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 S-1 (n-pi*), T-1 (n-pi*), and T-2 (pi-pi*) reaction paths for deazetization (via alpha C-N cleavage) and rearrangement reaction to azirane (via beta 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-pi*) and triplet (n-pi*) species and a stable excited state (3)(n-pi*)-(3)(pi-pi*) intermediate. It is demonstrated that the singlet (1)(n-pi*) intermediate can decay directly to S-0 or undergo ISC to generate the 3(n-pi*)-(3)(pi-pi*) intermediate or/and the (3)(n-pi*) intermediate. The (3)(n-pi*) intermediate can directly decay to the T-1 diazenyl biradical or undergo IC to generate the (3)(n-pi*)(-)3(pi-pi*) intermediate. Finally, the much more stable (3)(n-pi*)(3)(pi-pi*) intermediate cannot be converted to the other excited state intermediates but can only react via either alpha C-N and beta C-C cleavage. Our computed energetics suggest that the (3)(n-pi*)-(3)(pi-pi*) intermediate is the best candidate for the experimentally observed transient triplet intermediate.