Exocyclic push–pull conjugated compounds. Part 4: rotational barriers in poorly polarized push–pull ethylenes

Abstract

The rotational barrier for substituted ethylenes was calculated with MO ab initio methods, both in the Hartree–Fock (HF) single determinant scheme and with multiconfigurational self-consistent field (MCSCF) approaches. The HF model affords reliable results only when applied to molecules substituted with strong electron donor groups, assigning a push–pull character to the molecule. The MCSCF approach was employed for calculating rotational barriers in poorly polarized ethylenes not directly amenable to the HF methods; however, this method was found hard to handle for low symmetry molecules with substituents interacting with the double bond. A method is proposed based on the interpolation of the energy of the perpendicular conformation from a Fourier truncated function constructed with HF molecular energies calculated for frozen conformations twisted up to 60°. The application of HF theory for studying internal rotation in substituted ethylenes with poorly polarized character is discussed and an upper limit of 35–40kcal/mol can be set up for having reliable barriers from the calculated energy of the rotational transition state at this level of theory.