Advanced Organic Chemistry
AQA A Level Chemistry — 7.10 Nuclear Magnetic Resonance Spectroscopy — AQA A Level Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 11 of 12
Lesson summary
Carbon-13 NMR: count distinct carbon environments to predict the number of resonances and use molecular symmetry to explain equivalent atoms. In ¹³C NMR, carbon atoms in different chemical environments produce resonances at different chemical shifts.
- two signals
- 1,3-dihydroxybenzene
- four distinct carbon environments
- four ¹³C resonances
- not reliably proportional
Part 1 of 4What a signal means
¹³C NMR
In ¹³C NMR, carbon atoms in different chemical environments produce resonances at different chemical shifts. Chemically equivalent carbon atoms share one environment and normally give one signal.
Ppm
The chemical shift is reported in ppm relative to tetramethylsilane, TMS, at 0 ppm. Counting signals is often the first step in analysing a spectrum.
Part 2 of 4Use symmetry
Draw the structure and look for a symmetry operation that exchanges carbon positions while leaving the same surroundings. Carbons exchanged in this way are equivalent and count as one environment.
Propanone
Propanone has two environments: its two equivalent methyl carbons give one signal, while the carbonyl carbon gives another. Its ¹³C spectrum therefore has two signals.
Part 3 of 4Worked ring example
1,3-dihydroxybenzene
In 1,3-dihydroxybenzene, the ring has a symmetry relationship that pairs some carbon positions. After grouping equivalent positions, there are four distinct carbon environments.
Four ¹³C resonances
Expect four ¹³C resonances for this molecule. Count environments rather than simply counting the six carbon atoms in the benzene ring.
Part 4 of 4Interpret with care
1
Routine proton-decoupled ¹³C spectra show relatively simple single signals; they do not show the same splitting patterns commonly analysed in ¹H NMR.
2Not reliably proportional
Signal height is not reliably proportional to the number of carbon atoms in an environment. Use the number and chemical shifts of signals, together with the structural formula, to compare possible isomers.
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