NMR Spectroscopy: Carbon-13 and Proton NMR

This guide covers subtopics 37.3, Carbon-13 NMR spectroscopy, and 37.4, Proton (¹H) NMR spectroscopy, from Topic 37, Analytical techniques, of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is A Level content — the technique Analytical Techniques: IR and Mass Spectrometry explicitly states is not required at AS Level.

Before studying this

This resource assumes IR and mass spectrometry interpretation from Analytical Techniques: IR and Mass Spectrometry — NMR is typically used alongside those techniques, not instead of them, to fully identify an unknown structure — and functional-group recognition from across the AS and A Level organic resources.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — A Level, Topic 37

37.3 Carbon-13 NMR spectroscopy — analysing and interpreting a carbon-13 NMR spectrum to deduce the different carbon environments present and possible structures; predicting or explaining the number of peaks for a given molecule.

37.4 Proton (¹H) NMR spectroscopy — analysing and interpreting a proton NMR spectrum to deduce proton environments (from chemical shift), relative numbers of each type of proton (from peak area), the number of equivalent protons on the adjacent carbon (from splitting pattern, using the n+1 rule, limited to singlet/doublet/triplet/quartet/multiplet), and possible structures; predicting chemical shifts and splitting patterns; describing the use of TMS as the chemical-shift standard; stating the need for deuterated solvents; describing O–H/N–H identification by proton exchange with D₂O.

Carbon-13 NMR

Carbon-13 NMR detects the different chemical environments that carbon atoms occupy in a molecule — carbons in genuinely different structural positions (different neighbouring atoms/groups) give separate peaks; carbons in equivalent, symmetry-related positions give the same peak.

Number of peaks. The number of peaks in a carbon-13 spectrum equals the number of chemically distinct carbon environments, not the total number of carbon atoms.

Worked example. Predict the number of peaks in the carbon-13 NMR spectrum of propan-1-ol, CH₃CH₂CH₂OH, and of propan-2-ol, (CH₃)₂CHOH.

Propan-1-ol has three carbons, each in a different environment (the terminal CH₃, the central CH₂, and the CH₂ attached to OH) — 3 peaks.

Propan-2-ol has three carbons, but the two CH₃ groups are equivalent by the molecule’s own symmetry (both attached identically to the central CHOH carbon) — so there are only 2 distinct environments: the two equivalent CH₃ carbons (one peak) and the central CH carbon (a second peak).

This distinction — same molecular formula, different number of carbon-13 peaks — is exactly why carbon-13 NMR is useful for distinguishing structural isomers that other techniques might not separate cleanly.

Proton (¹H) NMR

Proton NMR gives more information again, because it reports not just how many distinct proton environments exist, but how many protons are in each, and what’s directly next to each one.

Chemical shift identifies each proton’s environment — different functional-group environments (e.g. a proton on a carbon next to a carbonyl, versus a proton on a carbon next to an oxygen, versus an aromatic proton) absorb at different, characteristic chemical shift values, interpreted using reference data (given in an exam, not memorised precisely — the same approach already established for IR wavenumbers).

Relative peak area gives the relative number of protons of each type — the ratio of the areas under each peak directly matches the ratio of protons contributing to each environment.

Splitting pattern (the n+1 rule) reveals how many equivalent protons sit on the carbon atom(s) adjacent to the one giving that peak: a peak splits into (n + 1) smaller lines, where n is the number of equivalent protons on the neighbouring carbon(s). At this level, splitting patterns are limited to:

Neighbouring protons (n)	Splitting pattern
0	singlet
1	doublet
2	triplet
3	quartet
more, or non-equivalent neighbours	multiplet

Worked example. Predict the proton NMR spectrum of ethanol, CH₃CH₂OH (three proton environments: CH₃, CH₂, OH).

• CH₃ protons: neighbouring carbon is the CH₂, which has 2 protons → n + 1 = 3 → triplet; relative area 3 (three equivalent protons).
• CH₂ protons: neighbouring carbons are the CH₃ (3 protons) and the OH oxygen — OH protons are usually not counted for splitting, since proton exchange (see below) usually averages this coupling away → n + 1 = 4 → quartet; relative area 2.
• OH proton: typically appears as a broad singlet (its coupling to neighbouring protons is usually not observed, again due to exchange); relative area 1; and — distinctively — its exact chemical shift can vary between samples, unlike C–H protons.

