¹³C NMR
How to see the carbon skeleton itself: one signal for every different carbon in the molecule. Proton NMR shows you the hydrogens and lets you infer the skeleton they are attached to. Carbon NMR looks at the carbon skeleton directly; the spectra are far easier to read, usually a set of single sharp lines.
Why ¹³CDecouplingShift scaleWhat it tells youCoupled spectraEquivalenceShiftsStep by stepExamplesPractice
01Why carbon-13, and why the spectra are weak
A nucleus only gives an NMR signal if it has spin. The common isotope of carbon, ¹²C, has six protons and six neutrons and no nuclear spin, so it is invisible in NMR. The signal comes entirely from ¹³C, which has spin ½ and behaves like a proton does: two orientations in a magnetic field, a small energy gap, resonance in the radiofrequency range.
Two facts make the experiment itself more difficult than ¹H NMR. First, ¹³C is only about 1.1 % of natural carbon. Second, its magnetic moment is roughly four times smaller than a proton’s, so each nucleus absorbs less strongly. Together these make ¹³C some ten thousand times less sensitive, which is why a carbon spectrum is collected by adding together many hundreds or even thousands of repeated scans rather than a single sweep.
One useful consequence of low abundance. Because only about one carbon in ninety is a ¹³C, the chance of two ¹³C atoms sitting next to each other in the same molecule is tiny. Carbon nuclei therefore never appear to couple to each other, and every signal is a clean line rather than a multiplet split by neighbouring carbons.
02Proton decoupling: one line per carbon
Each ¹³C nucleus is coupled to the hydrogens attached to it, and those couplings are large. Left alone, a CH₃ carbon would appear as a quartet and a CH as a doublet, and the resulting overlapping multiplets would be almost unreadable in a spectrum already short of signal.
Routine carbon spectra are therefore recorded proton-decoupled: the sample is irradiated continuously across the whole proton frequency range while the carbon spectrum is measured. The protons flip so rapidly between states that each carbon feels only their average field, and the coupling vanishes. The result is the standard spectrum you will be asked to interpret; one single sharp line for each carbon environment, with no splitting anywhere.
03The chemical shift scale
Shifts are quoted exactly as in ¹H NMR, as delta (δ) in parts per million relative to tetramethylsilane (TMS) at 0 ppm, and the axis again runs right to left with downfield, more deshielded carbons on the left.
The one big difference is the range. Carbon shifts spread over roughly 0 to 220 ppm, about twenty times wider than the proton scale. This is a considerable advantage: signals almost never overlap, and two carbons in slightly different environments are usually resolved cleanly. It also means small errors in reading the axis matter less, so it is worth learning the regions rather than exact numbers.
One extra line appears in nearly every real spectrum. Samples are dissolved in a deuterated solvent, most often CDCl₃, whose own carbon shows up at 77 ppm as a small three-line pattern (deuterium is spin 1, and it is not removed by proton decoupling). Learn to ignore it.
The proton-decoupled ¹³C NMR spectrum of ethyl ethanoate, CH₃COOCH₂CH₃, has four carbons, four environments, four single lines: the ester carbonyl far downfield at 171 ppm, the O–CH₂ at 60 ppm, and the two methyls at 21 and 14 ppm. Peak heights are not proportional to the number of carbons.
04What a carbon spectrum tells you, and what it does not
A decoupled carbon spectrum carries two pieces of information, not four. Knowing what is missing is as important as knowing what is there.
A rough guide worth knowing: carbons with no attached hydrogens, such as a carbonyl carbon or a substituted aromatic carbon, tend to give noticeably weaker lines than CH, CH₂ or CH₃ carbons. Use this as a hint about which peak is which, never as a count.
05Proton-coupled spectra: counting the hydrogens on each carbon
Decoupling removes information on purpose. Switch the decoupler off and each carbon signal splits again, this time telling you how many hydrogens are bonded to that carbon. The one-bond C–H couplings (about 125 to 170 Hz) are far larger than any longer-range coupling, so in practice each carbon is split only by its own attached hydrogens. The splitting follows the same n + 1 rule observed in ¹H NMR, but here n is the number of hydrogens on the carbon, not on its neighbours.
| Carbon | H attached (n) | Coupled pattern | Typical example |
|---|---|---|---|
| Quaternary C | 0 | singlet (s) | C=O of a ketone or ester; substituted ring carbon |
| CH (methine) | 1 | doublet (d) | Aromatic CH; the CH of an isopropyl group |
| CH₂ (methylene) | 2 | triplet (t) | The O–CH₂ of an ethyl ester |
| CH₃ (methyl) | 3 | quartet (q) | Any methyl group |
Example. Ethyl ethanoate or methyl propanoate?
