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

Abstract

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.