Abstract
Low temperature crystal structural studies of small cyclopropyl‐conjugated unsaturated hydrocarbons reveal significant differences in bond lengths between carbon atoms mainly within the three‐membered rings but also for adjacent CC single bonds. The strongest influence of conjugation was observed for spiro[2.4]hepta‐4,6‐diene (1), where the difference in the lengths of three‐membered ring bonds is 0.050 Å (X‐ray). The same effect is also obvious from ab initio calculations at HF/6‐31G(d) (to a minor extent) and MP2/6‐31G(d) levels, as shown by a difference analysis of bond lengths which was performed in order to cancel out systematic errors and differences for both methods. Static difference electron density maps were derived from the experimental data by multipole refinements. These maps showed shifts of electron density to exocyclic locations within the planes of the three‐membered rings and significant bond ellipticities for the adjacent single bonds. For 1,1‐diethenylcyclopropane (4) a nonsymmetric conformation was found in the crystal lattice, very close to the global minimum structure calculated by ab initio methods.