Polyenes molecular orbitals

Solutiorn Electrocyclic reactions are concerted, therefore they are completely stereospecific. The exact stereochemistry of the product depends upon the number of double bonds in the polyene molecular orbital theory allows us to predict this stereochemistry. Let us look at the electron configuration of butadiene, a four-ir-electron system, in the ground state and in the first excited state (achieved by the absorption of radiation) ... 

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

A conjugated polyene, as we saw in Section 14.1, is one with alternating double and single bonds. According to molecular orbital (MO) theory, the p orbitals on the sp2-hybridized carbons of a conjugated polyene interact to form a set of... 

Conjugated polyene, electrocyclic reactions of, 1181-1186 molecular orbitals of, 1179-1180 Conjugated tricne, electrocyclic reactions of, 1182 Conjugation, 482... 

Nonbenzenoid cyclic conjugated hydrocarbons are conveniently classified into two categories conjugated hydrocarbons composed of odd-membered rings called, in terminology of molecular orbital theory, nonalternant hydrocarbons, and cyclic polyenes currently known as annulenes. 

In the 1930 s HiickeP proposed, on the basis of molecular-orbital calculations, a theoretical criterion for aromaticity of cyclic polyenes, known as Hiickers rule, which states that cyclic polyenes should be aromatic if, and only if, they contain 4n- -2 Jt-electrons. At that time only two of such cyclic polyenes were known benzene and cyclo-pentadienyl anion, each having six rc-electrons and satisfying Huckel s rule. Since then, the validity of Hiickel s rule had not been challenged... 

The 327-670 GHz EPR spectra of canthaxanthin radical cation were resolved into two principal components of the g-tensor (Konovalova et al. 1999). Spectral simulations indicated this to be the result of g-anisotropy where gn=2.0032 and gi=2.0023. This type of g-tensor is consistent with the theory for polyacene rc-radical cations (Stone 1964), which states that the difference gxx gyy decreases with increasing chain length. When gxx-gyy approaches zero, the g-tensor becomes cylindrically symmetrical with gxx=gyy=g and gzz=gn. The cylindrical symmetry for the all-trans carotenoids is not surprising because these molecules are long straight chain polyenes. This also demonstrates that the symmetrical unresolved EPR line at 9 GHz is due to a carotenoid Jt-radical cation with electron density distributed throughout the whole chain of double bonds as predicted by RHF-INDO/SP molecular orbital calculations. The lack of temperature... 

In 1937, Coulson s mentor, Lennard-Jones, wrote a series of papers on molecular orbitals applied to polyenes and aromatic molecules, discussing the variations in bond lengths in conjugated and aromatic molecules from a theoretical point of view. 92 He involved his student in the work. Coulson defined... 

Review Section 2.2. The MO theory focuses attention on the interacting p AO s of the delocalized tr systems, such as conjugated polyenes. The theory states that the number of interacting p AO s is the same as the number of rr molecular orbitals formed. The molecular orbitals are considered to be stationary waves and their relative energies increase as the numbers of nodal points in the corresponding waves increase. Nodes may appear at a C atom, as indicated with a 0 rather than a +... 

This equivalence of the valence bond and molecular orbital descriptions of the bonding in these complexes arises from the alternant1 properties of the metal-butadiene bonding network. A similar equivalence between the two theories occurs for benzene and other polyenes that have alternant 7r-systems (73, 140). 

It is in this area that qualitative MO procedures have great success because there are general characteristics of the 77 molecular orbitals of mono-cyclic, conjugated polyene systems that predict differences in the properties of cyclobutadiene, benzene, cyclooctatraene, and other similar compounds that are not obvious from the simple VB method. 

Estimations [21] give / [ 3.7 eV and /i2 2.8 eV. In view of these estimations according to expression (10) the difference AEX— AE reaches value 0.1 eV at decrease of the polymer chain length (IV- - oo) down to value N0A equal, 40 dimer units or 80 bonds C-C. At the same time, experimental data for polyenes give the value A0.i 20 bonds C-C [22]. Thus, the model of noninteracting molecular orbitals delocalized along the whole chain overestimates increase AE with reduction of length of a chain. 

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