Polyene linear

Linear polyenes (butadiene, hexatriene, etc.) absorb ultraviolet radiation. They have absorption maxima at the approximate wavelengths given in Table 6-1. 

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. 

The most easily obtained information from such calculations is the relative orderings of the eneigy levels and the atomic coefficients. Solutions are readily available for a number of frequently encountered delocalized systems, which we will illustrate by referring to some typical examples. Consider, first, linear polyenes of formula C H 2 such as 1,3-butadiene, 1,3,5-hexatriene, and so forth. The energy levels for such compounds are given by the expression... 

Although the Hiickel method has now been supplanted by more complete treatments for theoretical analysis of organic reactions, the pictures of the n orbitals of both linear and cyclic conjugated polyene systems that it provides are correct as to symmetry and the relative energy of the orbitals. In many reactions where the n system is the primary site of reactivity, these orbitals correctly describe the behavior of the systems. For that reason, the reader should develop a familiarity with the qualitative description of the n orbitals of typical linear polyenes and conjugated cyclic hydrocarbons. These orbitals will be the basis for further discussion in Chapters 9 and 11. 

When we recall the symmetry patterns for linear polyenes that were discussed in Chapter 1 (see p. 33), we can fiorther generalize the predictions based on the symmetry of the polyene HOMO. Systems with 4 n electrons will undergo electrocyclic reactions by conrotatoiy motion, whereas systems with 4 4- 2 n electrons will react by the disrotatoiy mode. 

In 1982 the present author discovered cyclic orbital interactions in acyclic conjugation, and showed that the orbital phase continuity controls acyclic systems as well as the cyclic systems [23]. The orbital phase theory has thus far expanded and is still expanding the scope of its applications. Among some typical examples are included relative stabilities of cross vs linear polyenes and conjugated diradicals in the singlet and triplet states, spin preference of diradicals, regioselectivities, conformational stabilities, acute coordination angle in metal complexes, and so on. 

Rimai, L., Heyde, M.E., and Gill, D. 1973. Vibrational spectra of some carotenoids and related linear polyenes. A Raman spectroscopic study. J. Am. Chem. Soc. 95 4493 4501. 

This approach allows linear polyenes to be converted to functionalized polycyclic systems bearing up to six stereogenic centers. Another interesting use of the method deals with the synthesis of azapropellanes [45]. 

Birge RR, Pierce BM (1979) A theoretical analysis of two-photon properties of linear polyenes and the visual chromophores. J Chem Phys 70 165-178... 

Zhang, J., Van Lanen, S.G., Ju, J. et al. (2008) A phosphopantetheinylatingpolyketide synthase producing a linear polyene to initiate enediyne antitumor antibiotic biosynthesis. Proceedings of the National Academy of Sciences of the United States of America, 105, 1460. 

Linear polyenes. The measurement of membrane processes requires fluorophores inserted into cell membranes. Suitable fluorophores are polyene compounds such as diphenylhexatriene (DPH) (see Fig. 6.8) or parinaric acid, used in several occasions as... 

Sklar, L. A., Hudson, B. S. and Simoni, R. D. (1976a). Conjugated linear polyene fatty-acids as membrane probes. Fed. Proc. 35,1531-1531. 

Dienes and polyenes have been a subject of great interest due to their important role in biology, materials science and organic synthesis. The mechanism of vision involves cis-trans photoisomerization of 11 -civ-retinal, an aldehyde formed from a linear polyene. Moreover, this kind of molecule exhibits high linear and non-linear electrical and optical properties. Short polyenes are also involved in pericyclic reactions, one of the most important classes of organic reactions. 

The structure of the ground state of linear polyenes has been the subject of several theoretical studies12-37. Molecular geometries and vibrational frequencies for polyenes up to C18H20 have been reported. Much emphasis has been placed on the calculation of force constants that can be used in the construction of force fields. 

We will now consider the studies devoted to the next two linear polyenes 1,3,5-hexatriene and 1,3,5,7-octatetraene. First, we will present the results corresponding to geometries and conformational energies computed for these compounds. We will then discuss the computed frequencies and force fields. 

The linear polyenes are divided into the following three groups ... 

Ultraviolet/visible absorption, fluorescence, infrared and Raman spectroscopies are useful for studying structures (configuration, conformation, symmetry etc.) of electronically ground and excited states of linear polyenes, which have attracted much attention of... 

The ultraviolet/visible absorption spectrum of a polyene shows an intense absorption band and an extremely weak absorption band which is located below the strong absorption band, as described in the following section. This spectral pattern is a general property of linear polyenes of all chain lengths independent of local symmetry and/or the presence of cis bonds. This is the reason why in the literature on polyenes the labels 1 kg for So, 2 kg for Si and 1 feu for Si are used even in cases where Cih symmetry is not realized. The ordering that the 2 kg excited state is located below the 1 feu excited state is peculiar to linear polyenes. 

FIGURE 2. Some configurations describing the low-energy singlet states of linear polyenes... 

The spectra of linear polyenes are modelled well as one-dimensional free-electron systems. The cyanine dyes are a classical example. They constitute a class of long chain conjugated systems with an even number n of 7r-electrons distributed over an odd number N = n — 1 of chain atoms. The cyanine absorption of longest wavelength corresponds to promotion of an electron from the highest occupied energy level, En/2 to the lowest unoccupied level, such that in terms of a free-electron model... 

With these approximations the secular determinant for the energy eigenvalue e is reduced to sparse banded form for example, for a linear polyene... 

The LUMO of a conjugated linear polyene has a symmetry opposite to that of HOMO. Thus the excitation of an electron into the LUMO will reverse the direction of ring closure or ring opening... 

A significantly higher barrier (ca 32 kcalmol-1) is observed for iron migration in linear polyene complexes, e.g. 118 - 118 (Scheme 22). This isomerization is believed to proceed via rf -> rj1 coordination (118 - 119) followed by migration of the iron in the r]2 coordination mode (119 -> 119 ) (Scheme 22)122. While racemization of acyclic (diene)Fe(CO)3 does not occur at ambient temperatures, it is observed at elevated temperatures. This process is also proposed to occur via r/2 coordination (Scheme 23) however the rate for racemization (ca 2.3-2.7 x 104 at 119 °C) is approximately half the rate of polyene migration122. 

The DRE calculation scheme takes into account only two types of CC bonds, not accounting for the fact that the energy of an acyclic polyene depends on its branching. The 7r-energy of a branched acyclic polyene Ev (BP) is related to the energy of an isomeric linear polyene E (LP) by (75JCP3399)... 

Thus for the case of the linear polyene, the p-th transition energy Ep is given by... 

---