Trans-hexatriene, conformations

Photochemical cw,fraws-conformational interconversion is also known to occur in larger polyenes. Brouwer and Jacobs have reported the results of irradiation of E- and Z-2,5-dimethyl-l,3,5-hexatriene (14) in argon matrices at 10 K . Irradiation of the E-isomer gives rise to various rotamers, while irradiation of the Z-isomer results only in E,Z-isomerization. Photochemical trans/cis conformer interconversion has also been observed for E,E-l,3,5,7-octatriene in matrices at temperatures below 10... 

Namjesnik-Dejanovic K, Maurice PA (2001) Conformational and aggregate structures of sorbed natural organic matter on muscovite and hematite. Geochim Cosmochim Acta 65 1047-1057 Naves de Brito A, Svensson S, Correia N, Keane MP, Agren H, Sairanen OP, Kivimaki A, Aksela S (1992) X-ray induced electron yield spectrum of thin films of 1,3-trans-butadiene and 1,3,5-trans-hexatriene. J Electron Spect Rel Phen 59 293-305... 

Demonstrations of photophysical manifestations of NEER include the resolution of the fluorescence of aU-tra s-l,6-diphenyl-l,3,5-hexatriene into s-trans,s-trans and s cis,s-trans contributions and analogous resolutions of fluorescence and absorption spectra of styrylarene conformers.The conformer-specific adiabatic cis trans photoisomerizations of 2-styrylnaphthalene" (c/s- and tra s-NPE) and 2-styrylanthracene are especially striking examples. In each case, the more extended s-trans-like conformer undergoes adiabatic cis trans photoisomerization whereas the less extended s-c/s-like conformer undergoes selective photocyclization (Scheme In this sense, these cis-diarylethenes can be regarded as... 

The most stable conformation of both hexatriene and octatetraene is the al -s-trans one. Figure 2 represents these structures schematically. 

Unstable conformers of trans- and cis-hexatriene have been detected by means of the combination of matrix-isolation infrared spectroscopy and photoexcitation (or the high-temperature nozzle technique)84. Ab initio MO calculations at the HF/6-31G level have been performed for several conformers of 1,3,5-hexatriene93. The observed infrared bands of unstable conformers have been attributed to the gTt (major species) and gTg (minor species) conformers of /raw.s -hexalricne and the gCt conformer of cw-hexatriene93. It is noted that, in the previous paper93, the notation c is used for twisted structures for the sake of simplicity. The calculated torsional angles around C—C bonds for the gTt, gTg and gCt conformers are in the range between 32° and 45°. The observed and calculated vibrational frequencies of gTt and gCt are reported in Reference 93. 

Fig. II. (a) Schematic representation of hydrocarbons adsorbed on the [111] plane of platinum. Intersections of the lines of triangular net denote positions of the centers of platinum atoms. (1) Cyclohexane (2) all-cis conformation of cij-l,3,5-hexatriene (3) transoid conformations of cis- and trans-1,3,5-hexatriene (S4). (b) Adsorption configurations of acetylene and ethylene found most probable according to LEED studies 141).

As an application, let us compare the energies of two isomers of hexatrienes. The linear s-trans conformation can be described as a resonance between the canonical structure 15 and long-bond structures 16-18 (Scheme 3.7), where one short bond is replaced by a long one. On the other hand, the branched isomer is made of only structures 19-21, since it lacks an analogous structure to 18. 

There are two isomers of hexatriene a cis form and a trans form. The name refers to the geometry about the central double bond. The two isomers have different chemical and physical properties. Rotation is still possible about the single bonds (although slightly more difficult than around a normal single bond) and there are three different planar conformations possible for each isomer. Keeping the central black double bond the same, we can rotate about each of the green o bonds in turn. Each row simply shows different ways to draw the same compound. 

The photocyclization of (Z)-1,3,5-hexatriene represents a well-studied example of the influence of ground-state conformational equilibrium on the product composition (Havinga, 1973). The unsubstituted triene exists mainly in the s-trans-s-trans conformation and therefore cannot undergo photocyclization only Z-E isomerization has been observed. With the 2,5-dimethyl derivative the s-cis-s-cis conformation predominates and photocyclization yields dime-thylcyclohexadiene and some dimethylvinylcyclobutene. Finally, with the 2-methyl derivative the s-cis-s-trans conformation is preferred, and a relatively good yield of methylbicyclo 3.l.0]hexene as well as methylvinylcyclobutene and methyIhexatriene-1,2,4 is found. These results are collected in Scheme I. 

