= Solution
The relevant quantity is the H–C–C–H <dihedral angle>, through the <Karplus relation>, rather than the labels cis and trans alone. Couplings tend to be larger near eclipsed or antiperiplanar arrangements and smaller nearer a right angle. The observed value averages the accessible conformations.
In A, the saturated five-membered ring is puckered. A preferred envelope-like <conformation> can place the trans <hydrogens> closer to an antiperiplanar arrangement, giving a slightly larger coupling than the cis pair. In B, the double bond restricts ring geometry toward a flatter arrangement: the cis <hydrogens> are more nearly eclipsed, whereas the trans pair has an oblique <dihedral angle>. The cis coupling is then larger. Approximate Newman projections illustrate these trends; exact angles cannot be recovered from four coupling constants without a calibrated relation and conformational populations.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2025/paper-1-karplus-projections.png]
{title=Illustrative Newman projections explaining why cis/trans coupling order depends on five-membered ring geometry}
Back to article page
