Cyclic polyenes structure

A more general classification considers the phase of the total electronic wave function [13]. We have treated the case of cyclic polyenes in detail [28,48,49] and showed that for Hiickel systems the ground state may be considered as the combination of two Kekule structures. If the number of electron pairs in the system is odd, the ground state is the in-phase combination, and the system is aromatic. If the number of electron pairs is even (as in cyclobutadiene, pentalene, etc.), the ground state is the out-of-phase combination, and the system is antiaromatic. These ideas are in line with previous work on specific systems [40,50]. 

Later in this chapter we 11 explore the criteria for aromaticity in more detail to see how they apply to cyclic polyenes of different ring sizes The next several sections intro duce us to the chemistry of compounds that contain a benzene ring as a structural unit We 11 start with how we name them... 

By contrast, the Dewar resonance energy represents solely the contribution coming from the cyclic electron (bond) delocalization since the model reference structure is represented not by a system of isolated 7r-bonds, but by a hypothetical cyclic polyene with the number of tr- and tr-bonds equal to that in a given molecule. Making use of the additivity of bond energies in acyclic polyenes (65JA692), one may calculate the total energy... 

Planar cyclic polyenes containing (4rt + 2) rt-electrons obey Huckel s rule for aromaticity and show greater stability than that predicted from their classical structures. 

Kollmar [19] has developed a procedure to estimate, at the SCF level, the energy of one Kekule structure of a cyclic polyene. Thus, the QMRE is simply the difference between the latter energy and the energy of the molecule as calculated by the regular SCF method, which allows the electrons to delocalize themselves. 

A comparison of the exact FCI total jr-electron energies E(FCI) (in eV) with those obtained by the PPP-VB method, employing either the optimal [E(VB, e opt )] or average [U(VB, e( v ))] OEAO 61 basis set and the minimum number N of VB structures, for the ground states of cyclic polyenes C rH r with an odd number of sites N, N = 2n + 1, and of their ions. The degeneracy (Deg.) of these states is indicated by the type of the symmetry species involved (E for doubly degenerate and A for totally symmetric nondegnerate ones). 

Torsion about one of the formal double bonds is invariably the most efficient excited singlet state decay process of acyclic polyenes, and also often occurs efficiently in cyclic systems of moderate-to-large ring size- . E.Z-isomerization in the excited singlet state manifold takes place about only one of the double bonds per photon, as was initially demonstrated for 2,4-hexadiene (5) by Saltiel and coworkers and has since been shown to be quite general. Table 1 contains a summary of quantum yields for the direct E,Z-photoisomerization, in solution, of acyclic and cyclic polyenes 1, 42, 43, 5-18 bearing various substituents. For the most part, quantum yields for direct E,Z-photoisomerization of aliphatic dienes are not highly dependent on the structure of the system (i.e. acyclic, cyclic or exocyclic). 

TT-Allyl compounds are also readily formed via the interaction of more complex polymeric systems. Thus, the cyclic polyene cyclododeca-1,5,9-triene (104) with Ru3(CO),2 gives a good yield (70%) of the ir-allyl species shown in Fig. 14a. This structure is closely related to a complex obtained by Fischer et al. (105) by reaction of phenyllithium with Ru3(CO),2 (Fig. 14b). 

I, 3-butadiene delocalization energy as only 4 kcal/mol. Moreover, this method predicts substantial delocalization stabilization for certain cyclic polyenes that experiment reveals to be unstable, with no aromatic character. (A cyclic conjugated polyene is said to be aromatic when it shows substantially more stability than a hypothetical structure in which the double bonds do not interact with one another and when it undergoes substitution, rather than addition, when treated with electrophilic reagents like Br2.)... 

Semiempirical calculations on the cyclic polyenes C4 ,+2H4m+2 that included electron correlation by a variety of methods predict bond-length alternation for large m [X. Li and J. Paldus, Int J. Quantum Chem., 60,513 (1996) and earlier papers].Hie fact that we can write two equivalent VB resonance structures for C4, + 2H4 + 2 such that a given bond is single in one structure and double in the other is no guarantee of equal bond lengths. 

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