spin-spin coupling

Meaning: Spin-spin coupling is the interaction between the nuclear spins of nonequivalent hydrogens on neighboring carbons, usually through three bonds (H–C–C–H). Each neighbor's spin is aligned with or against the applied field, slightly raising or lowering the field the observed proton feels. One signal therefore splits into a multiplet.

Equation: Number of peaks = n + 1, where n is the number of equivalent neighboring hydrogens (a unitless count). The spacing between peaks is the coupling constant J, in hertz (Hz = s⁻¹).

On the MCAT: You read a ¹H NMR spectrum, or predict one from a structure. Chemical shift shows the environment, integration shows relative hydrogen counts, and splitting shows the neighbors. An ethyl group gives a triplet (CH₃, 2 neighbors) and a quartet (CH₂, 3 neighbors). An isopropyl group gives a 6H doublet and a 1H septet. Peak heights follow Pascal's triangle (1:2:1, 1:3:3:1).

Common confusion: Students count a signal's own hydrogens instead of its neighbors. Hydrogens that are equivalent do not split each other, so an isolated CH₃ is a singlet. O–H and N–H protons often appear as singlets because of rapid exchange, so they usually neither split nor are split.