Nuclear Magnetic Resonance Spectroscopy
High-Yield Summary
- NMR — the most important spectroscopy technique for the MCAT — studies nuclei via nuclear spin (odd number of protons and/or neutrons) in a magnetic field. Proton NMR (¹H NMR) examines hydrogen environments to map connectivity.
- Surrounding electrons shield a nucleus via an induced magnetic field. In an external field, nuclei align with it (lower-energy alpha state) or against it (higher-energy beta state); radiofrequency radiation flips them between states — that absorbed energy is what NMR measures.
- Spectrum plots chemical shift (ppm) vs. energy absorption, calibrated to tetramethylsilane (TMS) at 0 ppm. More deshielded protons appear further downfield (higher ppm).
- Four things to read from any spectrum: (1) number of signals — unique proton environments, (2) location — chemical shift/shielding, (3) integration — relative proton ratio from peak area, (4) splitting — n+1 rule from neighboring protons.
- Splitting follows the n+1 rule: n equivalent neighboring protons produce n+1 peaks (e.g., 3 neighbors → quartet, 1 neighbor → doublet).
n+1 Splitting Rule
peaks = n + 1
- n = Number of equivalent protons on neighboring carbon(s)
- peaks = Number of peaks the signal splits into (multiplicity)
- 0 neighbors → singlet (1 peak). 1 neighbor → doublet (2). 2 neighbors → triplet (3). 3 neighbors → quartet (4).
Chemical Shift Landmarks (Upfield → Downfield)
| Proton type | Approx. shift |
|---|---|
| Methyl (R–CH₃) | ~0.9 ppm |
| Methylene (CH₂) / Methine (CH) | ~1.2 / ~1.7 ppm |
| Alkyl halide H (next to Cl/Br) | 2–4 ppm |
| Alcohol O–H | 2–5 ppm (varies with H-bonding) |
| Vinyl H (on C=C) | 4.5–6.5 ppm |
| Aromatic H (benzene ring) | 6.5–8 ppm |
| Aldehyde H (–CHO) | ~10 ppm |
| Carboxylic acid H (–COOH) | ~12 ppm (furthest downfield) |
Key Terms
- Nuclear spin
- Quantum mechanical property of any nucleus with an odd number of protons or neutrons (or both), giving it an internal magnetic moment.
- Shielding
- Reduction of the magnetic field a nucleus experiences due to surrounding electron density; more shielded = more upfield (lower ppm).
- TMS (tetramethylsilane)
- Calibration standard for chemical shift; very shielded, appears far upfield, defined as 0 ppm.
- Integration
- Area under an NMR signal; reveals the ratio of protons between signals, not the absolute count.
- Coupling
- Interaction between protons on neighboring carbons that splits a signal into multiple peaks (multiplicity).
Common MCAT Trap
- Integration gives a ratio, not an absolute proton count — a 1:3 integration ratio could mean 1 and 3 protons, or 2 and 6, etc.
- The n+1 rule counts neighboring protons on adjacent carbons, not protons on the same carbon as the signal itself — don't include the signal's own protons in n.
- More deshielded = further downfield = higher ppm, not lower — carboxylic acid protons (~12 ppm) are the most deshielded, appearing furthest from TMS (0 ppm), not closest.
Quick Recall
A proton is adjacent to 3 equivalent neighboring hydrogens. What splitting pattern appears, and how many peaks?
Why is TMS used as the 0 ppm reference?
What are the four features analyzed in an NMR spectrum?