NMR Chemical Shift
Understanding ¹H and ¹³C NMR spectroscopy: chemical shifts, shielding, and structure determination
Chemical Shift Formula
δ = (νsample - νref) / νspectrometer × 10⁶
Where:
- δ = Chemical shift in ppm (parts per million)
- νsample = Frequency of sample signal (Hz)
- νref = Frequency of reference signal (Hz)
- νspectrometer = Operating frequency of NMR (Hz)
Standard References:
- • ¹H NMR: TMS (tetramethylsilane, (CH₃)₄Si) = 0.00 ppm
- • ¹³C NMR: TMS = 0.00 ppm
- • Other solvents: CDCl₃ (7.26 ppm for ¹H, 77.0 ppm for ¹³C)
¹H NMR Chemical Shift Ranges
Alkyl C-H
0.8-1.5 ppmRCH₃, R₂CH₂, R₃CH
Highly shielded, upfield
Allylic C-H
1.6-2.0 ppmC-CH₂-C=C
Related Calculators
Alpha to C=O
2.0-2.5 ppmO=C-CH₂-R, O=C-CH₃
Deshielded by carbonyl
Alkyl halide C-H
3.0-4.0 ppmR-CH₂-Cl, R-CH₂-Br
Halogen withdraws electrons
Ether/Alcohol C-H
3.3-4.0 ppmR-O-CH₂-R, R-CH₂-OH
Vinylic C-H
4.6-6.0 ppmR₂C=CH₂, RC=CH-R
Alkene protons
Aromatic C-H
6.5-8.5 ppmBenzene ring protons
Ring current effect, deshielded
Aldehyde C-H
9.0-10.0 ppmR-CHO
Very deshielded, downfield
Carboxylic acid O-H
10-13 ppmR-COOH
Extremely deshielded, broad peak
¹³C NMR Chemical Shift Ranges
Aliphatic C
CH₃: 10-30 ppm
CH₂: 20-40 ppm
CH: 30-50 ppm
Quaternary C: 30-40 ppm
C-O, C-N
C-O (ether/alcohol): 50-90 ppm
C-N (amine): 30-65 ppm
sp² Carbons
Alkene C=C: 100-150 ppm
Aromatic C: 110-160 ppm
Nitrile C≡N: 115-125 ppm
Carbonyl C=O
Ketone/Aldehyde: 190-220 ppm
Ester: 160-180 ppm
Carboxylic acid: 170-185 ppm
Amide: 160-180 ppm
Shielding vs Deshielding
Shielding (Upfield, Lower δ)
Cause: Electron density around nucleus increases
Effect: Nucleus experiences weaker applied field
Result: Signal appears at lower ppm (right side of spectrum)
Example: Si(CH₃)₄ (TMS) has 12 equivalent H's at 0 ppm - most shielded
Deshielding (Downfield, Higher δ)
Cause: Electron density around nucleus decreases
Effect: Nucleus experiences stronger applied field
Result: Signal appears at higher ppm (left side of spectrum)
Factors causing deshielding:
- • Electron-withdrawing groups (O, N, Cl, Br)
- • Aromatic ring current
- • C=O groups (carbonyl)
- • sp² hybridization (alkene, aromatic)
Worked Examples
Example 1: Ethyl Acetate (CH₃COOCH₂CH₃)
¹H NMR Predictions:
| CH₃CO- | ~2.0 ppm | Singlet (3H) | Alpha to C=O |
| -OCH₂- | ~4.1 ppm | Quartet (2H) | Adjacent to O |
| -CH₃ | ~1.2 ppm | Triplet (3H) | Alkyl CH₃ |
Example 2: Benzaldehyde (C₆H₅CHO)
¹H NMR Predictions:
| -CHO | ~9.9 ppm | Singlet (1H) | Aldehyde proton |
| Ar-H | ~7.5-8.0 ppm | Multiplet (5H) | Aromatic |
Example 3: 2-Propanol ((CH₃)₂CHOH)
¹H NMR Predictions:
| (CH₃)₂- | ~1.2 ppm | Doublet (6H) | Coupled to CH |
| -CH- | ~4.0 ppm | Septet (1H) | Adjacent to OH |
| -OH | ~2-3 ppm | Broad singlet (1H) | Exchangeable |
Common Mistakes
Confusing Upfield/Downfield
Downfield = higher δ (left), deshielded. Upfield = lower δ (right), shielded.
Ignoring Integration
Peak area tells you # of equivalent protons! Always check integration.
Forgetting OH/NH Exchange
O-H and N-H peaks are broad and variable (D₂O exchange test confirms).
Pro Tip: n+1 Rule
Splitting pattern = n+1, where n = # of adjacent equivalent H's.