Pick a structure
Ring-fusion geometry
Click a row to select those atoms in the model. At a fusion carbon the bond to the other ring is either equatorial (trans fusion) or axial (cis fusion) with respect to the first ring.
Exercise: count the gauche interactions
Compare trans- and cis-decalin: the difference in gauche count × 0.9 kcal/mol is the textbook explanation of why trans-decalin is more stable (≈ 2.7 kcal/mol).
Why the lattice matters
Every six-membered ring that can be drawn on the diamond lattice is automatically a perfect chair, and any number of chairs can be fused as long as each fusion bond is a lattice bond. A five-membered ring can never be placed on the lattice, which is why the steroid D ring is missing here (see the ring-strain module).
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C H
Click atoms to measure.