Difficulty: Intermediate | Prerequisites: General Chemistry, Lewis structures, hybridisation, ¹H NMR basics.
Carbon-13 NMR tells you about the carbon skeleton of a molecule. While ¹H NMR focuses on hydrogen environments, ¹³C NMR reveals how many chemically distinct carbon atoms are present and what type each one is (alkyl, aromatic, carbonyl, etc.). In a typical Organic Chemistry II structure-determination problem, you will use ¹³C NMR alongside ¹H NMR and IR to identify unknowns. The number of peaks in a ¹³C spectrum directly tells you the number of unique carbon environments, which is powerful for assessing molecular symmetry. If you are not yet comfortable reading ¹H NMR spectra, work through those notes first.
¹³C NMR chemical shifts span about 0 – 220 ppm. Saturated sp³ carbons appear upfield (0 – 50 ppm), carbons bonded to electronegative atoms shift to 50 – 90 ppm, sp² carbons (alkenes, aromatics) fall in 100 – 150 ppm, and carbonyl carbons appear furthest downfield (150 – 220 ppm). The number of distinct peaks equals the number of chemically non-equivalent carbon environments in the molecule.
¹³C NMR (carbon-13 nuclear magnetic resonance)
An NMR technique that detects the ¹³C isotope (natural abundance ~1.1%) to reveal the carbon environments in a molecule. In simple terms, it is the carbon counterpart of proton NMR: instead of seeing hydrogen environments, you see carbon environments.
DEPT (distortionless enhancement by polarisation transfer)
An experiment that distinguishes CH₃, CH₂, CH, and quaternary C signals. DEPT-135 shows CH₃ and CH pointing up, CH₂ pointing down, and quaternary carbons absent. This is often provided alongside a standard ¹³C spectrum to help you assign peaks.
Broadband-decoupled ¹³C spectrum
The standard ¹³C spectrum in which all C-H coupling is removed, so each unique carbon environment appears as a single line. This simplifies the spectrum but means you lose splitting information (which DEPT recovers).
Chemical shift range (0 – 220 ppm)
¹³C chemical shifts span a much wider range than ¹H (which covers roughly 0 – 12 ppm). This wider spread means peaks are less likely to overlap, making it easier to count distinct carbon environments.
Equivalent carbons
Carbons that are in identical chemical environments (related by symmetry) give a single peak. For example, the two methyl carbons in acetone are equivalent, so acetone shows only two ¹³C peaks despite having three carbons.
Primary CH₃: δ ~8 – 30 ppm. Methyl groups on simple alkanes sit at the low end of this range.
Secondary CH₂: δ ~15 – 55 ppm (most commonly 20 – 40 ppm). Cyclopropane carbons appear unusually upfield (~0 – 5 ppm) owing to ring-strain effects.
Tertiary CH and quaternary C: δ ~20 – 50 ppm. Branching pushes chemical shifts slightly downfield within the sp³ range.
C-O (alcohols, ethers): δ ~50 – 90 ppm. The electronegative oxygen deshields the carbon substantially.
C-N (amines): δ ~30 – 65 ppm.
C-X (haloalkanes):
C-F: δ ~70 – 90 ppm (fluorine is the most electronegative halogen)
C-Cl: δ ~25 – 50 ppm
C-Br: δ ~25 – 35 ppm
C-I: δ ~0 – 35 ppm (iodine is large but not very electronegative, so the shift is modest)
Alkylthiols C-S: δ ~15 – 45 ppm.
Alkynes (C≡C): δ ~65 – 90 ppm. Despite the high s-character of sp hybridisation, alkyne carbons appear in a relatively narrow window.
Alkene carbons (C=C): δ ~100 – 150 ppm, most commonly 110 – 140 ppm.
Aromatic carbons: δ ~110 – 160 ppm. Substituted positions (bearing electronegative groups) can push beyond 150 ppm.
C≡N: δ ~110 – 125 ppm. This overlaps somewhat with the aromatic/alkene region, so context matters.
Carboxylic acids and derivatives (esters, amides, carboxyl C=O): δ ~160 – 180 ppm. The resonance donation from O or N moderates the shift.
Aldehydes (R-CHO): δ ~190 – 205 ppm.
Ketones (R₂C=O): δ ~195 – 220 ppm. These are the most downfield common carbon signals.
"The number of peaks equals the number of carbon atoms." Only if every carbon is in a unique chemical environment. Symmetry reduces the peak count. Benzene has six carbons but shows one ¹³C peak because all six are equivalent.
"¹³C NMR peak heights are proportional to the number of carbons." Unlike ¹H NMR, ¹³C peak heights are not reliably proportional to the number of carbons producing them. Quaternary carbons, in particular, tend to give weaker signals because they lack directly attached hydrogens and relax more slowly. Do not use ¹³C peak height the way you use ¹H integration.
