splitting

Meaning: In ¹H NMR, the tiny magnetic field of a neighboring proton's spin adds to or subtracts from the field a proton feels. This spin-spin coupling splits one signal into several peaks. A proton with n equivalent neighboring protons, typically on the adjacent carbon, gives n + 1 peaks: singlet, doublet, triplet, quartet. Peak heights follow Pascal's triangle (1:1, 1:2:1, 1:3:3:1). Chemically equivalent protons do not split one another.

Equation: number of peaks = n + 1, where n is the number of equivalent neighboring protons (a dimensionless count). The spacing between peaks is the coupling constant J, measured in Hz (s⁻¹), and it is the same for the two coupled sets.

On the MCAT: You may be given a structure and asked to predict the pattern, or given a spectrum and asked to build the fragments. A classic case is an ethyl group CH₃CH₂– attached to an atom with no hydrogens: the CH₃ is a triplet (3H) and the CH₂ is a quartet (2H). Combine splitting with chemical shift (the environment) and integration (the relative number of H).

Common confusion: Students count the hydrogens on the same carbon, or all hydrogens in the molecule. Splitting depends on the neighbors, not on the proton being observed. Protons separated from the signal by an atom with no hydrogen, such as O, usually do not split it. O–H and N–H protons often appear as broad singlets because of rapid exchange, and they typically do not split neighbors.