Substituent effect on the sudden polarization in simply substituted ethylenes

The sudden polarization effect occurring in the excited states of simply-substituted ethylenes is analyzed in both molecular orbital and valence bond terms for the substituents R = CH₃, F, HO and NH₂. Ab initio level computations (4—31G + 100 × 100 CI) indicate that at 90° twist the ground states of all these structures are diradical in character with very little substituent effect on the ground state cis—trans isomerization barrier. On the other hand, the energies of both the Z₁ and Z₂ states are strongly affected by the substituent. In all cases the Z₁ states have a nearly pure R---C⁺H---C⁻H₂ zwitterionic character. The stabilization energies of the Z₁ states as compared to ethylene roughly correspond to the order expected from the analogous carbonium ions with the exception of the fluoro substituent.