proton spectrum.

The signal from those hydrogens disappears from the second spectrum.

Use the missing peak to identify the OH or NH group in the structure.

Question: Predict the proton NMR spectrum of ethyl ethanoate, CH₃COOCH₂CH₃, giving the integration and splitting of each signal.

There are three hydrogen environments: CH₃CO, OCH₂ and CH₂CH₃.
CH₃CO has 3 H and no neighbouring hydrogens, so it is a singlet at about 2.0 ppm.
OCH₂ has 2 H and is next to CH₃ (3 H), so it is a quartet (3 + 1) at about 4.1 ppm.
The CH₃ of the ethyl group has 3 H and is next to CH₂ (2 H), so it is a triplet (2 + 1) at about 1.3 ppm.

Answer: Singlet 3H, quartet 2H and triplet 3H, giving an integration ratio of 3 : 2 : 3.

6.9.5 – Key Points for the Exam

Chemical shift: The position of a signal relative to TMS in ppm.
Integration: The relative area under a signal, indicating the number of hydrogens.
Splitting: The division of a signal into several lines caused by adjacent hydrogens.
D₂O shake: A test that removes O–H and N–H signals by deuterium exchange.

Common Mistakes

Using the n + 1 rule with the number of hydrogens on the same carbon instead of the adjacent carbon.
Treating O–H and N–H hydrogens as part of the splitting of neighbouring groups.
Reading integration as the exact number of hydrogens rather than a simple ratio.
Counting two equivalent CH₃ groups as two signals.