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Three radiationless decay pathways for the photochemical decomposition of diazirine and diazomethane have been characterized using the MC-SCF method with a 6-31G* basis. From diazirine, two almost barrierless paths exist on SI. One leads, via a diradicaloid IDur conical intersection at a bent, in-plane, diazomethane-like structure, to ground-state diazomethane; the other leads, via another I D . . conical intersection at a ring-opened diazirine diradicaloid geometry, directly to lCH2 + Nz. The triplet pathway starts at a )u-u* diazirine minimum, passing over a 9 kcal mol-' barrier to a %-7r* ' D, bent diazomethane-like minimum from which the barrier to Nz extrustion is 7 kea1 mol-'. In the absence of sensitizers, this triplet path can be entered from the singlet manifold via intersystem crossing at a point that has been characterized by finding the lowest energy point on the singlet-triplet crossing surface. This crossing point occurs at a geometry that is very similar to the transition state that occurs on the singlet path between diazirine and ground-state diazomethane. However, the efficiency of intersystem crossing (spin-orbit coupling) is predicted to be low. These data rationalize the temperature dependence of the fluorescence, the fact that diazomethanes and diazirines are observed as products of photolysis of diazirines and diazomethanes, respectively, the fact that there is CHz + N2 formation from both diazirines and diazomethanes, and the fact that no triplet states seem to be involved in the reaction.

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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.