13C NMR Spectroscopy

Fundamental Principles

  • 13C^{13}C is a spin 12\tfrac{1}{2} nucleus; NMR‐active but only ≈1.1%\approx1.1\% natural abundance (vs. 12C^{12}C ≈98.9%\approx98.9\%, NMR-silent).
  • Low abundance ⇒ weak signals; spectra acquired via Fourier Transform NMR (FT-NMR) using many scans.
  • 13C–13C^{13}C–^{13}C coupling rare (most neighbours are 12C^{12}C), so carbon–carbon splittings usually absent.

Proton Coupling & Splitting

  • Each 13C^{13}C bonded to nn protons splits into (n+1)(n+1) lines.
  • Excessive splitting from adjacent protons complicates spectra; controlled by decoupling techniques.

Decoupling Modes

  • Off-resonance decoupling:
    • Irradiate near 1H^{1}H frequency.
    • Removes long-range 13C–1H^{13}C–^{1}H couplings; retains direct 13C–1H^{13}C–^{1}H splittings.
  • Broadband (full) decoupling:
    • Irradiate at all 1H^{1}H frequencies.
    • Eliminates all 13C–1H^{13}C–^{1}H couplings ⇒ each carbon appears as a singlet located at midpoint of original multiplet.

Chemical Shifts

  • Same ppm scale as 1H^{1}H, but spread ≈ 00 to 250 ppm250\,\text{ppm} (vs. ≈10 ppm\approx10\,\text{ppm} for 1H^{1}H).
  • TMS set to 0 ppm0\,\text{ppm}.

DEPT (Distortionless Enhancement by Polarisation Transfer)

  • Provides 13C^{13}C signals with 1H^{1}H multiplicity information without overcrowded spectra.
  • In decoupled DEPT:
    • CH, CH<em>3\text{CH}<em>3 ⇒ upright peaks. • CH</em>2\text{CH}</em>2 ⇒ inverted peaks.
    • Quaternary C (no hydrogens) ⇒ absent.

Data Assignment Strategy

  • Count number of unique 13C^{13}C signals ⇒ number of distinct carbon environments.
  • Use chemical shift tables to deduce functional groups.
  • Combine with DEPT (or off-resonance) to assign number of attached hydrogens.
  • Peak integrals unreliable (relaxation varies; quaternary & C=OC=O carbons often weaker).

Practical Considerations

  • Solvents: CDCl<em>3<em>3 common; appears as 1:1:1 triplet at 78–79 ppm78\text{–}79\,\text{ppm}. D</em>2</em>2O for water-soluble samples.
  • FT-NMR essential due to low signal intensity and long T1T_1 relaxation times.
  • Reference libraries facilitate spectral matching of unknown compounds.