Core Concept
Key Features of ¹³C NMR
- Number of peaks = Number of chemically non-equivalent carbon environments.
- No spin-spin splitting: All peaks in routine broadband-decoupled ¹³C NMR appear as singlets.
- Chemical shift (\(\delta\)): Indicates hybridization and electronegative atom attachments (e.g. \(C=O\) carbonyl carbon at \(160\text{--}220\text{ ppm}\)).
¹³C Chemical Shift Ranges
| Environment | δ / ppm |
|---|---|
| C-C (alkyl) | 5-40 |
| C-O (alcohol/ether) | 50-90 |
| C=C (alkene/aromatic) | 110-160 |
| C=O (carbonyl) | 160-220 |
Worked Example
Counting Carbon Environments in Propanone
Propanone (CH₃COCH₃) has a plane of symmetry through the carbonyl group:
- The two methyl carbons (
-CH₃) are chemically equivalent → 1 peak (~30 ppm) - The carbonyl carbon (
C=O) is unique → 1 peak (~206 ppm)
Total: 2 peaks on the ¹³C NMR spectrum.
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