An MC-SCF/MP2 Study of the Photochemistry of 2,3-Diazabicyclo[2.2.1]hept-2-ene: Production and Fate of Diazenyl and Hydrazonyl Biradicals

Abstract

A CAS-SCF/MP2 study of the photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) has been carried out with use of a 6-31G* basis. The S1 (n−π*), T1 (n−π*), and T2 (π−π*) reaction paths for deazetization (via α C−N cleavage) and rearrangement reaction to azirane (via β C−C cleavage) have been investigated along with the associated reaction pathways for cyclization and rearrangement of the photoproduct, 1,3-cyclopentanediyl biradical. It is shown that singlet and triplet photoexcited DBH evolve along a network of 18 ground and excited-state intermediates, 17 transition structures and 10 “funnels”, where internal conversion (IC) or intersystem crossing (ISC) occurs. Three cyclic excited-state species are reached following evolution from the Franck−Condon region: two metastable singlet (n−π*) and triplet (n−π*) species and a stable excited state 3(n−π*)−3(π−π*) intermediate. It is demonstrated that the singlet 1(n−π*) intermediate can decay directly to S0 or undergo ISC to generate the 3(n−π*)−3(π−π*) intermediate or/and the 3(n−π*) intermediate. The 3(n−π*) intermediate can directly decay to the T1 diazenyl biradical or undergo IC to generate the 3(n−π*)−3(π−π*) intermediate. Finally, the much more stable 3(n−π*)−3(π−π*) intermediate cannot be converted to the other excited state intermediates but can only react via either α C−N and β C−C cleavage. Our computed energetics suggest that the 3(n−π*)−3(π−π*) intermediate is the best candidate for the experimentally observed transient triplet intermediate.