Chemistry
The Principle of Spin-Spin Coupling in NMR
Quick fact
Spin-spin coupling is transmitted through the bonding electrons, not through space, so it can reveal which atoms are directly bonded or close in a molecular chain.
Why this is interesting
You've seen an NMR spectrum with a quartet instead of a single peak—why has one type of proton split into multiple signals? The answer lies in a quantum conversation between nearby nuclei.