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.