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Carbon-13 NMR spectroscopy reveals the carbon skeleton of an organic molecule. Where ¹H NMR (lesson 2) tells us about the hydrogen environments — how many, what neighbours, and what relative populations — ¹³C NMR addresses the orthogonal question: how many distinct carbon environments exist, and what functional-group type does each belong to. The technique exploits the spin-½ nature of the ¹³C isotope, but only 1.1 % of natural carbon is ¹³C, so signals are weak and acquisition is long; on the positive side, the very low ¹³C–¹³C coupling probability and routine use of broadband proton decoupling mean each unique carbon appears as a single sharp line, and the chemical-shift range stretches from 0 to about 220 ppm — nearly twenty times the width of the ¹H window — so environments rarely overlap. Integration, however, is not reliable in standard spectra: T₁ relaxation times and nuclear Overhauser enhancements differ wildly between carbons, so peak heights do not faithfully reflect carbon counts. In what follows we work through the principle, the shift table, peak-counting by symmetry, the comparison to ¹H NMR, four full structure-elucidation worked spectra, and the synoptic links into Combined Spectroscopy (L6) and the Unknown-Identification Capstone (L7).
Spec mapping (AQA 7405): This lesson maps to §3.3.15 of the AQA A-Level Chemistry specification (NMR spectroscopy, ¹³C component). It builds on the spectroscopic foundations laid in L0 (Spectroscopy Overview), L1 (Mass Spectrometry) and L2 (¹H NMR) of this course, and feeds directly into L6 (Combined Techniques: IR + MS + ¹H + ¹³C) and L7 (Unknown-Identification Capstone). The official AQA data booklet supplies the ¹³C shift table reproduced (and extended) below; refer to that booklet for the exact ranges used in marking.
Assessment objectives: AO1 items include recall of the ¹³C shift ranges, the rationale for broadband proton decoupling, the use of TMS as the δ = 0 reference and the reasons why integration is unreliable. AO2 questions test the count of unique carbon environments in a given structure and the assignment of peaks at stated chemical shifts to functional-group types. AO3 work — the highest-yield part of Paper 2 — demands that ¹³C data be combined with ¹H NMR, IR and mass-spectrum information to deduce the structure of an unknown organic compound and to justify each structural feature against the observed spectroscopic evidence.
The ¹³C nucleus has nuclear spin I = ½, identical to ¹H, so the same Zeeman splitting in a magnetic field B₀ applies: the two spin states (mₛ = +½ and −½) differ in energy by ΔE = γₕB₀, and a radio-frequency pulse tuned to that gap induces transitions whose detection gives the NMR signal. Two facts make ¹³C very different from ¹H in practice:
The combined effect is that ¹³C NMR is roughly 5 700 times less sensitive than ¹H NMR for the same molar concentration of nucleus. Modern instruments compensate by signal averaging: tens, hundreds or thousands of pulse-and-acquire cycles are summed, the noise (random) averaging to zero while the signal (coherent) accumulates. A routine ¹³C spectrum therefore typically takes 5–30 minutes of instrument time, against seconds for ¹H NMR. Larger samples (50–100 mg in 0.6 mL of deuterated solvent) are common.
A key benefit of the low ¹³C abundance is the near-total absence of ¹³C–¹³C coupling. The probability that two directly bonded carbons are both ¹³C is (0.011)² ≈ 0.012 % — below the routine noise floor — so spin–spin splitting between adjacent carbons is not observed in normal spectra. What would still be observed, however, is ¹³C–¹H coupling: a ¹³C bonded to one H would appear as a doublet, to two H as a triplet, and so on (the same n+1 rule as ¹H NMR, but with much larger J-couplings of 120–250 Hz for one-bond ¹JCH). Such coupling would shred the spectrum into a forest of overlapping multiplets across hundreds of Hz, ruining the resolution that the wide chemical-shift range otherwise provides. The standard fix is broadband ¹H decoupling: a second radio-frequency field is applied during acquisition that simultaneously saturates all ¹H frequencies. Each proton is flipping between its spin states so rapidly that on the time-scale of the ¹³C signal, its average coupling effect is zero. The result is one sharp singlet for each unique carbon environment — a clean, easily-counted spectrum.
