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

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', when 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.