8.16: Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)
Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The stereoelectronic interactions between the involved orbitals are maximized when the orbitals are synperiplanar at a dihedral angle of zero and minimized when the orbitals are orthogonal. Strong coupling is also seen at 180° when the back lobes of the sp3 orbitals overlap.
The Karplus equation predicts the approximate variation of 3JHH as a function of the dihedral angle.
This relationship is evident in the vicinal coupling constants of cyclohexane derivatives, where ring flipping is restricted by the presence of bulky substituents. In acyclic systems, however, single bonds rotate rapidly and the observed coupling constant is an average over all the conformations.
