NMR Spectroscopy: Carbon-13 NMR

Overview

• Focus on Carbon-13 NMR, useful for analyzing carbon atoms in an organic compound.
• Advantage: Fewer carbon atoms than hydrogen atoms, leading to simpler spectra.
• Each equivalent carbon atom produces one signal.

Chemical Shift Range

• Proton NMR: 0 to 13 ppm.
• Carbon NMR: 0 to 220 ppm.
• Broad range helps distinguish similar carbons, unlike proton NMR where signals can overlap.

Sensitivity Challenges

• Carbon-12 is the most abundant carbon isotope; Carbon-13 only makes up 1%.
• Carbon-13 NMR is less sensitive than proton NMR.
• Requires more sample and longer experiment time (hours or overnight).

Spectral Characteristics

• Low abundance of Carbon-13 leads to unique signals for non-equivalent carbons (singlets).
• Proton decoupled Carbon-13 NMR: No splitting by bonded protons.
• Proton coupled Carbon-13 NMR: Displays carbon-hydrogen splitting following the N+1 rule.

Functional Group Identification

• Benzene ring carbons: 100-170 ppm.
• Carbonyl carbons: 160-200 ppm.

Example Analysis: 2-butanol

• Not symmetrical, resulting in distinct carbon signals.
• Four signals expected: Two CH3, one CH2, and one CH bonded to an alcohol group.
• Proton decoupled spectrum: Displays four colored signals.
• Height of signals in Carbon-13 NMR is not indicative of number of carbons but related to relaxation time.

Interpretation of Spectra

• Proton coupled spectrum: Shows splitting patterns
  • CH3: Quartet (due to 3 hydrogens)
  • CH2: Triplet (due to 2 hydrogens)
  • CH: Doublet (due to 1 hydrogen)
• Solvent (deuterated chloroform - CdCl3) observable in spectra.

Application

• Carbon-13 NMR supports structural analysis developed from IR and proton NMR.
• Main disadvantage: Low sensitivity due to low natural abundance of Carbon-13.

Conclusion

• Serves as a confirmatory tool in conjunction with IR and proton NMR.
• May add additional resources if problems arise integrating various methods to solve structures.