Excited potential energy hypersurfaces for H4. 2. "Triply right" (C2v) tetrahedral geometries. A possible relation to photochemical "cross-bonding" processes

Gerhartz, Wolfgang; Poshusta, R. D.; Michl, Josef

doi:10.1021/ja00455a009

Cited by 59 publications

(22 citation statements)

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“…In the previously reported bicyclobutane6a and benzvalene6b conical intersection structures (Figures 2 b and 2 c, respectively), the interactions of the four singly occupied orbitals adopt different patterns (rhomboid (Figure 2 b) and triangular (Figure 2 c)). In the past, the same interaction patterns have been documented for the conical intersections of H 4 clusters 19. Thus our results support the conjecture by Michl and co‐workers19 that the structure of the conical intersections of conjugated hydrocarbons conforms to the conical intersections of elementary systems.…”

Section: Methodssupporting

confidence: 92%

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Intrinsically Competitive Photoinduced Polycyclization and Double-Bond Shift through a Boatlike Conical Intersection

et al. 2001

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The production of polycyclic carbon backbones through the direct irradiation of simple conjugated hydrocarbons is a remarkably simple way of forming complex molecular architectures. [1] Examples of these processes are the production of bicyclobutanes from butadienes, [2] tetrahedranes from cyclobutadienes, [3] benzvalenes from benzenes, [4] and semibullvalenes from cyclooctatetraenes. [5] We have demonstrated [6] that the photogeneration of 1,2-di-tert-butylbicyclobutane and benzvalene involves the deactivation at different types of S 1 /S 0 conical intersections, which prompts a concerted (simultaneous formation of two new s bonds) [6a] or stepwise (initial formation of the prefulvene diradical) [6b] mechanism, respectively. Herein, we discuss our investigations into the mechanisms of the photochemical production of polycyclic hydrocarbons, specifically the conversion of cycloocta-1,3,5,7tetraene (COT) into semibullvalene (SBV). We show that a novel type of conical intersection is involved in this process.Although SBV was first isolated after the sensitized irradiation of barrelene, [5a] it can also be synthesized by the direct irradiation of COT. [5b,c] In particular, direct irradiation in solution [5b] affords SBV and benzene (as a by-product), whereas vapor-phase UV irradiation is of preparative value. [5c] Experimental evidence was obtained for the direct formation of SBV, [5b] and a two-photon mechanism for SBV photogeneration was proposed. [5b] According to this model, the first photon is absorbed by the stable (cis,cis,cis,cis) configuration of COT, thus leading to the production of the strained trans,cis,cis,cis isomer, which then absorbs a second photon to produce SBV. The SBV is produced more rapidly than the strained isomer.Double-bond shifting (DBS) is an alternative process that occurs in antiaromatic [4n] systems such as COT. It has been established that the reversible interconversion between DBS isomers (a p-skeletal rearrangement) may be induced either thermally or photochemically. [7±9] Although DBS is of no practical interest for the parent COT, it may be used to turn on and off the conjugation between p substituents located at the vicinal position. The DBS reaction may thus be exploited to design photochemically driven switches. [9] We have found computational evidence that SBV and DBS originate from the same (excited-state) reaction path. In particular, the calculated S 1 path [10] (Scheme 1; light lines) goes through a novel type of boatlike S 1 /S 0 conical intersection (CI), which provides the locus for decay and branching [11] by means of three independent S 0 paths (dark lines). Scheme 1. Schematic representation of the reaction paths of COT*. Symmetry labels and relative CASPT2 energies (kcal mol À1 in parenthesis) are also reported for each calculated structure. S 1 COT is a planar D 8h -symmetric minimum (COT*). This structure (Figure 1) corresponds to the expected (aromatic) geometry, which characterizes the S 1 state of an antiaromatic system. [7] The S 1 ± S 0 energy ...

