NMR spectrum
Summary
An NMR spectrum plots radio-frequency absorption against chemical shift (δ, ppm) for nuclei such as ¹H or ¹³C in a magnetic field. Each set of equivalent nuclei gives one signal, and its position, area and splitting reveal structure; n equivalent neighbor hydrogens split a ¹H signal into n + 1 peaks.
Detail
Meaning: Nuclei with spin, usually ¹H or ¹³C, absorb radio waves in a strong magnetic field. Each set of chemically equivalent nuclei gives one signal. Four features carry the information: number of signals (distinct environments), chemical shift (electronic surroundings), integration (relative number of H) and splitting (neighboring H). Electronegative atoms and carbonyls deshield nearby nuclei and move signals downfield (larger δ). TMS is the 0 ppm reference.
Equation: δ = (νsample − νref)/ν0 × 10⁶, where νsample and νref are resonance frequencies in Hz, ν0 is the spectrometer operating frequency in Hz, and δ is in ppm (dimensionless). Splitting: peaks = n + 1, where n is the number of equivalent H on adjacent atoms.
On the MCAT: A passage gives a molecular formula and a spectrum, and you pick the structure. Typical ¹H shifts: alkyl about 1, alkene about 5, aromatic about 7, aldehyde about 9–10, carboxylic acid about 10–12 ppm. A triplet plus a quartet with a 3:2 area ratio suggests an ethyl group. In ¹³C spectra, each unique carbon gives one signal, and carbonyl carbons appear far downfield.
Common confusion: A larger δ means more deshielded and further downfield (left), not more shielded. Hydrogens split their neighbors' signals, not their own equivalent partners.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
- Organic Chemistry (Clayden)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.