Practical-skills box. Running a routine ¹³C spectrum: dissolve 30–100 mg of sample in 0.6–0.7 mL of deuterated solvent (CDCl₃ is standard; D₂O for water-soluble analytes; DMSO-d₆ for polar solids) and transfer to a 5 mm NMR tube. Acquire 1 024–16 384 scans with a 30–60° pulse and a recycle delay of 1–2 seconds for organic compounds, longer for compounds with quaternary carbons (e.g. CDCl₃ carbonyls). To speed up acquisition and — if needed — obtain reliable integrations, the paramagnetic relaxation agent chromium(III) acetylacetonate, Cr(acac)₃, can be added at ≈10 mM; it shortens T₁ relaxation times of all carbons fairly uniformly and quenches the variable nuclear-Overhauser enhancement that makes standard ¹³C integration unreliable. The combination of Cr(acac)₃ doping with an inverse-gated decoupling pulse sequence (decoupler on only during acquisition, off during the recycle delay) allows quantitative ¹³C NMR — useful for measuring isomer ratios in research, but not part of the routine A-Level technique.
The ¹³C chemical-shift scale spans roughly 0–220 ppm, in contrast to the 0–12 ppm window of ¹H NMR. This is not because the underlying physical effect is different — chemical shielding by electron density is the cause in both cases — but because the electron environment around a carbon nucleus changes much more dramatically with bonding and hybridisation than the environment around a peripheral hydrogen. A carbonyl carbon, for example, has its σ and π electrons heavily withdrawn by the oxygen; an alkyl sp³ carbon has a near-symmetric tetrahedral electron cloud. The difference in shielding constant between these extremes is far larger than the difference between, say, an aldehyde proton (δ ≈ 9.7) and a TMS proton (δ = 0). The practical advantage is enormous: peaks rarely overlap in ¹³C NMR, and the position of a peak is by itself a strong diagnostic of functional-group type. The cost is the much weaker signal and longer acquisition.
TMS reference. As in ¹H NMR, tetramethylsilane, (CH₃)₄Si (TMS), defines δ = 0. The same compound is used because (a) all four methyl-carbon environments in TMS are equivalent, giving a single ¹³C peak; (b) silicon is more electropositive than carbon, so the methyl carbons are heavily shielded and resonate at lower frequency than virtually any organic carbon — placing them sensibly at one end of the scale; (c) TMS is chemically inert, volatile (easy to remove) and miscible with most organic solvents. Industry has largely moved to referencing against the solvent peak (CDCl₃ at δ = 77.16 ppm, DMSO-d₆ at δ = 39.52 ppm, etc.), but the AQA specification still discusses TMS, and any A-Level question will assume it.
