The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved in homonuclear coupling, nuclei of different elements interact in heteronuclear coupling.
Consider non-equivalent protons A and X that have excitation energies of hνA and hνX. The term J = 0 is used to indicate that they do not interact via coupling. Coupling between A and X results in the modification of their nuclear spin energy levels and is expressed as J ≠ 0. For the coupled spins of A and X, the energy required to excite proton A to a spin state parallel to proton X is slightly different from the energy required for it to become antiparallel to spin X.
