Exocyclic push-pull conjugated compounds. Part 2. The effect of donor and acceptor substituents on the rotational barrier of push-pull ethylenes

The energy of the rotational barriers and electronic structure of the trans ition state in substituted ethylenes are discussed in the light of the resu lts obtained from different theoretical MO ab initio approaches. The 6-31G* basis set at Hartree-Fock (HF) level and with second-order Moller-Plesset perturbation theory (MP2) was employed, critical points were localized thro ugh Full geometry relaxation and characterized by vibrational analysis. A m ulticonfigurational approach (MCSCF) with different active spaces was also employed. For alkenes the correct rotational transition state is obtained o nly from the MCSCF approach, whereas for push-pull olefins the HF approach with correlation corrections at MP2 level provides correct answers for the internal rotation around C(sp(2))-C(sp(2)) bonds. The choice is more critic al when only acceptor or donor groups are present, especially when change o f hybridization occurs at the atoms at the edges of the C-C bond in the cri tical points. (C) 2000 Published by Elsevier Science B.V. All rights reserv ed.