An MS-CASPT2 study of the photodecomposition of 4-methoxyphenyl azide: role of internal conversion and intersystem crossing.

The photochemical decomposition of 4-methoxyphenyl azide (CH3O-Ph-N3) is investigated using multiconfigurational second-order perturbation theory (MS-CASPT2). In addition, the multi-state resonance Raman spectra of the reactant, intermediates, and product are computed with a multi-state version of the vibronic theory of Albrecht. The results support that the key step of the photolysis of the parent azide is a 21A'/23A'' intersystem crossing which in a second step decays through a 23A''/13A'' conical intersection to give directly the formation of triplet 4-methoxyphenyl nitrene (CH3O-Ph-N) in its lowest electronic state, 13A''. It is found that the efficiency of the cited intersystem crossing is enhanced by the close presence of a 21A'/21A'' conical intersection. On the other hand, the calculated spectra suggest that the only two species which would be observed in the gas phase experiments are the triplet nitrene plus 4,4'-dimethoxyazobenzene.