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Spectroscopy and Structure Determination · Reading each technique

Carbon-13 NMR, DEPT and 2D NMR

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What you'll learn

  • Explain why a routine ¹³C spectrum shows one singlet per carbon environment, why its peak heights are not integrals, and how solvent carbons split by deuterium.
  • Assign ¹³C chemical shifts by carbon type and explain conjugation, resonance-donation and heavy-atom effects on them.
  • Count carbon environments from molecular symmetry, including diastereotopic groups.
  • Classify carbons as C, CH, CH₂ or CH₃ from broadband, DEPT-135 and DEPT-90 spectra and use the result to choose between isomers.
  • Read COSY, HSQC, HMBC and NOESY cross peaks and choose the experiment that answers a given structural question.

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The ¹³C experiment

Only ¹³C (spin ½, natural abundance about 1.1%) is NMR-active; ¹²C has no spin. Carbon resonates at about a quarter of the proton frequency, so a 400 MHz spectrometer observes ¹³C near 100 MHz, and shifts span about 0–220 ppm from TMS. Because so few molecules carry two ¹³C atoms next to each other (0.011² ≈ 1 in 8000 pairs), ¹³C–¹³C splitting is not seen. One-bond ¹³C–¹H couplings are large, ¹J(CH) ≈ 500 × (fraction of s character) Hz: about 125 Hz at sp³, 160 Hz at sp² and 250 Hz at sp carbon. A routine spectrum is recorded with broadband proton decoupling, which removes these couplings, so every carbon environment appears as a single line. In a proton-coupled spectrum each carbon is split only by its own hydrogens (n + 1 lines): CH₃ a quartet, CH₂ a triplet, CH a doublet, C a singlet. Deuterium (spin 1) is not decoupled, and a carbon bearing n deuterons shows 2n + 1 lines: CDCl₃ gives a 1:1:1 triplet at δ 77.16, and the CD₃ carbons of DMSO-d₆ a septet at δ 39.5. Decoupling also transfers intensity to carbons that carry hydrogens (the nuclear Overhauser effect, up to roughly threefold), and carbons without hydrogens relax slowly, so quaternary and carbonyl carbons give small peaks and routine ¹³C peak areas are not proportional to the number of carbons.

Chemical shift regions

Approximate ranges: alkyl C 0–50 (ethane ≈ 6); C–N 30–65; C–O 50–90 (methanol ≈ 50, OCH₃ of an ester ≈ 52, OCH₂ of an ethyl ester ≈ 60); alkyne C 65–90; nitrile C 115–125; alkene and aromatic C 100–160 (benzene 128.4); acid, ester and amide C=O 165–185 (ethyl acetate 171, acetic acid ≈ 178); aldehyde C=O 190–205; ketone C=O 205–220 (acetone 207), lowered to about 196–200 by conjugation with C=C or an aryl ring (acetophenone 198, cyclohex-2-en-1-one 200). Electronegative substituents deshield the attached carbon and their effects add: CH₄ ≈ −2, CH₃Cl ≈ 25, CH₂Cl₂ ≈ 54, CHCl₃ 77, CCl₄ ≈ 96. Iodine is the exception: its heavy-atom (spin–orbit) effect shields the attached carbon, so CH₃I appears near −21 ppm. A lone pair on O or N next to a C=O donates into it and lowers the positive character of the carbonyl carbon, which is why esters, acids and amides sit 30–40 ppm below ketones. Resonance moves charge in conjugated systems too: in cyclohex-2-en-1-one the β-carbon (δ ≈ 151) is deshielded relative to the α-carbon (δ ≈ 130), and in anisole the electron-donating OCH₃ shields the ortho (δ 114) and para (δ 121) carbons relative to benzene, while the oxygen-bearing ipso carbon is at δ 160.

Counting carbon environments

Carbons related by a symmetry operation of the molecule (or by fast rotation) give one signal. Pentan-3-one has five carbons but three signals; p-xylene has eight carbons but three signals; toluene shows five (CH₃, ipso, ortho, meta, para). The three dichlorobenzenes give 3 (1,2-), 4 (1,3-) and 2 (1,4-) signals. A stereocentre removes symmetry nearby: the two methyl carbons of the isopropyl group in 4-methylpentan-2-ol are diastereotopic and give two signals. Enantiomers are not distinguished in an achiral solvent, so a racemate and a single enantiomer give identical spectra.

