Ground- and Excited-State Reactions of Norbornene and Isomers: A CASSCF Study and Comparison with Femtosecond Experiments

1999-06-06T00:00:00Z (GMT) by Sarah Wilsey K. N. Houk A. H. Zewail
The ground-state and <sup>1</sup>(ππ*)-state potential energy surfaces of norbornene and isomeric C<sub>7</sub>H<sub>10</sub> 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 <sup>1</sup>(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<sup>3,7</sup>]heptane ([1,2]-sigmatropic shift); and (d) collapse of a <i>gauche-in</i> 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<sup>3,7</sup>]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.