Ab initio study on the photochemical isomerization of thiazole derivatives

The photochemical isomerization reactions of 2-phenyl and 2-acetylthiazole were studied using ab initio methods. The results are in agreement with the previously reported data obtained through semiempirical methods. Triplet excited 2-phenylthiazole is a π,π∗ triplet with LSOMO at −9.47 eV and HSOMO at −6.84 eV. In this case, the singlet excited state can evolve giving the Dewar thiazole while the corresponding excited triplet state cannot be obtained. Furthermore, the triplet state cannot be converted into the biradical intermediates because these intermediates show a higher energy than the triplet state, thus preventing the formation of the cyclopropenyl derivatives. Triplet excited 2-acetylthiazole is a π,π∗ species. It shows the LSOMO at −10.70 eV and the HSOMO at −8.14 eV. In this case, the direct irradiation involves the population of the excited singlet state, and then the formation of the Dewar isomer is possible. The intersystem crossing to the triplet state can occur; the intersystem crossing quantum yield for this conversion is nearly quantitative. The triplet state cannot convert into Dewar thiazole but it can give the corresponding biradicals. These biradicals are not responsible for the isomerization reactions but they are able to give only decomposition products.