The reaction pathways both for single- and double-bond isomerization enter the funnel region at less than one-third of the way toward the products, suggesting that the majority of excited-state molecules should decay back to the ground-state reactant. In addition, the excited-state barrier for singlebond isomerization is smaller than that for double-bond isomerization, reducing the quantum yield for cis-trans isomerization (since the product of s-cis-s-trans isomerization is just a different conformer, not a new cis-trans isomer of the reactant). Thus the low experimental value of = 0.034 reported for the quantum yield of /ran.v-hexatriene from r/.v-hexatriene (Jacobs... 

High energy stable rotamers of hexatriene have also been theoretically studied . Two possible Cis/Trans isomers are possible with respect to the Cl=C2 bond (see Figure 2). For each of them, the rotation around the Cl—C3 and C2—C4 bonds can lead to s-trans and gauche conformations. The gauche-Trans-trans, trans-Cis-trans gauche-Cis-trans conformers have been found to be 3.0, 2.0 and 5.1 kcalmol above the most stable all-trans conformation, respectively. ... 

Photoisomerization of s-cis fixed conjugated dienes under UV irradiation usually gives cyclobutenes or, in the case of six-membered rings, hexatrienes, from bond cleavage. Such reactions are virtually impossible with a diene held in an s-trans conformation then, quite different reactions occur. ... 

Molecular orbital calculations on fluorinated butadienes and hexatrienes were used to model the effects of fluorination on the properties of poly(acetylene). Like poly(acetylene), "head-to-head" poly(fluoro- acetylene), (-CH=CF-CF=CH-), is predicted to adopt a planar, all trans structure, but poly(difluoro-acetylene) favors a non-planar skewed chain conformation. "Head-to-tail" poly(fluoroacetylene), (-CH=CF-CH=CF-) is predicted to favor a nearly planar cis structure stabilized by intramolecular CF-HC hydrogen binding. Calculations on 2-fluoroethanol and on both 2-fluoroacetaldehyde enol and its alkali metal (Li, Na, K) enolates reveal moderately strong intramolecular CF—HO hydrogen bonds(1.9 and 3.2 kcal/mol, respectively) and even stronger intramolecular coordination of CF to alkali metal cations (9-12 kcal/mol). 

We now consider the relative stability of 1,3,5-hexatriene in its all-trans and all-cis planar conformations. 1,3,S-hexatriene can be viewed, theoretically, as the result of the union of a central ethylenic fragment A and two ethylenic fragments B and... 

An alternative approach for determining the relative pi electronic stabilization of two torsional isomers utilizes a molecular dissection into two open shell radical fragments. This approach is illustrated by examining torsional isomerism in butadiene and 1,3,5 hexatriene. The n MO s of the conformational isomers of 1,3,5-hexatriene can be constructed from the union of the n MO s of two formal allyl radicals. The two regiochemical modes of union of interest wiU be designated cis and trans ... 

It has long been known that the interconversion of cyclooctatetraene (COT) and bycyclo[4.2.0]octa-2,4,7-triene (BCO) is rapid in both directions at relatively low temperatures (ca. 100°C) [19, 20] the product has a cis junction, so the disro-tatory pathway is evidently preferred. The conventional procedure would be to freeze one tt bond and regard the reaction as being an allowed disrotatory six-electron cyclization of the hexatriene moiety. One could, of course, freeze one pair of adjacent tt bonds instead, and produce the imns-joined product by a no less allowed conrotatory four-electron cyclization of the residual butadiene system. In view of the fact that adjacent double bonds of cyclooctatetraene (COT) are twisted away from each other in its stable tub conformation, the latter mode would seem to be the more facile, so the failure to observe the tran - o ned isomer has to be rationalized in terms of its lesser thermodynamic stability, rather than a symmetry-imposed potential barrier along the conrotatory pathway. 

Subtle predictions can be made about the relative energies of two isomers having comparable Kekule structures, such as the linear s-trans conformation and branched conformation of hexatriene, 18 and 22 in Scheme 9 above. For each of these isomers, we shall consider that the total n energy is a perturbation of... 

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