NMR
Summary
NMR (nuclear magnetic resonance) spectroscopy uses a magnetic field and radio waves to probe the environments of nuclei such as ¹H and ¹³C. Most testable: chemical shift, integration and splitting reveal the type, number and neighbors of hydrogens; deshielded protons appear downfield (higher δ).
Detail
Meaning: NMR spectroscopy places a sample in a strong magnetic field and uses radio waves to flip the spins of nuclei such as ¹H and ¹³C. Each set of chemically equivalent nuclei absorbs at a slightly different frequency depending on its electron environment. Electronegative atoms, carbonyls and pi systems withdraw electron density, deshielding the nucleus and moving its signal downfield (higher δ in ppm, with TMS at 0). Rough ¹H shifts: alkyl 0.9–1.5, next to O or halogen 3–4, alkene 5–6, aromatic 6.5–8, aldehyde 9–10, carboxylic acid 10–12.
Equation: number of peaks = n + 1, where n is the number of equivalent hydrogens on adjacent carbons (a pure count, no unit).
On the MCAT: You are given a molecular formula and a spectrum or list of shifts, and must choose the structure. Shift gives the environment, integration gives the relative number of hydrogens, and splitting gives the neighbors. The number of signals equals the number of distinct hydrogen types.
Common confusion: Splitting comes from hydrogens on neighboring carbons, not from the hydrogens producing the signal, and equivalent hydrogens do not split each other. Integration gives a ratio, not an exact count. O–H and N–H signals are often broad singlets.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Spectrometric Identification of Organic Compounds (Silverstein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.