1 An interaction between two different parts of a system or between two or more systems. Examples of coupling in the spectra of atoms and nuclei are Russell-Saunders coupling, j-j coupling, and spin-orbit coupling. In the spectra of molecules there are five idealized ways (called the Hund coupling cases) in which the different types of angular momentum in a molecule (the electron orbital angular momentum L, the electron spin angular momentum S, and the angular momentum of nuclear rotation N) couple to form a resultant angular momentum J. (In practice, the coupling for many molecules is intermediate between Hund's cases due to interactions, which are ignored in the idealized cases.)
2 A type of chemical reaction in which two molecules join together; for example, the formation of an azo compound by coupling of a diazonium ion with a benzene ring.