The photochemical rearrangement pathways of imidazoles: a theoretical study.

The mechanisms of the two reaction pathways for the photochemical transformations of methyl substituted imidazoles (i.e., 1,4-dimethyl-imidazole and 1,4,5-trimethyl-imidazole) in their first excited state (1pi --> 1pi*) have been determined using the CASSCF (10-electron/8-orbital active space) and MP2-CAS methods with the 6-311(d) basis set. These two reaction pathways are denoted as the conical intersection path (path 1) and the internal cyclization-isomerization path (path 2). Our model investigations suggest that conical intersections play a crucial role in the photorearrangements of imidazoles. Additionally, the present theoretical findings suggest that photoisomerizations of imidazoles via path 1 should adopt the reaction path as follows: imidazole --> Franck-Condon region --> conical intersection --> photoproduct. Moreover, we have examined the alternative mechanism, the internal cyclization-isomerization path (path 2), which consists of a sequence of small geometric rearrangements. Our theoretical investigations suggest that for the photorearrangement of 1,4-dimethyl-imidazole both mechanisms are comparable. On the other hand, for the photorearrangement of 1,4,5-trimethyl-imidazole path 1 should be favored over path 2. Our present theoretical results agree with the available experimental observations.