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Section: Methodssupporting

confidence: 92%

“…In the past, the same interaction patterns have been documented for the conical intersections of H 4 clusters 19. Thus our results support the conjecture by Michl and co‐workers19 that the structure of the conical intersections of conjugated hydrocarbons conforms to the conical intersections of elementary systems.…”

Section: Methodssupporting

confidence: 92%

Intrinsically Competitive Photoinduced Polycyclization and Double-Bond Shift through a Boatlike Conical Intersection

et al. 2001

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“…So occurs through a conical intersection at a distorted pericyclic geometry involving diagonal interactions was first proposed on the basis of model calculations for H4. [12] The various ways of recoupling the electrons of the tetraradical depicted in Scheme 1 demonstrate how different photoproducts may evolve from the same ' 1…”

Section: Conical Intersections and The Mechanism Of Singlet Photoreacmentioning

confidence: 99%

Conical Intersections and the Mechanism of Singlet Photoreactions

Klessinger

1995

Angew. Chem. Int. Ed. Engl.

153

114

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“…B. Within the two-electron two-orbital model, one obtains Equation (30) for E(T), where E:, given by Equation (31), is the triplet energy at the orthogonal reference geometry (y=O) and Ka"B. KOAE are also evaluated at this geometry (see Appendix 3). For the purposes of qualitative discussion, the difference terms in parentheses can be neglected.…”

Section: E(s2)=e(t)+3mentioning

confidence: 99%

Neutral and Charged Biradicals, Zwitterions, Funnels in S₁, and Proton Translocation: Their Role in Photochemistry, Photophysics, and Vision

Bonačić‐Koutecký

Koutecký

Michl

1987

Angew. Chem. Int. Ed. Engl.

Self Cite

412

333

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A knowledge of the geometries at which excited molecules return to the electronic ground state (So) is essential for the understanding of the structures of photoproducts. Particularly good candidates are geometries corresponding to local minima on the S, (lowest excited singlet) and TI (lowest triplet) surfaces, as well as So-Sl conical intersections (funnels). Given sufficient effort, such geometries can nowadays be found numerically for small enough molecules. Still, it is interesting to ask whether more approximate, but also more general, statements can be made concerning the geometries at which the So and S, surfaces closely approach each other. Since many of these are biradicaloid geometries, it is logical to examine the properties of biradicals and related species at some length. After reviewing the two-electron two-orbital model for molecules at biradicaloid geometries, we formulate the conditions under which the So and S, surfaces touch. The results obtained for the simple model are supported by a b initio large-scale configuration interaction (CI) calculations for the twisting of ethylene in the polarizing field of a positive charge and for the twisting of charged double bonds and n-donor-to-n-acceptor single bonds, and by similar calculations for "push-pull" perturbed cyclobutadienes, some of which are predicted to have nearly degenerate So, S,, and TI states. The likely consequences of these results for the detailed description of the mechanisms of cisstruns isomerization, the formation of twisted internal charge-transfer (TICT) states, proton translocation, and possibly of the initial step in vision, as well as for the understanding of the regiospecificity of singlet photocycloaddition, are summarized.

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Excited potential energy hypersurfaces for H4. 2. "Triply right" (C2v) tetrahedral geometries. A possible relation to photochemical "cross-bonding" processes

Cited by 59 publications

References 2 publications

Intrinsically Competitive Photoinduced Polycyclization and Double-Bond Shift through a Boatlike Conical Intersection

Intrinsically Competitive Photoinduced Polycyclization and Double-Bond Shift through a Boatlike Conical Intersection

Conical Intersections and the Mechanism of Singlet Photoreactions

Neutral and Charged Biradicals, Zwitterions, Funnels in S₁, and Proton Translocation: Their Role in Photochemistry, Photophysics, and Vision

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Excited potential energy hypersurfaces for H4. 2. "Triply right" (C2v) tetrahedral geometries. A possible relation to photochemical "cross-bonding" processes

Cited by 59 publications

References 2 publications

Intrinsically Competitive Photoinduced Polycyclization and Double-Bond Shift through a Boatlike Conical Intersection

Intrinsically Competitive Photoinduced Polycyclization and Double-Bond Shift through a Boatlike Conical Intersection

Conical Intersections and the Mechanism of Singlet Photoreactions

Neutral and Charged Biradicals, Zwitterions, Funnels in S1, and Proton Translocation: Their Role in Photochemistry, Photophysics, and Vision

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Neutral and Charged Biradicals, Zwitterions, Funnels in S₁, and Proton Translocation: Their Role in Photochemistry, Photophysics, and Vision