spin-spin coupling
/ spin-spin KUP-ling /
Spin-spin coupling is how neighboring nuclei in NMR feel one another and gossip about it. Each magnetic nucleus is like a tiny bar magnet that can point up or down; a nucleus senses whether its neighbor is pointing up or down, and that nudges its own resonance frequency slightly. The result is that a single NMR peak splits into a little cluster of peaks.
More precisely, spin-spin coupling is the interaction between the magnetic moments of nuclei transmitted through the bonding electrons between them. A nucleus with n equivalent neighbors splits into n + 1 lines, with characteristic intensity ratios, separated by a spacing called the coupling constant J that does not change with magnet strength.
It matters because the splitting pattern directly counts a nucleus's neighbors: a triplet says "two neighbors," a quartet says "three," letting a chemist trace how atoms are connected. The honest caveat is that coupling patterns can get tangled in complex molecules with many inequivalent neighbors, so real spectra are not always the tidy textbook triplets and quartets.
In ethanol, the CH₃ group sits next to a CH₂ with two hydrogens, so its peak splits into a triplet (2 + 1). The CH₂, next to the three CH₃ hydrogens, splits into a quartet (3 + 1).
The number of split lines counts a nucleus's neighboring hydrogens.
Coupling constant J is measured in hertz and stays the same on any magnet, whereas chemical shift is measured in parts per million. The two carry independent information: shift tells you the environment, coupling tells you the neighbors.