"Alkyne carbons should appear downfield of alkene carbons because sp is more s-character than sp²." Alkyne carbons (65 – 90 ppm) are actually upfield of alkene carbons (100 – 150 ppm). The anisotropic shielding from the cylindrical pi cloud of the triple bond offsets the electronegativity of sp hybridisation, just as it does for the alkyne proton in ¹H NMR.
"A peak at 170 ppm must be a carboxylic acid." Esters and amides also appear in the 160 – 180 ppm range. You need IR (to check for broad O-H or N-H) and ¹H NMR (to check for a -COOH signal near 11 ppm) to distinguish among them.
⚠️ Counting ¹³C peaks to determine symmetry is a very common exam question. If a compound with molecular formula C₄H₈O₂ shows only two ¹³C peaks, it must be highly symmetric.
⚠️ Be able to distinguish a ketone carbon (δ 195 – 220) from a carboxylic acid/ester/amide carbon (δ 160 – 180). The difference is large enough to be diagnostic.
⚠️ Know that DEPT-135 separates CH₃ (up), CH₂ (down), CH (up), and C (absent). Exams frequently provide a DEPT alongside a standard ¹³C spectrum.
⚠️ The absence of peaks in the 100 – 160 ppm region rules out alkenes, aromatics, and most unsaturated functional groups. This negative evidence is as useful as a positive peak.
True or false: A ¹³C NMR peak at δ 205 ppm is consistent with a ketone carbonyl.
True. Ketone C=O carbons appear at δ 195 – 220 ppm.
Fill in the blank: The ¹³C chemical shift range for sp² carbons (alkenes and aromatics) is approximately ______ to ______ ppm.
100 to 150.
True or false: Benzene shows six ¹³C peaks because it has six carbon atoms.
False. All six carbons are equivalent, so benzene shows one ¹³C peak.
Fill in the blank: In a DEPT-135 experiment, CH₂ groups point ______ and CH₃ groups point ______.
Down; up.
True or false: Peak heights in a ¹³C NMR spectrum can be used to count the number of each type of carbon, the same way integration works in ¹H NMR.
False. ¹³C peak heights are not reliably proportional to the number of carbons. Quaternary carbons in particular give weak signals.
Q: A compound with molecular formula C₃H₆O shows two ¹³C peaks: one at δ 30 ppm and one at δ 206 ppm. What is the compound?
A: Acetone (propan-2-one). The peak at 206 ppm is a ketone carbonyl. The peak at 30 ppm corresponds to the two equivalent methyl groups. Two peaks for a three-carbon compound confirms high symmetry.
Q: How many ¹³C peaks would you expect for para-xylene (1,4-dimethylbenzene)?
A: Three. The two methyl carbons are equivalent (one peak), the two ring carbons bearing the methyl groups are equivalent (one peak), and the four remaining ring C-H carbons form two pairs of equivalent carbons (but by symmetry all four are equivalent), giving a total of three peaks.
Q: A ¹³C spectrum shows a peak at δ 170 ppm and no peak above 200 ppm. The IR spectrum shows a strong C=O near 1740 cm⁻¹ and no broad O-H. What functional group is present?
A: An ester. The ¹³C shift at 170 ppm falls in the carboxylic acid/ester/amide carbonyl range (160 – 180 ppm), and the IR at 1740 cm⁻¹ is characteristic of an ester. The absence of a broad O-H rules out a carboxylic acid.
Q: A compound shows five ¹³C peaks, including signals at δ 128 and δ 137 ppm. What structural feature do these peaks suggest?
A: An aromatic ring. Aromatic carbons appear in the 110 – 160 ppm range. The peak at 137 likely represents a substituted ring carbon, and 128 likely represents unsubstituted ring C-H carbons.
Q: In a DEPT-135 experiment, a peak at δ 45 ppm points downward. What does this tell you?
A: The carbon is a CH₂ (methylene). In DEPT-135, CH₂ groups point down, CH₃ and CH point up, and quaternary carbons are absent.
¹³C NMR pairs directly with ¹H NMR. A carbonyl carbon at δ 200 in the ¹³C spectrum paired with an aldehyde proton at δ 9.8 in the ¹H spectrum confirms an aldehyde. Without the ¹H data, you might confuse an aldehyde carbon with a ketone carbon, since both fall in the 190 – 220 range.
DEPT experiments bridge the two: DEPT tells you how many hydrogens are on each carbon, which is the same information the ¹H NMR integration and splitting give from the hydrogen side.
IR spectroscopy confirms functional group identity where ¹³C and ¹H data overlap. An ester and a carboxylic acid both show a carbonyl carbon near 170 ppm, but IR distinguishes them by the presence or absence of a broad O-H.
¹³C NMR, carbon-13 NMR, carbon NMR chemical shift table, DEPT, DEPT-135, broadband decoupled, carbonyl carbon shift, aromatic carbon shift, sp3 carbon shift, organic chemistry II spectroscopy, structure determination, NMR correlation table, equivalent carbons, molecular symmetry NMR