Structural aspects of cyclopropyl homoconjugation. Experimental studies and ab initio calculations.
Low-temp. crystal structural studies of a series of satd. and unsatd. bicyclo[2.2.1]heptadienes, heptenes, and heptanes with 7-spirocyclopropyl substitution reveal differences in the bond lengths of the 3-membered rings and in the C-C single bonds of the bicyclic fragment. A complex interplay of strain and different types of conjugation affects the mol. structure of 7-cyclopropylidenebicyclo[2.2.1]hepta-2,5-diene, where the difference in the length of the 3-membered ring bonds is 0.040 .ANG. and all C-C single bonds in the bicyclic fragment are lengthened. Ab initio calcns. at the HF/6-31G(d) (to a minor extent) and MP2/6-31G(d) levels are in good agreement with the exptl. data. Calcd. charge distributions and dipole moments further support the relevance of cyclopropyl homoconjugation in the investigated prototype of Walsh and through-space p-orbital interaction. Static difference electron d. maps were derived from the exptl. data by multipole refinements which showed exocyclic shifts of electron d. in the planes of the 3-membered rings and bond ellipticities at the C-C single bonds in the unsatd. bicyclic units.
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