| Carbon environment | Chemical shift δ / ppm | Comment / example |
|---|---|---|
| Alkyl C (sp³, R–CH₃, R–CH₂–R, R₃C–H) | 5–40 | Most aliphatic carbons. Ethane CH₃ ≈ 6, propane CH₂ ≈ 16, cyclohexane ≈ 27. |
| C–N (amine, sp³) | 30–60 | Methylamine CH₃ ≈ 28, triethylamine CH₂ ≈ 47. |
| C–O (alcohol, ether, ester O–CH₃) | 50–90 | Methanol ≈ 50, ethanol CH₂ ≈ 58, t-butanol C(CH₃)₃ ≈ 69. |
| C–Cl (chloroalkane) | 25–50 | Chloroethane CH₂Cl ≈ 40. |
| C–Br (bromoalkane) | 20–40 | Bromoethane CH₂Br ≈ 28. |
| Alkene C (sp², C=C) | 100–150 | Ethene = 123, 1-hexene =CH₂ ≈ 114, internal CH ≈ 139. |
| Aromatic C (benzene ring) | 110–160 | Benzene = 128.5, methylbenzene ipso ≈ 137. |
| Nitrile C≡N | 110–125 | Ethanenitrile CH₃–C≡N: CN ≈ 117, CH₃ ≈ 1. |
| Ester C=O (RCOOR′) | 165–175 | Methyl ethanoate ≈ 171. |
| Amide C=O (RCONR₂) | 165–180 | Ethanamide ≈ 172. |
| Carboxylic acid C=O (RCOOH) | 170–185 | Ethanoic acid ≈ 178. |
| Ketone / aldehyde C=O | 195–220 | Propanone ≈ 207, ethanal ≈ 200, benzaldehyde ≈ 192. |
Exam Tip. The single most-tested diagnostic in this lesson is the carbonyl-carbon split: ketones and aldehydes sit at δ = 195–220 ppm, comfortably above the acids, esters and amides at δ = 165–185 ppm. The physical reason: in an acid, ester or amide, the adjacent O–H, O–R or N–R₂ has a lone pair that donates electron density into the C=O π* system via resonance (the mesomeric +M effect), increasing the electron density at the carbonyl carbon and therefore increasing its shielding (→ lower δ). A ketone or aldehyde has no such donor next door; its carbonyl carbon is more deshielded.
The number of peaks in a (proton-decoupled) ¹³C spectrum equals the number of chemically distinct carbon environments in the molecule — not the number of carbon atoms. Two carbons are equivalent if they are related by a symmetry operation of the molecule (a mirror plane, a Cₙ rotation axis or, in averaged form, free rotation about a bond). Spotting that symmetry quickly is one of the two main ¹³C exam skills.
The two carbons are clearly different: one bears three H (a primary methyl), the other bears two H and an OH (a primary alcohol). Predicted peaks: CH₃ at δ ≈ 18 ppm, CH₂–OH at δ ≈ 58 ppm. Two peaks, well separated, the higher one diagnostic of a C–O environment.
The two methyl carbons are equivalent by the C₂ axis through the C=O. The carbonyl carbon is its own environment. Two peaks: methyl at δ ≈ 30 ppm, C=O at δ ≈ 207 ppm. Note how peak height does not report the 2 : 1 carbon ratio in a routine spectrum — the C=O peak is typically much smaller than the methyl peak because the quaternary-style carbonyl has a long T₁ and no attached ¹H to feed NOE enhancement.
All three methyl groups are equivalent (free rotation about each C–C bond combined with the C₃ axis through the central carbon and the OH). The central, quaternary C–OH carbon is its own environment. Two peaks: methyl at δ ≈ 31 ppm, C–OH (quaternary) at δ ≈ 69 ppm.
The two methyl groups on C-2 are equivalent (mirror plane through the CH–CH₂–OH chain). Three environments in total: CH₃ (δ ≈ 19 ppm), CH (δ ≈ 31 ppm), CH₂OH (δ ≈ 70 ppm). Three peaks.
The D₆ₕ symmetry of benzene makes all six carbons equivalent. One peak at δ = 128.5 ppm.
The methyl group breaks the D₆ₕ symmetry of benzene down to C₂ᵥ. There are now four kinds of ring carbon — ipso (C-1, attached to CH₃), ortho (C-2 and C-6, equivalent), meta (C-3 and C-5, equivalent) and para (C-4) — plus the CH₃ carbon, giving five peaks: δ ≈ 21 (CH₃), 125 (para), 128 (meta), 129 (ortho), 137 (ipso). The ipso carbon is quaternary (no attached H) and is typically the smallest peak in the spectrum.
