Ab Initio Study on One-Way Photoisomerization of the Maleic Acid and Fumaric Acid Anion Radical System as a Model System of Their Esters

Potential energy surfaces (PESs) of the maleic acid anion radical (MA^-•:  cis isomer)/fumaric acid anion radical (FA^-•:  trans isomer) system as a model system of their esters have been studied in detail using CASSCF method. The results suggest the following:  The photoisomerization is initiated with the H−C−C−H dihedral angle distortion [hydrogen out of plain (HOOP) motion] on the D₁ PES. The C−C−C−C dihedral angle distortion occurs on the D₀ PES after the deactivation from D₁ to D₀. A large fraction of the net motion along the isomerization coordinate occurs on the D₀ PES. The D₀ state is responsible for the one-way nature of the photoisomerization.