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

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.

A direct dynamics study of protonated alcohol dehydration and the Diels-Alder reaction

“…45 In addition, the AM1 method yields relatively small energy values for deformation from the symmetric structure, which is consistent with some ab initio and density functional theory calculations. 46,47 Thus, the dynamics of this reaction using the AM1 method should provide at least qualitative results consistent with higher level electronic structure methods. …”

Femtochemistry of Norrish Type-I Reactions: III. Highly Excited Ketones—Theoretical

“…[36±43] Since the process of funneling through a CI is very efficient, the nonradiative decay from the upper surface to the lower one usually occurs within a single vibrational period. [42,43] Furthermore, the structural information of a CI may provide significant insight into the mechanism of product formation from a ™photochemically heated∫ ground-state species. [2, 43,44] Schematically, the bifurcated character (with respect to the gradient difference (x 1 ) and nonadiabatic coupling (x 2 ) vectors) of a CI can be represented by a single crossing point of a double cone formed by two intersecting surfaces along the (n À 2)-dimensional hyperline, where n is the number of the degrees of freedom in the molecule.…”

Photoinduced Intramolecular Charge Transfer and S2 Fluorescence in Thiophene‐π‐Conjugated Donor–Acceptor Systems: Experimental and TDDFT Studies

“…After the finding of the importance of photoinduced ICT processes in biological systems such as photosynthesis, [5] interest in molecular donor-acceptor systems has increased significantly. [6][7][8][9][10][11][12][13][14] For several years, the dual-fluorescence properties of donor-acceptor systems have been used for studying molecular switches. [15][16][17][18][19] It was found that changes in molecular structure and conjugated system can induce very different optical and physical properties in D-p-A compounds.…”