The mechanism of ground-state-forbidden photochemical pericyclic reactions: evidence for real conical intersections

Cyclobutene photochemistry. Adiabatic photochemical ring opening of alkylcyclobutenes

“…At the 2'A$11A, conical intersection calculated for s-cis butadiene, the molecule is twisted about all three carbon+ar-bon bonds; the termini (the C1-C2 and C3-C4 bonds in 1,3-butadiene) are twisted in disrotatory fashion, while the central (C2-C3) bond is effectively twisted by ca. 52" as a result of pyramidalization at C2 or C3 (16,18,19). One particularly intriguing aspect of this mechanism is the suggestion that torsion about the terminal double bonds of a diene is accompanied by torsional motions about the central (single) bond.…”

“…This is the mechanistic model that is traditionally employed to explain, for example, why conjugated dienes undergo photochemical ring closure with a high degree of disrotatory stereospecificity (15). If it is valid -and recent theoretical work suggests it is not (16)(17)(18)(19) -then the ultimate formation of formally-forbidden diene isomers must be due to some intervening process that competes with internal conversion at the avoided crossing for disrotatory interconversion. The possibilities that have been considered include (1,9): (i) internal conversion to upper vibrational levels of the ground state (before the pericyclic minimum is reached), from which conrotatory ring opening ensues (1, 12); (ii) internal conversion to biradicaloid geometries on the diene ground state surface, from which torsional relaxation yields a mixture of isomeric dienes (20); and (iii) complete disrotatory ring opening on the excited state surface to yield fully open diene(s) in the first excited singlet state (i.e., adiabatically), followed by decay to the ground state by cis,trans isomerization (1, 9,20).…”

Accidental conical intersections in H2Cl+(1A′)

“…The classic paper by Longuet-Higgins in 1975 (1) appears to be definitive as to the existence of such intersections and their dimensionality. Since then, these intersections have become well established both from the theoretical and computational perspectives (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12) and have important implications for the theory of reaction dynamics (13)(14)(15)(16)(17)(18)(19)(20)(21)(22). Within a specified area, the existence of a conical intersection can be demonstrated without actually locating it, if one exploits the geometric phase factor (Berry's phase): the eigenvectors of the intersecting states change their signs on being transported in a closed curve around the point of intersection (23)(24)(25)(26)(27)(28).…”

Toward a computational photobiology

“…In 1990, Olivucci, Bernardi, and Robb [15,16] used these methodologies to explain the photochemical cycloaddition of ethylene: both a vital material for the chemical industry and a plant hormone. It was shown that i a CI exists right at the bottom of the excited-state energy surface of two interacting ethylene molecules, and ii the molecular structure of the CI was intimately related to the observed production of cyclobutane, a hydrocarbon which is square in shape.…”