Trans conformation, potential energy surfaces

The import of diabatic electronic states for dynamical treatments of conical intersecting BO potential energy surfaces is well acknowledged. This intersection is characterized by the non-existence of symmetry element determining its location in nuclear space [25]. This problem is absent in the GED approach. Because the symmetries of the cis and trans conformer are irreducible to each other, a regularization method without a correct reaction coordinate does not make sense. The slope at the (conic) intersection is well defined in the GED scheme. Observe, however, that for closed shell structures, the direct coupling of both states is zero. A configuration interaction is necessary to obtain an appropriate description in other words, correlation states such as diradical ones and the full excited BB state in the AA local minimum cannot be left out the scheme. 

An intriguing competition arises in the context of cation radical cycloadditions (as in the context of Diels-Alder cycloadditions) which involve at least one conjugated diene component. Since both cyclobutanation and Diels-Alder addition are extremely facile reactions on the cation radical potential energy surface, it would not be surprising to find a mixture of cyclobutane (CB) and Diels-Alder (DA) addition to the diene component in such cases. Even in the cyclodimerization of 1,3-cyclohexadiene, syn and anti cyclobutadimers are observed as 1 % of the total dimeric product. Incidentally, the DA dimers have been shown not to arise indirectly via the CB dimers in this case [58]. The cross addition of tw 5-anethole to 1,3-cyclohexadiene also proceeds directly and essentially exclusively to the Diels-Alder adducts [endo > exo). Similarly, additions to 1,3-cyclopentadiene yield essentially only Diels-Alder adducts. However, additions to acyclic dienes, which typically exist predominantly in the s-trans conformation which is inherently unsuitable for Diels Alder cycloaddition, can yield either exclusively CB adducts, a mixture of CB and DA adducts or essentially exclusively DA adducts (Scheme 26) [59]. 

In order to discuss the potential energy surfaces of isomerization one can assume the following points, On the triplet potential energy surface potential energy minima could exist at only three conformations cis ( c ), perpendicular ( p ), and trans (H ). In addition, the deactivation can take place only from c, p, and with different contributions depending on the shape of the potential energy surface. 

When porphyrins with much higher triplet energies such as palladium octaethylporphyrin (17 Et = 44.8 kcal mol" ) were used as sensitizers, even the cis trans isomerization of stilbene took place as a quantum chain process = 1-6) [95]. The high quantum efficiencies were explained by a quantum chain process in which the metalloporphyrin serves as both an energy donor and an acceptor. Since the quantum yield of cis trans isomerization of 1,2-diphenylpropene = 0-37) remained as a normal value under the same experimental conditions as those of stilbene, the potential energy surface of the triplet state is an important factor for occurrence of the quantum chain cis-trans isomerization. That is, in 1,2-diphenylpropene the triplet state exists exclusively as a perpendicular conformation, where the triplet state and the ground state lay very close in energy and the deactivation can only take place thermally. 

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