37.4 Proton NMR spectroscopy

A proton NMR spectrum reveals different proton environments. The integration trace gives relative numbers of protons, and splitting shows coupling to neighbouring non-equivalent protons.

Build a structure by combining three clues: number of signals, relative integrals and chemical-shift regions. Use the n+1 rule for simple adjacent proton sets, then check the total hydrogen count.

An ethyl group gives a triplet for CH₃ and a quartet for CH₂ with an integral ratio 3:2; an isolated OH may appear as a broad singlet.

A quartet does not mean four protons: it usually means one proton set is split by three neighbouring protons.

Chemical shift depends on the proton’s electronic environment: electronegative atoms and π systems usually deshield nearby protons. Splitting depends on neighbouring non-equivalent hydrogens.

Assign each proton set to a shift region, then count adjacent hydrogens and apply n+1 only when the sets are sufficiently equivalent for the simple model.

Protons next to an oxygen appear downfield from an ordinary alkyl group; a CH₂ next to CH₃ is commonly split into a quartet while the CH₃ is split into a triplet.

Do not predict splitting from the total number of hydrogens in the molecule or treat exchangeable OH/NH protons as fixed n+1 partners.

Tetramethylsilane (TMS) is used as the reference compound assigned δ = 0 ppm in NMR. Sample peaks are reported relative to this standard, making chemical shifts comparable.

TMS gives one sharp signal because its twelve protons (and four carbons in 13C NMR) are equivalent. It is chemically inert and easy to remove from the spectrum.

A proton signal at 2.1 ppm lies 2.1 ppm downfield from TMS under the same instrument conditions.

The TMS peak is not a sample impurity to interpret as an unknown structure; it defines the scale.

A deuterated solvent such as CDCl₃ replaces most solvent hydrogen atoms with deuterium, which is not detected in ordinary proton NMR in the same way. This prevents a huge solvent ¹H signal masking the sample.

The solvent must dissolve the sample and be sufficiently non-reactive. Small residual protonated-solvent peaks can still appear and should not be mistaken for the compound.

Dissolving an organic sample in CDCl₃ allows its proton signals to be observed while the deuterated solvent also provides a lock signal for the instrument.

Deuterated does not mean proton-free in an absolute sense; residual solvent peaks and exchangeable protons may remain visible.

Exchangeable O–H and N–H protons can be replaced by deuterium when D₂O is added. Because deuterium is not normally observed in a proton NMR spectrum, the corresponding signal disappears or weakens.

Compare spectra before and after adding D₂O. A disappearing broad signal supports an O–H or N–H assignment, but it does not by itself distinguish the two.

An alcohol spectrum with a broad signal that vanishes after D₂O treatment provides evidence for an O–H proton.

Do not expect ordinary C–H peaks to disappear; their hydrogens do not exchange rapidly with D₂O under the test conditions.