Ground- and excited-state reactions of norbornene and isomers: A CASSCF study and comparison with femtosecond experiments

The ground-state and (1)(pi pi*)-state potential energy surfaces of norborn ene and isomeric C7H10 species were mapped using CASSCF theory and the 6-31 G* basis set and compared with the results of femtosecond experiments on no rbornene retro Diels-Alder reactions. Computations explored stepwise and co ncerted retro Diels-Alder pathways, [1,3]-sigmatropic shifts, and [1,2]-sig matropic shifts originating from the (1)(pi pi*)-state or ground-state surf aces. Extremely efficient decay occurs from the excited state to the ground state via two different conical intersections (surface crossings). The fir st of these crossing points is accessed by one-bond cleavage of C1-C6 (or C 4-C5). Several possible subsequent ground-state reaction paths have been id entified: (a) ring-closure to form norbornene; (b) ring-closure to form bic yclo[3.2.0]hept-2-ene ([1,3]sigmatropic shift); (c) formation of a metastab le 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 vibratio nally excited cyclopentadiene and ethylene, or norbornene. Excited-state on e-bond cleavage of C4-C7 (or C1-C7) leads to the second conical intersectio n. 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]-s igmatropic shift product). The vibrationally excited cyclopentadiene is the 220 fs lifetime species of mass 66 amu, consistent with the retro Diels-Al der 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.