Modeling Photochemical [4 + 4] Cycloadditions:  Conical Intersections Located with CASSCF for Butadiene + Butadiene

Abstract

The [4 + 4] photocycloaddition of butadiene + butadiene has been studied at the CASSCF /4-31G level, as a prototype for a class of photochromic systems. For this model system, minima and transition structures are characterized by analytic frequency calculations, and conical intersections are located. Our results indicate that the standard model for the [4 + 4] addition (based on H4) needs to be revised. The reorganization of all 8π electrons is crucial (i.e., it is not always the same 4π electrons that are important). Efficient nonradiative decay of butadiene + butadiene can be explained by the presence of two distinct S 1/S 0 conical intersections. The first-the lowest-energy point on S 1 overall-is preceded by a barrier for the formation of a new σ bond. The resulting structure is similar to those previously characterized for methyl migration in but-1-ene and the addition of ethylene to benzene. A higher-energy barrier leads to a second crossing which resembles the rhomboidal funnel for the [2 + 2] addition of ethylene + ethylene, but which involves only one double bond from each butadiene. Both reaction paths commence at a true pericyclic minimum, at which the (S 0−S 1) energy gap of ∼37 kcal mol-1 prohibits decay.

Synthesis, Fluorescence Properties, and Head-to-Tail Regioselectivity in the Photodimerization of a Donor–Acceptor-Substituted Anthracene

“…[4 ϩ 4] photocyclization of 1,3-butadiene on the basis of Acetic acid was added until pH ഠ 4Ϫ5 and the solution was stirred CASSCF calculations. [21] This model should also be valid for 1 h at room temperature. On addition of 100 mL of water with for other laterally mono-substituted anthracenes; however, vigorous stirring, a brown solid precipitated.…”

New Dyes Based on Amino-Substituted Acridizinium Salts-Synthesis and Exceptional Photochemical Properties

“…From detailed theoretical studies on the [22] photodimerization of substituted alkenes, a conical intersection has been detected with an energy minimum for a head-to-tail arrangement, which determines the regioselectivity of the reaction. [36] Since the same theoretical considerations have been used to explain the electronic influence of substituents on the head-to-tail photocyclization of anthracene derivatives, [36] and a conical intersection has also been found recently for the [44] photocyclization of 1,3-butadiene, [37] it is tempting to assume that a conical intersection with a head-to-tail geometry of the ground and excited states of salt 5 a exist on the hypersurfaces. This conical intersection thus determines the regioselectivity of the photodimerization independently from the regioselectivity of the excimer formation.…”

Metal‐Organic Frameworks: Photoreactive Frameworks

“…The maximum distance criterion proposed in Schmidt's postulate may be related to maximum distance for interaction between the p z orbital clouds that can be slightly larger than the sum of van der Waal's radii. 22,23 This minimum movement of atoms or molecules in the solid state leading to the reaction is known as topochemical reaction. 24, 25 Since the structure of the product is correlated with the geometry of the reactant solid in the crystal lattice, the stereochemistry of the product obtained in a solid state reaction is, in general, predictable.…”

“…[23][24][25] The zwitter ionic Pb(II) complex [Pb(bpe-H) 2 (TFA) 4 ] (4) (where TFA = trifluoroacetate anion) comprises [Pb(TFA) 4 ] 2− anion and two bpe-H + cations. The coordinated monoprotonated bpe-H + cations and the neighboring oxygen atoms of the TFA ligands have complementary hydrogen bonds resulting in an infinitely organized 1D hydrogen-bonded aggregate in which pairs of bpe-H + cations are well-aligned in an HT manner as shown in Figure 4.…”