Logic Puzzle #12: Torsion Angle and Coupling … Solution

Couplings can be affected by the torsion angle. The couplings can be expressed mathematically with a cos θ relationship. At certain values of θ, the couplings are expected to be relatively weaker to non-existent.

Based on the torsion angle, fragment B is expected to exhibit a prominent coupling between the red and gold nuclei. Fragment A, exhibiting a torsion angle of 90º, will generally lead to a weak or non-evident coupling on an NMR spectrum.

At the initial stages of an elucidaton of an unknown structure, the torsion angle(s) is usually not known. As such, an elucidator must be prepared to expect the unexpected coupling(s).