Structural Aspects of Cyclopropyl Conjugation: Experimental Studies and Ab Initio Calculations

Low-temperature crystal structural studies of a series of saturated and unsaturated bicyclo[2.2.1]heptadiene, heptene and heptane compounds with 7-spirocyclopropyl substitution reveal significant differences in the bond lengths of the three-membered rings and in the CC single bonds of the bicyclic fragment. A complex interplay of strain and different types of conjugation influence the molecular structure of the bicycloheptadiene derivative 1, where the difference in the length of the three-membered ring bonds is 0.040 A and all CC single bonds in the bicyclic fragment are lengthened significantly. Ab initio calculations at the HF/6-31G(d) (to a minor extent) and MP2/6-31G(d) levels are in good agreement with the experimental data. Calculated charge distributions and dipole moments further support the relevance of cyclopropyl homoconjugation in the investigated prototype of Walsh and through-space π-orbital interaction. Static difference electron density maps have been derived from the experimental data by multipole refinements which showed exocyclic shifts of electron density in the planes of the three-membered rings and significant bond ellipticities at the CC single bonds in the unsaturated bicyclic units.