DEPT: how many hydrogens on each carbon

DEPT transfers polarisation from protons to the carbons they are directly bonded to, so carbons without hydrogens (quaternary carbons, C=O of ketones and esters, C–OH of a tertiary alcohol, nitrile carbons) disappear; so does the CDCl₃ solvent peak. In DEPT-135, CH and CH₃ peaks point up and CH₂ peaks point down, whether the CH₂ is sp³ or a terminal =CH₂. DEPT-90 shows only CH carbons. Comparing the broadband spectrum (all carbons), DEPT-135 and DEPT-90 sorts every signal: present only in broadband → C; up in DEPT-90 → CH; down in DEPT-135 → CH₂; up in DEPT-135 but absent in DEPT-90 → CH₃.

Two-dimensional correlations

Each cross peak in a 2D spectrum connects two nuclei. COSY (¹H–¹H) links protons coupled through two or three bonds, so it traces chains of CH groups and stops at a heteroatom or a carbon with no hydrogens. HSQC (¹H–¹³C) links each carbon to the hydrogens directly bonded to it; quaternary carbons give no peak, and a CH₂ carbon whose two hydrogens are diastereotopic shows two cross peaks at one δC. In multiplicity-edited HSQC the CH₂ cross peaks have the opposite sign to CH and CH₃, as in DEPT-135. HMBC (¹H–¹³C) shows couplings over two and three bonds (occasionally four) while one-bond peaks are suppressed; it is the experiment that reaches carbonyl and quaternary carbons and links fragments across O, N or C=O, for example the OCH₂ protons of an ethyl ester to the ester carbonyl carbon. NOESY (¹H–¹H) shows protons close in space, within about 5 Å, regardless of bonds, so it settles alkene geometry and relative configuration when coupling constants cannot.

What to carry forward

Read a ¹³C spectrum in four passes: count the signals against the symmetry of each candidate, place each signal in its shift region, sort carbons into C, CH, CH₂ and CH₃ with DEPT, then assemble the fragments with COSY (H–C–C–H chains), HSQC (which H is on which C), HMBC (two- and three-bond links to quaternary and carbonyl carbons) and NOESY (what is close in space).

Practice problems (40)

Free to practice with an account.

Predict the product 2

The ¹³C experiment

  • Multiplicity in a proton-coupled ¹³C spectrumPredict the one-bond multiplicity of a carbon in a proton-coupled ¹³C spectrum from the number of hydrogens attached to it.

Reading carbon chemical shifts

  • The most shielded ring carbon of anisolePredict which ring carbon of an aryl ether is most shielded from the resonance donation of the OCH₃ group.

Classify 4

DEPT and carbon multiplicity

  • A peak pointing down in DEPT-135Classify a carbon from the sign of its DEPT-135 peak.

Symmetry and signal counting

  • Racemic versus single-enantiomer butan-2-olClassify how the ¹³C spectra of a single enantiomer and its racemate compare in an achiral solvent.

Two-dimensional correlations

  • Which 2D experiment pairs each H with its carbon?Identify the 2D experiment that correlates each carbon with the hydrogens directly bonded to it.
  • What a NOESY cross peak meansClassify a NOESY cross peak as evidence of spatial proximity rather than bonding.

Compare 3

Reading carbon chemical shifts

  • Rank the chloromethanes by ¹³C shiftPredict how successive electronegative substituents on one carbon change its ¹³C shift.
  • Rank carbons by chemical shiftOrder carbons of different types by ¹³C chemical shift using the standard shift regions.

Symmetry and signal counting

  • Rank by number of ¹³C signalsRank compounds by their number of distinct carbon environments using symmetry.

Calculate 8

DEPT and carbon multiplicity

  • Negative DEPT-135 peaks of limonenePredict the number of negative DEPT-135 peaks of a terpene, counting sp² as well as sp³ CH₂ carbons.

The ¹³C experiment

  • How many ethanol molecules carry a ¹³C?Use the 1.1% natural abundance of ¹³C to find the fraction of molecules that contain at least one ¹³C atom.
  • Lines in the DMSO-d₆ solvent signalApply the 2nI + 1 rule to predict the multiplicity of a carbon bonded to several deuterium atoms.
  • Convert a ¹³C frequency offset to ppmConvert a ¹³C frequency offset in Hz to a chemical shift using the carbon observe frequency.
  • Estimate ¹J(CH) for an alkyne C–HEstimate a one-bond C–H coupling constant from the s character of the carbon hybrid orbital.

