Theoretical Investigations of the Photochemical Isomerizations of Indoxazene and Isoxazole

The mechanisms of the photochemical isomerization reactions were investigated using two model systems of indoxazene (1) and isoazole-3-carboxylate (5) with the CASSCF and MP2-CAS methods using the 6-311G(d,p) basis set. The active space of the former consists of 14 electrons in 11 orbitals, while that of the latter consists of 10 electrons in seven orbitals. Two reaction pathways were examined in the present work. They are referred to as the internal cyclization-isomerization path (paths I and III) and the direct path (paths II and IV). Our model investigations suggest that the preferred reaction route for these species is as follows: reactant --> Franck-Condon region --> conical intersection --> photoproduct. In particular, the direct (conical intersection) mechanism found in this work gives a better explanation than the previous proposed mechanism and also supports the available experimental observations. Additionally, a simple p-pi orbital topology model is proposed that can be used as a diagnostic tool to predict the location of the conical intersections as well as the geometries of the phototransposition products of various heterocycles.