Both esters, C₄H₈O₂, give four lines in the decoupled spectrum. The coupled spectrum settles it. In ethyl ethanoate, CH₃COOCH₂CH₃, the C–O carbon at 60 ppm is a triplet (an O–CH₂), the carbonyl at 171 ppm is a singlet, and both methyls at 21 and 14 ppm are quartets. In methyl propanoate, CH₃CH₂COOCH₃, the C–O carbon near 52 ppm is a quartet (an O–CH₃) and the triplet moves to the CH₂ next to the carbonyl, near 27 ppm.
The cost, and the modern alternative. Coupled spectra are slow to record: each signal is spread over several lines and loses the intensity boost that decoupling provides. Most laboratories therefore get the same information from DEPT experiments, which keep the clean single lines: DEPT-90 shows only CH carbons, and DEPT-135 shows CH and CH₃ pointing up, CH₂ pointing down, and no quaternary carbons at all.
06Equivalent carbons: counting the signals
Carbons in identical environments are equivalent and give one shared line. Counting environments is the first thing you should do with any structure, and symmetry is what you are looking for.
- Propanone, CH₃COCH₃, has three carbons but two signals; the two methyls are equivalent (30 ppm) and the carbonyl is alone (206 ppm).
- Benzene has six carbons but one signal, at 128 ppm.
- Methylbenzene (toluene) has seven carbons and five signals: the CH₃, the substituted ring carbon, and the ortho, meta and para carbons, each pair equivalent.
- 1,4-dimethylbenzene has eight carbons and only three signals, because its symmetry makes both methyls equivalent, all four CH ring carbons equivalent, and both substituted ring carbons equivalent.
The classic use of a carbon count. Disubstituted benzenes are separated instantly by the number of aromatic signals. In the 1,4- (para) isomer of a symmetrically substituted ring the four ring environments collapse to two; the 1,2- (ortho) isomer gives three, and the 1,3- (meta) isomer gives four.
07Characteristic chemical shifts
Four broad regions cover almost everything you will meet. Learn the boundaries and you can name a carbon from its shift alone.
| Type of carbon | δ (ppm) | Notes |
|---|---|---|
| Alkane C (C–C only) | 5 – 40 | Simple saturated chains; methyls are at the low end |
| C next to a carbonyl (CH₃–C=O) | 20 – 40 | Only modestly shifted; the carbonyl carbon itself is far away |
| C–N (amine) | 30 – 60 | Below the C–O region, as nitrogen is less electronegative |
| C–Cl, C–Br | 30 – 70 | Overlaps the C–O region; use other evidence too |
| C–O (alcohol, ether, ester) | 50 – 90 | A reliable marker for a saturated carbon bearing oxygen |
| Alkyne C (C≡C) | 65 – 90 | Sits in the same window as C–O; often weak |
| Alkene C (C=C) | 110 – 150 | Overlaps the aromatic region; count the signals to distinguish |
| Aromatic C | 110 – 160 | Benzene itself is 128; substituted ring carbons are usually weak |
| C=O of ester, amide, acid | 160 – 185 | Lower than ketones; oxygen or nitrogen donates electron density |
| C=O of aldehyde, ketone | 190 – 220 | The most downfield region; nothing else appears there |
Notice that the two carbonyl bands separate ketones and aldehydes (above 190 ppm) from esters, acids and amides (160 to 185 ppm). That single distinction, which IR makes only through careful reading of the C=O stretch, is unmistakable in a carbon spectrum.
08How to interpret a spectrum, step by step
- Discard the solvent line. Cross out the small three-line pattern at 77 ppm (CDCl₃) or the multi-line signal at 40 ppm (DMSO-d₆) before you count anything.
- Count the signals and compare with the formula. If there are fewer signals than carbons, the molecule has some symmetry, be it local (e.g. in a t-butyl group) or global (there is a mirror plane within the molecule).
- Check above 160 ppm first. A line there means a carbonyl, and its exact position says which kind: ester, acid or amide below 185 ppm, ketone or aldehyde above 190 ppm.
- Then the 110 to 160 window. Lines here mean alkene or carbons in aromatic cycles; the number of them tells you the ring’s substitution pattern.
- Then 50 to 90 ppm. A line here is a saturated carbon attached to oxygen, one of the most useful single observations in the spectrum.
- Assign the rest and cross-check. Everything below about 50 ppm is plain alkyl carbon. Then combine with ¹H NMR for connectivity, IR for functional groups, and MS for the molecular formula.
