Polyene molecules

Woodward and Hoffmann have first disclosed that the thermal (4M+2)-cyclization (and also the photochemical (4M)-cyclization) takes place via Type I process, and the photochemical (4m+2)-cyclization (and also the thermal (4m)-cyclization) via Type II process 51>. They called the former (Type I) process "disrotatory", while the latter (Type II) process was referred to as "conrotatory". They attributed this difference in selectivity to the symmetry of HO and SO MO in the ground-state and excited-state polyene molecules, respectively (Fig. 7.33). The former is symmetric with respect to the middle of the chain, and the latter antisymmetric, so that the intramolecular overlapping of the end regions having the same sign might lead to the Type I and Type II interactions, respectively. 

Longuet-Higgins phase-based treatment, two-dimensional two-surface system, scattering calculation, 154-155 three-state molecular system, 134-137 two-state molecular system, single conical intersection solution, 98-101 permutational symmetry, degenerate/near-degenerate vibrational levels, 730-733 Polyene molecules ... 

Nevertheless, during inspection of the above-mentioned reviews and many of the works cited therein, many excellent analytical methods which can be applied to other polyene molecules were found. 

Very recently, MacGillivray et al. succeeded in the supramolecular construction of molecular ladders in the solid state using a linear template approach [48]. They designed the cocrystals 1,3-benzenediol (resorcinol) or a derivative with an all-trans-bis(4-pyridyl)butadiene or hexatriene, in which two resorcinol molecules preorganize two polyene molecules through two hydrogen bond interactions, for [2-1-2] photoaddition (Scheme 5). In this design, two polyenes would... 

Removal of one electron should make no difference to the relative stabilities of polyene molecule ions or even electron polyene fragments as compared to their neutral counterparts, e.g. butadiene and the allyl radical should have the same relative stabihties as the butadiene molecule ion, and the allyl cation. Removal of one electron will, however, alter the stabihties, and thus the reactivities of cychc polyenes. The molecule ions of aromatic hydrocarbons will be substantially less aromatic then their neutral counterparts. Correspondingly the molecule ions of antiaromatic hydrocarbons will not be as antiaromatic as their neutral analogs, e.g. cyclobutadiene + should be relatively more stable than cyclobutadiene. The largest charge effects in hydrocarbons will be observed in nonaltemant ) monocychc hydrocarbons. The cyclopropenium ion 7 and the tropillium ion 2 are both strongly aromatic as compared to their neutral analogs. Consequently CsHs is a very common ion in the mass spectra of hydrocarbons while cyclopropene is not a common product of hydrocarbon pyrolysis or photo-... 

SDS micelles were also employed for the light-induced cyclization of polyenes to steroids in low yield. No such reaction was observed in homogeneous organic solutions. Again, it is believed that only one polyene molecule is dissolved within one micelle. On the other hand, trans-cinnsimic acid produced dimeric products after UV irradiation in 1% aqueous CTAB, while no photodimers were formed in homogeneous solutions. ... 

No complex is formed with alpha- or beta-cyclodextrin as the polyene molecule does not fit in their smaller cavities. 

Correlation between the molecular P with the ground-state polarization and consequently with BLA was demonstrated through quantum chemical calculations in polyene molecules [17,18]. In these studies the ground-state polarization... 

Fig. 7. Neutral, cyanine and charge-separated form of a polyene molecule and corresponding BLA value...

Luo et al. have investigated the influence of the solvent polarity on the NLO properties of a simple donor-acceptor polyene molecule (Scheme 4) at the ab initio level of theory applying the continuum model of solvent [107]. It has been shown that the evolution of the TPA cross section with respect to the bond length alternation (BLA) closely follows that of the static first-order hyperpolarizability, p. The TPA cross section is strongly dependent on the geometrical changes. Moreover, these authors have noticed that the solvent effect on the TPA cross section (for the CT excited state) in tlie typical donor-acceptor polyene molecule exhibiting the positive solvatochromism is smaller than the influence of the solvent on the values of p. 

A sequence of H-abstractions due to the very reactive chlorine radical and / -decomposition gives rise to the formation of polyene molecules and HC1 which is released from the melting phase. Cross-linking reactions between polyenes lead to alkyl aromatic intermediates which can further decompose, releasing tar compounds and/or can polymerize with char formation. These transformations are experimentally observed as a second weight loss in PYC thermo gravimetric analysis. 

This mechanism is more complex with respect to other vinyl polymer ones and demands specific simplifications. One proposed approach carefully analyses all the reactions of the single monomer unit in the polymer chain (Marongiu et al., 2003). The initial PVC polymer is simply represented by the chlorinated reference unit P (CH2CHC1)-P. The successive steps of degradation form polyene molecules and these species, which have different molecular weights, are represented by the alkene reference unit P-(CH = CH)-P. The reference species (reported in bold characters in the brackets) are the reacting units and are placed inside the polymer chain, represented here by the P at the beginning and end. 

Figure 39 presents the characteristic three phases of PVC decomposition very clearly. The initial dehydrochlorination reactions form FIC1 and polyene structures. During this phase, benzene and some naphthalene and phenanthrene are also formed through Diels-Alder reactions and the successive dealkylation of polyene molecules. Then, when Cl has been quantitatively released... 

Huckel s rule (in its original form) stated that monocyclic polyenic molecules are aromatic only if their re-systems contain An + 2) re-el ec-trons, where n is an integer 1>. There have been many advances in LCAO-MO theory since Hiickel s original contributions (although the simplest approximation still bears his name, i.e. HMO), and today a more precise statement of the rule might read as follows. 

All-tra 5- -carotene is a one-dimensional polyene molecule which has a transition moment parallel to its molecular axis in the visible region. The 445 nm light used in determining the angular dependence of absorbance excites this particular transition moment. Denoting the molar concentration of j3-carotene in the film whose transition moment is parallel to the x-, y-, or z-axis of the substrate to be Q, C, or C, respectively, then the total concentration C can be described as... 

Photocyclization of polyenic molecules, such as divinylamine derivatives or divinylamides, may produce zwitterionic and polar intermediates according to the Woodward-Hoffmann rules. In the presence of chiral additives, the photoproducts can be formed enantioselectively. An intriguing example of this type of photoreaction is discussed in this section. 

Photocyclization of prochiral and polyenic molecules may lead to cyclic products according to the Woodward-Hoffman rules [55]. When asymmetric carbons are created in the cyclization step, asymmetric induction becomes possible during transformation of the excited state. Such reactions... 

A variety of routes have been documented for the synthesis of polyacetylene or linear conjugated polyene molecules and are collected in Table 1. 

Linear and nonlinear optical sepctra of polyene molecules... 

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