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.