Overall predicted pattern: a triplet (area 3), a quartet (area 2) and a broad singlet (area 1) — the classic, frequently tested ethanol spectrum, directly demonstrating the n+1 rule (CH₃’s triplet comes from its 2 neighbouring CH₂ protons; CH₂’s quartet comes from its 3 neighbouring CH₃ protons).

Deducing structure from a spectrum combines all three pieces of information together: the number of peaks gives the number of distinct proton environments; the chemical shift of each identifies what kind of environment it is; the relative area gives how many protons are in each environment; and the splitting pattern reveals how many protons are on each neighbouring carbon — together usually enough to piece together the full structure, especially combined with the molecular formula from mass spectrometry.

Practical requirements

Tetramethylsilane (TMS) is used as the standard reference for chemical shift measurements — by convention assigned a chemical shift of exactly zero, with essentially all other proton (and carbon) environments appearing at higher, positive shift values relative to it. TMS is chosen because it gives a single, sharp reference peak (all twelve of its protons are equivalent), it’s chemically unreactive, and it’s volatile enough to be removed easily from the sample afterwards.

Deuterated solvents (such as CDCl₃) are needed to dissolve the sample for the NMR measurement, because an ordinary hydrogen-containing solvent would itself give enormous, overwhelming proton NMR signals of its own, swamping the sample’s much smaller signal — replacing hydrogen with deuterium (²H) removes the solvent’s interfering ¹H signal, since deuterium is not detected in a ¹H NMR experiment.

Identifying O–H and N–H protons by proton exchange with D₂O. O–H and N–H protons exchange readily with the deuterium in D₂O, converting them to O–D or N–D — which are not detected in a ¹H NMR spectrum. Running the spectrum again after shaking the sample with D₂O and comparing the two spectra: any peak that has disappeared in the second spectrum corresponds to an O–H or N–H proton, providing a reliable way to identify (and distinguish) these often broad, variable-shift peaks from ordinary C–H peaks, which do not exchange and remain unchanged.

Common mistakes

Counting carbon-13 or proton peaks by counting atoms directly, without checking for molecular symmetry. Equivalent atoms related by the molecule’s own symmetry (as in propan-2-ol’s two identical CH₃ groups) give only one peak between them, not one peak each.

Including an OH or NH proton in a neighbouring carbon’s splitting count. Because O–H and N–H protons usually exchange rapidly, they don’t typically show the expected coupling to neighbouring C–H protons, and are excluded when applying the n+1 rule to a neighbouring CH₂ or CH₃ group’s splitting pattern.

Forgetting relative peak area is a ratio, not an absolute proton count. A 3:2:1 area ratio is consistent with 3, 2 and 1 protons, but equally with 6, 4 and 2, or any other multiple — area alone gives the ratio; the molecular formula (often from mass spectrometry) is needed to fix the actual numbers.

Assuming the D₂O exchange test removes a peak permanently from the molecule’s real structure. It only removes that peak from this particular NMR spectrum, by temporarily swapping one exchangeable proton for a deuterium — a diagnostic trick for spectrum interpretation, not a permanent chemical change relevant to the actual synthesis or use of the compound.

Quick revision checklist

• Number of carbon-13 (or proton) peaks = number of distinct environments, accounting for molecular symmetry
• Proton NMR: chemical shift → environment type; peak area → relative proton count; splitting → n+1 rule from neighbouring protons
• Splitting: 0 neighbours = singlet; 1 = doublet; 2 = triplet; 3 = quartet; more/mixed = multiplet
• TMS: zero-shift reference standard, single sharp peak, unreactive, volatile
• Deuterated solvent (e.g. CDCl₃): avoids swamping the spectrum with the solvent’s own ¹H signal
• O–H/N–H identified by peak disappearing after shaking with D₂O (proton exchange)

Related resources

• Analytical Techniques: IR and Mass Spectrometry — the AS-level techniques NMR is typically combined with
• Chromatography: Thin-Layer and Gas/Liquid Chromatography — the remaining two subtopics of Topic 37
• A Level Organic Synthesis: Multi-Step Routes with Aromatic Chemistry — using spectroscopic evidence to confirm a synthesis route’s product
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.