Part 1: NMR principles and carbon-13 spectra

Use chemical environments, symmetry, TMS and carbon-13 chemical shifts to interpret AQA NMR spectra.

What NMR measures

Nuclei such as 1H and 13C interact with a strong magnetic field. Radiofrequency energy can be absorbed at resonance. The electrons and nearby groups affect the field experienced by a nucleus, so nuclei in different chemical environments can resonate at different frequencies.

The spectrum plots signal against chemical shift, δ, in parts per million (ppm). A shift is measured relative to a reference, normally tetramethylsilane, TMS. Larger δ values are generally plotted towards the left. Use the actual axis labels rather than assuming left means “lower”.

Carbon-13 NMR tells you about carbon environments. Proton NMR tells you about hydrogen environments and also provides useful relative areas and splitting. Standard A-Level 13C spectra are treated as proton-decoupled: each distinct carbon environment gives a single signal. Do not apply the proton n + 1 rule to those carbon spectra.

Why TMS and deuterated solvents are used

TMS is Si(CH3)4. Its four carbon atoms are equivalent and its twelve hydrogen atoms are equivalent, so it gives one reference signal in each type of spectrum. The reference is assigned δ = 0 ppm. TMS is comparatively unreactive, volatile and gives a signal well separated from most ordinary organic signals. A reference peak is not an extra environment in the unknown.

A suitable solvent dissolves the sample without reacting with it. Deuterated solvents such as CDCl3 greatly reduce the solvent signal in a proton spectrum because deuterium is 2H rather than 1H. Real solvents can contain residual protonated solvent and water, so “no solvent peaks under any circumstances” is too absolute.

Deuterated carbon-containing solvents can still produce 13C signals. Recognise labelled solvent/reference signals and exclude them when counting sample environments. CCl4 contains no hydrogen and may appear in theoretical questions, but it is hazardous and is not a default recommendation for practical solvent choice.

Count environments, not just atoms

Equivalent carbons have the same chemical surroundings. Symmetry can make different positions equivalent, but simply being a CH3 carbon does not. Compare the whole molecular structure, including the positions of functional groups.

Propanone, CH3COCH3, has three carbons but only two environments: the two methyl groups are equivalent and the carbonyl carbon is different. Propanal, CH3CH2CHO, has three carbon environments.

Butan-2-one has four environments because its two methyl groups occupy different surroundings. 2-Methylpropanal, (CH3)2CHCHO, has three because its two methyl groups are equivalent. For ordinary benzene, symmetry makes all six ring carbons equivalent.

Use the carbon-13 shift ranges

Use the AQA data booklet supplied with the question. Ranges overlap and neighbouring groups can shift a signal, so a single δ value is evidence for an environment rather than a complete structural identification.

Worked deduction from carbon-13 data

Original teaching data: a compound has molecular formula C4H8O and four sample 13C signals at approximately δ 8, 29, 36 and 209 ppm. The high-shift signal supports an aldehyde or ketone carbonyl; the other three indicate saturated carbon environments.

Butan-2-one is consistent with four environments and a ketone carbonyl. The data do not justify identifying it uniquely on signal count alone: butanal also has four environments and a carbonyl. Additional proton NMR or aldehyde-test evidence can distinguish them.

An ordinary 13C peak area is not a reliable count of the number of carbons represented. Four signals do not mean the molecule necessarily contains exactly four carbons, and a weak or overlapping signal can complicate a real spectrum.

Give a structural reason for each assignment

AQA June 2023 Paper 2 Q05 separates counting carbon environments from explaining proton shifts and splitting. Its examiner report discusses difficulty recognising the environments and using incomplete spectra. Make a labelled structure and check all of its predicted environments against the data.

In an exam spectrum, use any stated simplifications. Do not add imagined solvent peaks or missing signals, but recognise explicitly stated overlap or incomplete information when evaluating whether a structure is uniquely determined.

Quick checks

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

Sources