Structures and Rotational Barriers of a Diiodobinorsnoutane: Energetic Preference for <i>Gauche</i> Conformation

The diiodobinorsnoutane, bi­(5-iodopentacyclo­[4.3.0.0^2,4.0^3,8.0^5,7]­non-4-yl) (<b>5</b>), exists in a sterically hindered <i>gauche</i> conformation rather than an <i>anti</i> or an averaged (freely rotating) <i>C</i>_2<i>v</i> structure. Density functional theory (DFT) predictions place the <i>gauche</i> conformation 11 kcal/mol more stable than the <i>anti</i> conformation with a barrier of 17 kcal/mol connecting the minima. These are consistent with variable-temperature NMR (17.1 ± 0.8 kcal/mol) estimates and X-ray analysis. Predictions of the torsional profiles of the yet-unsynthesized bromo-, chloro-, and fluoro- analogues show a progressive lowering of the barriers