13.12 DEPT 13C NMR Spectroscopy

DEPT 13C NMR spectroscopy is a powerful tool for analyzing carbon atoms in molecules. It differentiates carbons based on attached protons, helping identify CH, CH2, CH3, and quaternary carbons. This technique enhances our understanding of molecular structure.

Chemical shift analysis in 13C NMR provides crucial insights into carbon environments. Factors like hybridization, electronegativity, and conjugation affect shifts. Combining DEPT and chemical shift data allows for accurate structure determination, a key skill in organic chemistry.

DEPT 13C NMR Spectroscopy

Interpretation of DEPT NMR spectra

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• DEPT (Distortionless Enhancement by Polarization Transfer) differentiates between different types of carbon atoms in a molecule based on the number of attached protons
• DEPT-90 spectrum shows only CH carbons (methine) as positive peaks, does not show CH2, CH3, or quaternary carbons (carbon atoms with no attached protons)
• DEPT-135 spectrum shows CH and CH3 carbons as positive peaks, CH2 carbons as negative peaks, does not show quaternary carbons
• Quaternary carbons do not appear in either DEPT-90 or DEPT-135 spectra because they lack attached protons, making them distinguishable from other carbon types
• DEPT experiments utilize specific pulse sequences to manipulate nuclear spin states and enhance signal intensity

Analysis of 13C NMR chemical shifts

• 13C NMR chemical shift values are influenced by the electronic environment of the carbon atom, providing insights into the structural features of the molecule
• Factors affecting chemical shift include:
  1. Hybridization of the carbon atom
     • ###[sp](https://www.fiveableKeyTerm:sp)^3_0### carbons have lower chemical shifts (0-90 ppm) (alkanes, alcohols)
     • $[sp^2](https://www.fiveableKeyTerm:sp^2)$ carbons have higher chemical shifts (100-200 ppm) (alkenes, aromatics)
     • $sp$ carbons have the highest chemical shifts (70-100 ppm for alkynes, 100-150 ppm for nitriles)
  2. Electronegativity of neighboring atoms
     • Electronegative atoms (O, N, halogens) deshield the carbon, causing a higher chemical shift (alcohols, amines, alkyl halides)
  3. Conjugation and resonance effects
     • Conjugated systems and aromatic rings cause higher chemical shifts due to increased electron delocalization (benzene, conjugated dienes)
  4. Steric effects
     • Bulky substituents can shield the carbon, causing a lower chemical shift (tert-butyl groups)
• Chemical shift anisotropy contributes to the observed chemical shifts, especially in molecules with asymmetric electronic environments

Structure determination from NMR data

• Broadband decoupled 13C NMR spectrum shows all carbon atoms in the molecule as singlets, regardless of the number of attached protons, providing information about the total number of unique carbon atoms
• Combining information from broadband decoupled, DEPT-90, and DEPT-135 spectra allows for the determination of the number of CH, CH2, CH3, and quaternary carbons in the molecule
  1. Compare the number of peaks in each spectrum to determine the number of each carbon type
  2. Use chemical shift values to identify the structural environment of each carbon atom (hybridization, neighboring atoms, conjugation)
  3. Consider the molecular formula and degree of unsaturation to propose possible structures
  4. Eliminate structures that do not match the NMR data
• Example: For a compound with a molecular formula of $C_4H_8O$ and the following NMR data:
  • Broadband decoupled: 4 signals
  • DEPT-90: 1 signal
  • DEPT-135: 2 positive signals, 1 negative signal
  • The structure must contain 1 CH, 1 CH2, 1 CH3, and 1 quaternary carbon (possible structures: butanone, isobutyraldehyde)

Advanced NMR Concepts

• Magnetic resonance is the fundamental principle behind NMR spectroscopy, involving the interaction between nuclear spins and external magnetic fields
• Fourier transform techniques are used to convert time-domain NMR signals into frequency-domain spectra, enabling rapid data acquisition and improved sensitivity
• Relaxation times of nuclear spins influence signal intensity and resolution in NMR experiments, providing information about molecular dynamics and interactions