STRUCTURAL ASPECTS OF CYCLOPROPYL HOMOCONJUGATION - EXPERIMENTAL STUDIES AND AB-INITIO CALCULATIONS

Low-temperature crystal structural studies of a series of saturated an d 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 C-C single bo nds of the bicyclic fragment, A complex interplay of strain and differ ent types of conjugation influence the molecular structure of the bicy cloheptadiene derivative 1, where the difference in the length of the three-membered ring bonds is 0.040 Angstrom and all C-C single bonds i n the bicyclic fragment are lengthened significantly. Ab initio calcul ations 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 di stributions and dipole moments further support the relevance of cyclop ropyl homoconjugation in the investigated prototype of Walsh and throu gh-space pi-orbital interaction. Static difference electron density ma ps have been derived from the experimental data by multipole refinemen ts which showed exocyclic shifts of electron density in the planes of the three-membered rings and significant bond ellipticities at the C-C single bonds in the unsaturated bicyclic units.