Spin–Spin Coupling: One-Bond Coupling

Coupling interactions are strongest between bonded NMR-active nuclei, where spin information can be transmitted directly through the pair of bonding electrons.

Because nuclei polarize their electrons to the opposite spins, configurations with antiparallel nuclear spins are expected to be lower in energy.

When coupling makes antiparallel states more favorable, J is considered positive.

So, the one-bond coupling constant, ¹J, is usually positive with a large magnitude.

Spin information is transmitted more effectively with increasing s character of the involved orbitals.

This explains the increasing ¹J_C–H values for ethane, ethene, and ethyne.

Similarly, the increase in s character of exocyclic bonds in strained cyclic systems is responsible for the large C–H coupling constants.

 ¹J_C–H values also increase when the coupled carbon has an electronegative substituent.

Despite the strength of one-bond coupling, carbon–hydrogen coupling is rarely observed in proton NMR spectra because of the low abundance of carbon-13.