Model study of the photochemical rearrangement pathways of 1,2,4-oxadiazole

The mechanisms of photochemical isomerization reactions are investigated theoretically by using a model system of 1,2,4- oxadiazole with the CAS(14,9)/6-311G(d) and MP2-CAS-(14,9)/ 6-311++G(3df,3pd)//CAS(14,9)/6-311G(d) methods. Three reaction pathways are examined, including 1) the direct mechanism, 2) the ring contraction-ring expansion mechanism, and 3) the internal cyclization-isomerization mechanism, which lead to two types of photoisomers. The theoretical findings suggest that conical intersections play a crucial role in the photorearrangement of 1,2,4-oxadiazoles. These model investigations also indicate that the preferred reaction route for 1,2,4-oxadiazole, which leads to phototransposition products, is as follows: reactant → Franck-Condon region → conical intersection → photoproduct. In other words, the direct mechanism is a one-step process that has no barrier. These theoretical results agree with the available experimental observations.