No abstract

The ground-state and 1 (ππ*)-state potential energy surfaces of norbornene and isomeric C 7 H 10 species were mapped using CASSCF theory and the 6-31G* basis set and compared with the results of femtosecond experiments on norbornene retro Diels-Alder reactions. Computations explored stepwise and concerted retro Diels-Alder pathways, [1,3]-sigmatropic shifts, and [1,2]-sigmatropic shifts originating from the 1 (ππ*)-state or ground-state surfaces. Extremely efficient decay occurs from the excited state to the ground state via two different conical intersections (surface crossings). The first of these crossing points is accessed by one-bond cleavage of C1-C6 (or C4-C5). Several possible subsequent ground-state reaction paths have been identified: (a) ring-closure to form norbornene; (b) ring-closure to form bicyclo[3.2.0]hept-2-ene ([1,3]sigmatropic shift); (c) formation of a metastable 1,3-biradical which closes to form tricyclo[3.2.1.0 3,7 ]heptane ([1,2]-sigmatropic shift); and (d) collapse of a gauche-in biradical to a vibrationally excited cyclopentadiene and ethylene, or norbornene. Excited-state one-bond cleavage of C4-C7 (or C1-C7) leads to the second conical intersection. Possible ground-state reaction pathways from this structure lead to the formation of bicyclo-[4.1.0]hept-2-ene ([1,3]-sigmatropic shift product) or to a second 1,3-biradical leading to tricyclo[3.2.1.0 3,7 ]heptane ([1,2]-sigmatropic shift product). The vibrationally excited cyclopentadiene is the 220 fs lifetime species of mass 66 amu, consistent with the retro Diels-Alder reaction observed in the femtosecond laser experiments. It is proposed that biradicaloids formed after decay through the conical intersections are the 94 amu species, with an average lifetime of about 160 fs.

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.

The ground state (S0) and lowest energy triplet state (T1) energy surfaces of the parent dioxetane have been extensively explored using various CASSCF active spaces with MP2 corrections in several basis sets. In particular, the singlet/triplet surface crossing regions have been examined and the spin−orbit coupling and energetics computed. The computed energy barrier for the ring-opening of dioxetane is 16 kcal mol-1, which is lower than the experimentally observed threshold (22 kcal mol-1) for unsubstituted dioxetane decomposition. However, the surface topology is in good agreement with the experimental observations. The barrier for O−O cleavage on the ground state surface is found to lie at nearly the same energy as the transition structure for C−C biradical cleavage on the triplet energy surface. More significantly, the computational results indicate that the singlet and triplet surfaces do not cross along the minimum energy path (MEP) between the ground state O−O cleavage transition state and the singlet biradical, as previously thought. Instead, the S0 → 3(3π) surface crossing is prompted by a motion orthogonal to the reaction coordinate, which has components along both the OC−CO torsional and O−C−C asymmetric bending vibrational modes. In particular, we find evidence for a singlet/triplet crossing “line” that spans the ground state O−O cleavage valley and lies a few kcal mol-1 higher in energy. The computed spin−orbit coupling between the ground state and triplet 3(3π) surfaces is large (ca. 60 cm-1) throughout this crossing region. Therefore it is suggested that facile intersystem crossing (ISC) from the ground state to the triplet surface can occur anywhere along the MEP. ISC leads to production of a •OCH2−CH2O• triplet biradical that can either fragment to form triplet products or undergo ISC back to the ground state surface. The existence of a triplet/singlet crossing region located very close to the computed triplet biradical, suggests that this species is metastable with a short (picosecond) lifetime.