Symmetry and signal counting

  • Count the ¹³C signals of 4-methylpentan-2-olCount carbon environments when a stereocentre makes the two methyls of an isopropyl group diastereotopic.
  • Count the ¹³C signals of methyl 4-methylbenzoateCount carbon environments in a para-disubstituted benzene with two different substituents.
  • Count the ¹³C signals of 2-methylbutaneCount the carbon environments of a branched alkane by recognising equivalent methyl groups.

Find the error 2

DEPT and carbon multiplicity

  • Find the error in a DEPT assignmentFind the error when a positive DEPT-135 peak that is absent from DEPT-90 is assigned as CH.

Symmetry and signal counting

  • Find the error in a signal countFind the error in a ¹³C signal prediction that ignores molecular symmetry.

Interpret the evidence 10

DEPT and carbon multiplicity

  • Which C₄H₁₀O isomer is it?Assign a C₄H₁₀O isomer by reading the multiplicity of each carbon from DEPT-135 and DEPT-90.
  • Which C₅H₁₀O carbonyl compound is it?Combine broadband, DEPT-135 and DEPT-90 data to choose between C₅H₁₀O carbonyl isomers.

Reading carbon chemical shifts

  • Which C₄H₈O isomer is it?Identify a C₄H₈O carbonyl isomer from its number of ¹³C signals and their shift regions.
  • Which C₄H₈O₂ isomer is it?Use the carbonyl shift and the shift of the sp³ carbon bonded to oxygen to tell C₄H₈O₂ esters and acids apart.
  • Which C₆H₁₀ isomer is it?Use alkyne-region shifts together with the number of signals to identify a C₆H₁₀ isomer.

Symmetry and signal counting

  • Which dichlorobenzene gives four signals?Assign a disubstituted benzene isomer from the number of aromatic ¹³C signals.

Two-dimensional correlations

  • Read the COSY of ethyl propanoatePredict which proton pairs give COSY cross peaks in an ester, where the C=O and O break the coupling network.
  • Read a multiplicity-edited HSQCUse the sign of a cross peak in a multiplicity-edited HSQC to tell CH₂ from CH and CH₃ carbons.
  • Join the two halves of an ester with HMBCIdentify the HMBC cross peak that links an alkoxy group to an ester carbonyl across the ester oxygen.
  • Assign alkene geometry by NOESYAssign the E or Z geometry of a trisubstituted alkene from a NOESY cross peak when no vicinal alkene H–H coupling exists.

Work backward 1

  • Which reagents made this alcohol?Work back from DEPT data on a product alcohol to the regiochemistry of the alkene hydration used to make it.

Plan a synthesis 2

DEPT and carbon multiplicity

  • Assigning C, CH, CH₂ and CH₃Choose the set of one-dimensional carbon spectra that classifies every carbon by its number of hydrogens.

Two-dimensional correlations

  • Which methoxy singlet is the ester?Choose the 2D experiment that assigns two methoxy singlets by their three-bond connections to carbons without hydrogens.

Explain why 8

DEPT and carbon multiplicity

  • The missing C–OH peak of tert-butanolExplain why a carbon bearing an OH but no C–H disappears from DEPT spectra.

The ¹³C experiment

  • The three-line peak at δ 77Explain the 1:1:1 triplet of CDCl₃ from coupling to a spin-1 deuterium nucleus.
  • Why is every carbon a singlet?Explain why a routine ¹³C spectrum shows a single line for each carbon environment.
  • Why are two acetophenone peaks so small?Explain why carbons without attached hydrogens give weak peaks in a routine ¹³C spectrum.

Reading carbon chemical shifts

  • Why are ester carbonyls upfield of ketones?Explain the lower shift of ester and amide carbonyl carbons compared with ketones by lone-pair donation.
  • The two alkene carbons of cyclohexenoneExplain why the β-carbon of an α,β-unsaturated ketone is deshielded relative to the α-carbon.
  • Why is iodomethane below methane?Explain the shielding of a carbon bonded to iodine by the heavy-atom effect.

Two-dimensional correlations

  • One carbon, two HSQC cross peaksExplain two HSQC cross peaks at one carbon shift by diastereotopic protons on a CH₂ next to a stereocentre.

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