The two methyl carbons are equivalent by the C₂ axis perpendicular to the ring; the two ipso carbons (C-1, C-4) are equivalent; the four ring CH carbons (C-2, C-3, C-5, C-6) are equivalent. Three peaks: CH₃ (δ ≈ 21), CH ring (δ ≈ 129), ipso ring (δ ≈ 134). Compare with 1,2- and 1,3-dimethylbenzene — the ortho isomer shows four peaks, the meta isomer five.
A-Level Depth Box — why symmetric isomers give fewer peaks. ¹³C NMR is therefore a powerful structural-isomer discriminator. Para-xylene shows three peaks, meta-xylene shows five, ortho-xylene shows four — enough on its own (without IR, MS or ¹H NMR) to assign the substitution pattern of a disubstituted benzene. This synoptic point is regularly tested in Paper 2.
A common student error is to read the ¹³C peak heights as a ratio that gives the number of carbons in each environment, by analogy with ¹H NMR. Do not do this. Three effects conspire to make standard ¹³C integrations meaningless:
The clean fix — used in research but well beyond A-Level routine — is inverse-gated decoupling plus a long recycle delay (5× the longest T₁, often 30–60 s) plus optional Cr(acac)₃ doping. Under those conditions the integrals do reflect carbon counts. For exam purposes, treat ¹³C peak heights as qualitative only and never quote ratios.
| Feature | ¹H NMR | ¹³C NMR |
|---|---|---|
| Natural abundance of the observed isotope | 99.98 % | 1.1 % |
| Typical chemical-shift range | 0–12 ppm (organic) | 0–220 ppm |
| Number of peaks reports | Number of H environments | Number of C environments |
| Splitting / multiplicity | Yes — n + 1 rule from neighbouring H | None in standard broadband-decoupled spectrum |
| Integration | Reliable; gives the ratio of H in each environment | Not reliable in routine spectra (T₁ + NOE effects) |
| Reference compound | TMS, δ = 0 | TMS, δ = 0 |
| Sensitivity (relative, per nucleus) | ≈1 | ≈0.0002 (5 700× weaker) |
| Acquisition time | Seconds to minutes | Minutes to hours |
| Best at deducing | Number of H, neighbours, ratios | Number of C environments, functional-group types from shift |
| Strongest single diagnostic | Aldehyde H at δ = 9.7 ppm; OH/NH variable | Carbonyl C at δ = 165–220 ppm |
The two techniques are complementary: ¹H NMR gives the H-skeleton with quantitative ratios; ¹³C NMR gives the C-skeleton with high functional-group discrimination but no ratios. Together — with IR and mass spectrometry — they nail almost every A-Level unknown.
These four exercises model the kind of structure-deduction work tested in Paper 2.
Degree of unsaturation (DoU) = (2×3 + 2 − 6)/2 = 1 — consistent with one C=O or one ring. The peak at δ = 207 is squarely in the ketone/aldehyde range (195–220). With three carbons total, the formula C₃H₆O and one C=O carbon at δ = 207, the remaining two carbons account for all six hydrogens — i.e. two CH₃ groups. The peak at δ = 31 is in the alkyl range. Compound: propanone, CH₃COCH₃. The two methyl groups are equivalent by symmetry, so they give a single peak.
DoU = (2×4 + 2 − 8)/2 = 1. The peak at δ = 171 is in the ester/acid range (165–185), not the ketone range — so the C=O is an ester or carboxylic acid. With four carbons and three peaks, two carbons must be equivalent or there must be three distinct environments with one carbon each. The peak at δ = 51 is in the C–O range (50–90), suggesting an O–CH₃ or O–CH₂–. The peak at δ = 21 is alkyl. Putting it together: methyl ethanoate, CH₃COOCH₃. The four carbons are: ester carbonyl (δ ≈ 171), O–CH₃ (δ ≈ 51), –COO–CH₃ acetyl methyl (δ ≈ 21) — three environments, three peaks. The molecule is unambiguous.
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