13.12 DEPT 13C NMR Spectroscopy

DEPT 13C NMR Spectroscopy

Interpretation of DEPT NMR spectra

Standard (broadband decoupled) 13C NMR shows every unique carbon as a single peak, but it doesn't tell you how many hydrogens are attached to each carbon. DEPT solves this problem. DEPT (Distortionless Enhancement by Polarization Transfer) uses specific pulse angles to selectively reveal carbons based on their number of attached protons.

There are two DEPT experiments you need to know:

• DEPT-90 shows only CH carbons (methine) as positive peaks. Everything else disappears.
• DEPT-135 shows CH and CH₃ carbons as positive (up) peaks and CH₂ carbons as negative (down) peaks. Quaternary carbons still don't appear.

Quaternary carbons (C with no attached H) are absent from both DEPT-90 and DEPT-135 spectra. You identify them by finding peaks in the broadband decoupled spectrum that are missing from all DEPT spectra.

Analysis of 13C NMR chemical shifts

Chemical shift values in 13C NMR reflect the electronic environment around each carbon. Several factors determine where a carbon signal appears:

1. Hybridization

• $sp^3$ carbons typically resonate at 0–90 ppm (alkanes, alcohols, ethers)
• $sp^2$ carbons appear at 100–150 ppm for alkenes and aromatics, and 190–220 ppm for carbonyl carbons (aldehydes, ketones)
• $sp$ carbons fall around 65–90 ppm for alkynes and 115–120 ppm for nitriles

Note that $sp$ carbons in alkynes actually resonate at lower shifts than most $sp^2$ carbons. This can be counterintuitive, but it's due to the cylindrical electron density around the triple bond providing a shielding effect.

2. Electronegativity of neighboring atoms

Electronegative atoms (O, N, halogens) withdraw electron density from the carbon, deshielding it and pushing the signal to a higher chemical shift. For example, a carbon bonded directly to oxygen in an alcohol typically appears around 50–80 ppm, well above a simple alkyl carbon.

3. Conjugation and resonance effects

Aromatic rings and conjugated systems increase electron delocalization, which generally causes higher chemical shifts. Aromatic carbons typically appear in the 110–150 ppm range.

4. Steric effects

Bulky groups can slightly shield a carbon, nudging its chemical shift lower. This is a minor effect compared to the others but can explain small differences between similar carbons.

Structure determination from NMR data

Combining broadband decoupled 13C NMR with DEPT data is a systematic process. Here's how to work through a problem:

1. Count peaks in the broadband decoupled spectrum. This tells you the number of unique carbon environments.
2. Classify each carbon using DEPT data. Cross-reference the broadband spectrum with DEPT-90 and DEPT-135 to assign each peak as CH, CH₂, CH₃, or quaternary C.
3. Analyze chemical shifts. Use the shift values to determine hybridization and what atoms or functional groups are nearby.
4. Calculate the degree of unsaturation from the molecular formula. This tells you how many rings and/or pi bonds the molecule contains.
5. Propose candidate structures that match all the data, then eliminate any that conflict with the NMR evidence.

Worked example: A compound with molecular formula $C_4H_8O$ gives the following data:

• Broadband decoupled: 4 signals
• DEPT-90: 1 signal
• DEPT-135: 2 positive signals, 1 negative signal

The degree of unsaturation is 1 (one ring or one pi bond). From the DEPT data, the molecule contains 1 CH, 1 CH₃ (positive in DEPT-135 but absent in DEPT-90), 1 CH₂ (negative in DEPT-135), and 1 quaternary C (present only in broadband). A quaternary carbon with one degree of unsaturation and an oxygen points toward a carbonyl group. Butanone ($CH_3COCH_2CH_3$) fits: the carbonyl carbon is quaternary, and the remaining carbons match the CH₃, CH₂, and CH₃ pattern. (Note: butanone actually has two CH₃ groups, but two of them happen to be chemically inequivalent, producing 4 distinct signals in this case. Always verify your proposed structure accounts for every peak.)

Advanced NMR Concepts

• NMR spectroscopy relies on the interaction between nuclear spins (for nuclei with spin $I = \frac{1}{2}$, like $^1H$ and $^{13}C$) and an external magnetic field. Nuclei absorb radiofrequency energy and transition between spin states.
• Fourier transform (FT) NMR converts the raw time-domain signal (called a free induction decay, or FID) into the frequency-domain spectrum you actually read. FT methods allow all frequencies to be detected simultaneously, making data acquisition much faster than older continuous-wave techniques.
• Relaxation times ($T_1$ and $T_2$) describe how quickly nuclear spins return to equilibrium after excitation. These affect peak intensity and linewidth, and they're the reason quaternary carbons often appear as weaker peaks in 13C NMR: with no attached protons, they relax more slowly.