Rotational isomerism and charge density properties of vinylcyclopropane (VCP)

Abstract

Ab initio SCF calculations of more than double zeta quality have been carried out for nearly fully optimizing the s-trans, gauche and s-cis structures of VCP. The potential curve of internal rotation, which is obtained under (partial) consideration of the structural parameter relaxation, confirms the three minimum curve of preceding experimental and theoretical investigations. As was conjectured in a microwave study,] the dipole moment of gauche VCP (0.29 D) proves to be considerably smaller than the s-trans value (0.48 D). The different CC bonds (the ring bonds, the double bond, and the bond between ring and vinyl group) are mutually compared on the basis of bond lengths, force constants, population analyses (Mulliken and Davidson—Roby analyses), and bond path considerations. Especially, a π-conjugative interaction between the cyclopropyl ring and the vinyl group is thoroughly studied as a function of internal rotation. The occurrence of two gauche structures is interpreted as a compromise between the attempt to achieve maximal π-Conjugative interaction (which favours the s-cis structure) and to avoid steric interaction of the endstanding hydrogen atoms (which disfavours the s-cis structure).