Structural effects of spiroconjugation: Crystal structures of spiro[4.4]nonatetraene and spiro[4.4]nona-1,3,7-triene

Spiro-polyenes, compounds with two perpendicular pi-systems joined by a common spiro-atom, exhibit through-space spiroconjugation. One important representative of spiroconjugated systems is spiro[4.4]nonatetraene (3). The molecular structures of spiro[4.4]nona-1,3,7-triene (2) and (3) have been determined by X-ray diffraction and compared with the redetermined structure of cyclopentadiene (1) in order to detect the slight distortions expected from spiroconjugation. Comparison of bond lengths and angles reveals a slight shortening of the double bonds and a small elongation of the single bonds connecting the spiro atom in (3). The same effect is also found from ab initio calculations at Hartree-Fock level 6-31G*, although to a minor extent. This is shown by applying a difference analysis of structural parameters in order to cancel out the systematic errors and differences of both methods.