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Primer on ¹³C NMR Spectroscopy

Primer on ¹³C NMR Spectroscopy

¹³C NMR (carbon-13 nuclear magnetic resonance) spectroscopy is a powerful technique for analyzing the carbon framework of organic molecules. Unlike ¹H NMR, which focuses on hydrogen atoms, ¹³C NMR provides information about the carbon atoms in a molecule. This primer covers the key aspects of ¹³C NMR and includes a table of typical chemical shifts by functional group.

Introduction to ¹³C NMR

Key aspects of ¹³C NMR include:

  • Natural Abundance: Only ~1.1% of carbon atoms are ¹³C (the NMR-active isotope), making ¹³C NMR less sensitive than ¹H NMR.
  • Chemical Shifts: ¹³C chemical shifts are reported in ppm (parts per million) and are typically spread over a wider range (0–220 ppm) compared to ¹H NMR (0–12 ppm).
  • No Splitting: ¹³C NMR spectra are usually acquired using proton decoupling, so carbon signals appear as singlets (no splitting due to adjacent protons).

Key Features of ¹³C NMR Spectra

1. Chemical Shifts

  • ¹³C chemical shifts are highly dependent on the electronic environment of the carbon atom.
  • Electronegative atoms (e.g., oxygen, nitrogen) deshield carbons, causing downfield shifts (higher ppm).
  • sp²-hybridized carbons (e.g., alkenes, aromatics, carbonyls) resonate further downfield than sp³-hybridized carbons (e.g., alkanes).

2. Number of Signals

  • Each unique carbon environment in a molecule produces a distinct signal.
  • Symmetric molecules have fewer signals due to equivalent carbons.

3. Signal Intensity

  • Signal intensity in ¹³C NMR is not directly proportional to the number of carbons (unlike ¹H NMR).
  • Quaternary carbons (no attached hydrogens) often have weaker signals compared to carbons with attached hydrogens.

Table of Typical ¹³C Chemical Shifts by Functional Group

Below is a table of common ¹³C chemical shifts for various functional groups:

Functional Group Chemical Shift Range (ppm) Example
Alkanes 0–50 -CH₃ (methyl): ~10–30 ppm
Alkenes 100–150 C=C (alkene): ~110–150 ppm
Aromatics 120–160 Benzene ring: ~125–140 ppm
Carbonyls    
- Aldehydes 190–200 R-CHO: ~190–200 ppm
- Ketones 190–220 R₂C=O: ~190–220 ppm
- Carboxylic Acids/Esters 160–185 R-COOH/R-COOR: ~160–185 ppm
- Amides 165–180 R-CONH₂: ~165–180 ppm
Alcohols/Ethers 50–90 R-OH/R-O-R: ~50–90 ppm
Amines 30–65 R-NH₂: ~30–65 ppm
Nitriles 110–130 R-C≡N: ~110–130 ppm
Halides 10–80 R-X (X = Cl, Br, I): ~10–80 ppm

How to Interpret a ¹³C NMR Spectrum

  1. Identify the Number of Signals:
    • Count the number of unique carbon environments in the molecule.
    • Symmetric molecules will have fewer signals.
  2. Analyze Chemical Shifts:
    • Use the table above to assign signals to specific functional groups.
    • Look for characteristic regions (e.g., carbonyls at 160–220 ppm, aromatics at 120–160 ppm).
  3. Consider Symmetry and Intensity:
    • Symmetric molecules will have fewer signals due to equivalent carbons.
    • Quaternary carbons (no attached hydrogens) often appear as weaker signals.

Example: Interpreting a ¹³C NMR Spectrum

For a molecule like acetone (CH₃-CO-CH₃):

  • Number of Signals: 2 (one for the carbonyl carbon, one for the equivalent methyl carbons).
  • Chemical Shifts:
    • Carbonyl carbon (C=O): ~200 ppm.
    • Methyl carbons (CH₃): ~30 ppm.