Carbon-13 NMR Spectroscopy

Carbon-13 (¹³C) NMR spectroscopy provides information about the number and types of carbon environments in a molecule. While ¹H NMR tells us about hydrogen environments, ¹³C NMR is complementary, revealing the carbon skeleton of the molecule. This lesson covers the principles of ¹³C NMR, chemical shift ranges, and how to interpret ¹³C NMR spectra.

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Principles of ¹³C NMR

Why Carbon-13?

The most abundant carbon isotope, ¹²C (98.9% natural abundance), has no nuclear spin and is NMR-inactive. The ¹³C isotope (1.1% natural abundance) does have a nuclear spin of ½, making it NMR-active.

Because of its low abundance, ¹³C NMR is much less sensitive than ¹H NMR. This means:

• More scans (signal averaging) are needed, so acquisition takes longer.
• Larger samples may be required.
• Peak heights in ¹³C NMR are NOT proportional to the number of carbon atoms (unlike ¹H NMR where integration is proportional to the number of H atoms). You cannot reliably integrate ¹³C spectra.

Exam Tip: In ¹³C NMR, you can count the number of different carbon environments (number of peaks), but you CANNOT determine how many carbons are in each environment from peak heights. This is a key difference from ¹H NMR.

No Splitting in Routine ¹³C NMR

In the most common type of ¹³C NMR spectrum (called broadband proton-decoupled ¹³C NMR), all peaks appear as singlets. This is because:

• The probability of two ¹³C atoms being adjacent is very low (1.1% × 1.1% ≈ 0.01%), so ¹³C–¹³C coupling is negligible.
• Coupling between ¹³C and attached ¹H is deliberately removed by proton decoupling (irradiating all ¹H frequencies simultaneously), which simplifies the spectrum.

The result is a clean spectrum with one sharp peak for each unique carbon environment.

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¹³C Chemical Shift Ranges

The chemical shift range for ¹³C is much wider than for ¹H (0–220 ppm vs 0–12 ppm), which means peaks are generally more spread out and easier to distinguish.

Carbon Environment	Chemical Shift δ (ppm)	Example
R–CH₃ (alkyl, primary)	5–40	CH₃ in ethane
R–CH₂–R (alkyl, secondary)	15–55	CH₂ in propane
R₃CH (alkyl, tertiary)	25–60	CH in 2-methylpropane
C–Cl	25–50	Chloroalkanes
C–N	25–60	Amines
C–O (alcohols, ethers)	50–90	C in methanol (δ ≈ 50)
C=C (alkenes)	100–150	C in ethene
Aromatic C	110–160	C in benzene (δ ≈ 128)
C≡N (nitriles)	110–125	C in ethanenitrile
C=O (aldehydes, ketones)	190–220	C=O in propanone (δ ≈ 206)
C=O (carboxylic acids, esters)	160–185	C=O in ethanoic acid (δ ≈ 178)

Exam Tip: Note that carbonyl carbons in aldehydes and ketones appear at higher δ (190–220 ppm) than those in carboxylic acids and esters (160–185 ppm). This is because the lone pair on oxygen in –COOH and –COOR partially donates electron density into the C=O, increasing shielding.

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Determining the Number of Carbon Environments

Just as with ¹H NMR, symmetry is key:

Worked Example 1: Propanone (CH₃COCH₃)

Propanone has 2 carbon environments:

1. The two CH₃ carbons are equivalent (related by a mirror plane through the C=O) → one peak at ~δ 30 ppm.
2. The C=O carbon → one peak at ~δ 206 ppm.

The ¹³C NMR spectrum of propanone shows 2 peaks.

Worked Example 2: Ethanol (CH₃CH₂OH)

Ethanol has 2 carbon environments:

1. CH₃ → δ ≈ 18 ppm.
2. CH₂ (attached to O, so deshielded) → δ ≈ 58 ppm.

Worked Example 3: Ethanoic Acid (CH₃COOH)

Ethanoic acid has 2 carbon environments:

1. CH₃ → δ ≈ 20 ppm.
2. C=O (in COOH) → δ ≈ 178 ppm.

Worked Example 4: Benzene (C₆H₆)

All 6 carbons are equivalent → 1 peak at δ ≈ 128 ppm.

Worked Example 5: 1-Bromobutane (CH₃CH₂CH₂CH₂Br)

There are 4 carbon environments (no symmetry along the chain):

1. CH₃ → δ ≈ 13 ppm.
2. CH₂ (next to CH₃) → δ ≈ 20 ppm.
3. CH₂ (next to CH₂Br) → δ ≈ 34 ppm.
4. CH₂Br → δ ≈ 34 ppm.

(The exact shifts may vary, and the two CH₂ groups at C2 and C3 may have very similar shifts.)

Worked Example 6: Methylbenzene (C₆H₅CH₃)

Methylbenzene has 5 carbon environments (not 7):

1. CH₃ (δ ≈ 21 ppm).
2. C-1 (attached to CH₃, ipso carbon, δ ≈ 138 ppm).
3. C-2 and C-6 (equivalent, ortho positions, δ ≈ 125 ppm).
4. C-3 and C-5 (equivalent, meta positions, δ ≈ 128 ppm).
5. C-4 (para position, δ ≈ 126 ppm).

So the ¹³C NMR shows 5 peaks.

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DEPT Experiments

DEPT (Distortionless Enhancement by Polarisation Transfer) is a technique that helps distinguish between CH₃, CH₂, CH, and quaternary C: