A CASSCF Study of the Photochemical Isomerization of 2-Cyanothiophene

The study aims to discuss the photochemical isomerization of 2-cyanothiophene. The reaction was described several years ago, and the possible formation of Dewar 2-cyanothiophene was proposed. It has been suggested that the photochemical isomerization of pentatomic aromatic heterocycles occurring through the first excited singlet state proceeds via the formation of a Dewar isomer. CASSCF calculations have been performed to investigate this process. The CASSCF study of the photoisomerization of 2-cyanothiophene showed the presence of a conical intersection in the excited state, allowing the formation of Dewar 2-cyanothiophene. A transition state of 0.68 eV permits the formation of the isomeric Dewar 3-cyanothiophene, which can be converted into 3-cyanothiophene through a transition state of 1.40 eV. These results support the suggestion that all photochemical isomerizations of pentatomic aromatic heterocycles occurring through the first excited singlet state involve Dewar intermediates.