09Worked interpretation examples
Example 1. Ethyl ethanoate
Ignore the 77 ppm solvent lines and four signals remain, so there are four different carbons. The line at 171 ppm is a carbonyl, and because it lies below 185 ppm it belongs to an ester, acid or amide rather than a ketone; it is also the weakest line, consistent with a carbon carrying no hydrogens. The line at 60 ppm falls in the C–O window, so a saturated carbon is bonded to oxygen. The remaining two lines at 21 and 14 ppm are ordinary alkyl carbons. Assembling an ester with one O–CH₂ and two distinct methyls gives ethyl ethanoate, CH₃COOCH₂CH₃. Its isomer methyl propanoate would also show four lines, so here you would confirm the assignment with ¹H NMR, which shows the ethyl triplet and quartet.
Example 2. C₃H₆O with two signals
A compound of formula C₃H₆O gives two carbon signals, at 206 and 30 ppm. Two signals for three carbons means two of them are equivalent. The 206 ppm line is above 190 ppm, so it is a ketone or aldehyde carbonyl, and 30 ppm is a methyl next to that carbonyl. A symmetric structure with a carbonyl flanked by two identical methyls is propanone (acetone), CH₃COCH₃. Propanal, its isomer, would give three signals, one for each of its three inequivalent carbons.
Example 3. Two isomers of C₈H₈O₂
Methyl benzoate and phenyl ethanoate share a formula and both are esters, and their carbon spectra separate them by where the oxygen sits.
| Compound | Key signals | The give-away |
|---|---|---|
| Methyl benzoate, C₆H₅COOCH₃ | 167 (C=O), 133–128 (four aromatic), 52 (OCH₃) | The alkyl carbon sits in the C–O window at 52 ppm, so the methyl is on the oxygen |
| Phenyl ethanoate, CH₃COOC₆H₅ | 169 (C=O), 151 plus three other aromatic, 21 (CH₃) | The methyl is at 21 ppm, plain alkyl, and one ring carbon is pushed to 151 ppm by the oxygen |
10Practice questions
Try each before opening the answer.
Q1Why does ¹²C give no NMR signal, and why are ¹³C spectra built from many repeated scans?
¹²C has an even number of protons and neutrons and therefore no nuclear spin, so it has no magnetic states to flip between. ¹³C is NMR-active but makes up only about 1.1 % of carbon and has a weak magnetic moment, so a single scan is lost in the noise; adding many scans together builds the real signal up above it.
Q2How many signals appear in the ¹³C NMR spectrum of 1,4-dimethylbenzene, and roughly where?
Three. The two equivalent methyl carbons appear near 21 ppm; the four equivalent ring CH carbons near 129 ppm; and the two equivalent substituted ring carbons near 135 ppm, as a weaker line because they carry no hydrogens.
Q3A spectrum shows a single line at 197 ppm. What does that tell you, and what does it rule out?
A line above 190 ppm is the carbonyl carbon of a ketone or aldehyde. It rules out an ester, carboxylic acid or amide carbonyl, all of which fall between 160 and 185 ppm.
Q4An unknown of formula C₄H₁₀O gives two ¹³C signals, at 69 and 31 ppm. Identify it.
2-Methylpropan-2-ol (tert-butyl alcohol), (CH₃)₃COH. Two signals for four carbons demands high symmetry: three equivalent methyls (31 ppm) and one central carbon. That central carbon at 69 ppm sits in the C–O window, so it carries the OH.
Q5An unknown of formula C₃H₆O₂ shows a broad IR absorption near 3000 cm⁻¹ and a strong C=O stretch at 1710 cm⁻¹. Its ¹³C NMR spectrum has three lines, at 181, 28 and 9 ppm. Identify the compound.
Propanoic acid, CH₃CH₂COOH. The broad IR band and 1710 cm⁻¹ C=O point to a carboxylic acid, confirmed by the carbonyl carbon at 181 ppm (below 185). Three lines for three carbons means no symmetry; 28 ppm is the CH₂ next to the carbonyl and 9 ppm is the terminal CH₃. No line in the 50–90 window rules out an ester such as methyl ethanoate.
Key takeaways
- Only ¹³C, 1.1 % of natural carbon, is NMR-active; signals are weak and are built from many accumulated scans.
- Routine spectra are proton-decoupled, so each carbon environment gives one sharp line with no splitting.
- A proton-coupled spectrum (or DEPT) shows how many H atoms sit on each carbon: singlet C, doublet CH, triplet CH₂, quartet CH₃.
- Read two things only: the number of signals and the chemical shift. Peak size is not always a reliable count.
- Remember four regions: 5–50 alkyl, 50–90 C–O, 110–160 alkene and aromatic, 160–220 carbonyl, with esters and acids below 185 ppm and ketones and aldehydes above 190 ppm.
- Ignore the solvent: CDCl₃ at 77 ppm appears in nearly every spectrum.