The Photochemical Isomerization of Pyrazoles: An AB Initio Study

The photochemical isomerization of pyrazoles was studied using ab initio methods. The singlet excited singlet state can evolve to give the Dewar isomer and the corresponding triplet state. The latter shows a lower energy and probably can be obtained with higher efficiency. The triplet state can evolve to give the corresponding 1,2-biradical and, then, the isomerization product. The same behavior was obtained by using 1,5-dimethylpyrazole. However, 1-methyl-5-phenylpyrazole gave a different behavior. The triplet state cannot evolve to give the corresponding biradical. The isomerization product can be obtained only from the Dewar isomer.