Part 2: Proton NMR: integration, splitting and exchange

Combine environment count, chemical shift, relative area and neighbouring protons without overextending the n + 1 rule.

Four observations constrain one structure

A proton NMR spectrum supplies the number of distinct proton environments, their chemical shifts, their relative integrated areas and their splitting patterns. Each answers a different question. Environment count concerns equivalence; shift concerns local surroundings; area concerns relative numbers of H; splitting concerns coupling to nearby nonequivalent protons.

Do not use peak height as integration. A quartet spreads its intensity over four lines, but the integrated area covers the entire multiplet. A small-looking multiplet may represent more hydrogens than a taller narrow singlet.

Use the formula to scale an integration ratio. A 3:2 ratio might correspond to 3 H and 2 H or to 6 H and 4 H, depending on the total hydrogens and symmetry. Account separately for exchangeable protons when their integration is unreliable or the data says they are not observed.

The simple n + 1 rule has a defined setting

For a proton set coupled to n equivalent neighbouring protons in the simple first-order examples taught here, the signal has n + 1 lines. Zero gives a singlet, one a doublet, two a triplet and three a quartet. Common line ratios are 1:1, 1:2:1 and 1:3:3:1 respectively.

Equivalent protons do not split each other. The H atoms within an ordinary equivalent CH₃ group therefore do not make their own signal a quartet. Count protons in the relevant neighbouring environment instead.

If a proton set couples to two different neighbouring sets, a more complicated multiplet may result; simply adding every nearby H and applying n + 1 can fail. OH/NH exchange can remove expected coupling. OCR requires recognition of aromatic proton signals but not detailed aromatic splitting analysis.

Worked example: reciprocal splitting identifies an ethyl fragment

For an isolated CH₃CH₂– fragment in a suitable simple environment, CH₃ has two neighbouring CH₂ protons, so its signal is a triplet. CH₂ has three neighbouring CH₃ protons, so its signal is a quartet. Their integrated areas are 3:2.

The shifts then locate the fragment. A CH₂ next to oxygen is shifted downfield relative to a plain alkyl CH₂. In an ethoxy group, –OCH₂CH₃, the quartet is therefore more downfield than the methyl triplet.

A 6 H doublet with a 1 H septet can indicate two equivalent methyl groups adjacent to a single CH, as in a simple isopropyl pattern when other coupling does not complicate it. The 6 H area represents two equivalent methyl groups, not a chain of six hydrogen atoms.

Aromatic proton recognition is also required. An illustrative C₇H₈ compound with a 3 H singlet near 2.3 ppm and a group of aromatic signals around 7.1 ppm integrating to 5 H is consistent with methylbenzene. The five aromatic H atoms need not form one chemically equivalent set; use their combined integration and the aromatic region, without attempting detailed ring-splitting analysis beyond the required scope. Check the supplied data-sheet ranges because substituents can shift signals.

D₂O distinguishes exchangeable protons

OH and NH protons can exchange with deuterium from D₂O. After exchange, the corresponding ordinary-proton signal disappears or substantially decreases because the site now contains D rather than ¹H. Compare spectra before and after addition.

An OH/NH chemical shift can vary with solvent, concentration, temperature and hydrogen bonding. Do not insist that every alcohol OH has one fixed shift or an ideal n + 1 pattern. Use the exchange result and other evidence to identify it.

D₂O exchange supports an exchangeable H assignment but does not by itself decide whether the group is an alcohol, phenol, carboxylic acid or amine. IR, chemical tests and the remaining spectrum help distinguish these possibilities.

Predict before looking: ethanol as a connected example

Ethanol has CH₃, CH₂ and OH proton environments. In a simplified rapidly exchanging OH situation, CH₃ gives a triplet of relative area 3, CH₂ gives a quartet of area 2, and OH gives a variable often broad signal of area approximately 1. The OH signal disappears on D₂O exchange.

The CH₂ is more downfield because it is attached to oxygen. The CH₃ and CH₂ split one another; the OH is not simply counted as another neighbour under the stated exchange assumption. Different experimental exchange conditions can alter the OH coupling, so the assumption is part of the prediction.

For an unfamiliar spectrum, annotate each multiplet with area, shift and likely neighbours before joining fragments. Reject a structure if it fits the integration but predicts the wrong number of environments or splitting.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources