A computational study of photochemical isomerization reactions of thiophenes

The mechanisms of the photochemical isomerization reactions were investigated theoretically using three model systems; 2-methylthiophene, 2-cyanothiophene, and 2-phenylthiophene. The CASSCF (10-electron/eight-orbital active space) and MP2-CAS methods were employed with the 6-311(d) basis set. Three mechanisms, i.e., the internal cyclization-isomerization route (path A), the zwitterion-tricyclic route (path B), and the direct route (path C), have been used to explore the real photochemical reaction mechanism of these three model molecules. The structures of the conical intersections, which play a key role in such phototranspositions, were obtained. The intermediates and transition structures of the ground states were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred reaction route is as follows: reactant --> Franck-Condon region --> conical intersection --> photoproduct. In particular, the conical intersection mechanism described in this work gives a better explanation than either the previously proposed internal cyclization-isomerization (path A) or the zwitterion-tricyclic pathway (path B) mechanisms, and is supported by the experimental observations. The results obtained allow a number of predictions to be made.