8.15 : Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.

The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and ²J values are usually weaker than ¹J values. The energy of configurations with parallel spins is often lowered, resulting in negative ²J values, though the sign of J is not evident from the spectrum.

²J values become more positive as the s character on the involved orbitals increases, which is observed as a decrease in the magnitude of J in the spectrum. The angle formed by the involved atoms also influences the extent of geminal coupling. In cycloalkanes, as the H–C–H angle approaches 109°, the ²J values become more negative, which is observed as an increase in the extent of coupling. An electronegative substituent alpha to the coupled nuclei makes the ²J values more positive, as seen in fluoromethane and formaldehyde.