Conjugated polyenes planar

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... 

Among planar monocyclic fully conjugated polyenes only those possessing (4n + 2) TT electrons where n is a whole number will have special stability that be aromatic... 

Finally, before turning to a brief review of methodological and theoretical aspects, we mention that one of the distinguishing features of planar conjugated polyene radical cations (cf Section II.D) is that their EA spectra reveal a breakdown of the single-configuration picture for ionic excited states which had been used so successfully in interpreting their PE spectra (cf. Section II). 

The (An + 2) n electron standard follows from the pattern of orbital energies in monocyclic, completely conjugated polyenes. The tt energy levels were shown for benzene earlier in Figure 11.4 and are repeated in Figure 11.13b. Figure 11.13a and 11.13c show the T7 energy levels for square cyclobutadiene and planar cyclooctatetraene, respectively. 

Next we apply this simple model to the annulenes. These compounds are monocyclic conjugated polyenes with the general molecular formula (CH) with n even. Thus benzene may be considered as [6]-annulene. As n increases, essentially all [ ]-annulenes are non-planar. For instance, cyclooctatetraene, (CH)s, has the well-know tub structure. However, [18]-annulene is nearly planar, as shown below (bond lengths in picometers are displayed in bold italic font). 

The conjugated polyene, [18]annulene, IV, is a potential 4m+ 2 aromatic system. Because of steric repulsion between hydrogen atoms inside the ring, the molecule is distorted away from planarity. Nevertheless, its NMR shielding effects indicate an induced ring current in the mean molecular plane, once more in line with the decisive role of o-a-m. All evidence derived from the exclusion principle, conformational rigidity, aromaticity, electronic spectra and NMR shielding is therefore consistent with the alternative picture, and at variance with the conventional model. 

On the basis of his analysis Hiickel proposed that only certain numbers of tt electrons could lead to aromatic stabilization. Only when the number of tt electrons is 2, 6, 10, 14, and so on, can a closed-shell electron configuration be realized. These results are summarized in Hiickel s rule Among planar, monocyclic, fully conjugated polyenes, only those possessing (4 + 2) ti electrons, where n is an integer, will have special aromatic stability. 

Section 11.19 An additional requirement for aromaticity is that the number of tt electrons in conjugated, planar, monocyclic species must be equal to An + 2, where n is an integer. This is called Hiickel s rule. Benzene, with six TT electrons, satisfies Huckel s rule for n = 1. Cyclobutadiene (four tt electrons) and cyclooctatetraene (eight tt electrons) do not. Planar, mono-cyclic, completely conjugated polyenes are called annulenes. 

For monocyclic conjugated polyenes, high stabilization is found for systems with (4n + 2) TT electrons but not for systems with (4n) tt electrons. The relationship is formulated as Hiickel s rule, which states that completely conjugated planar hydrocarbons are strongly stabilized (aromatic) when they have 4n -I- 2) tt electrons. Benzene (6 tt electrons) is aromatic but cyclobutadiene (4 tt electrons) and cyclooc-tatetraene (8 tt electrons) are not. 

The eight-electron cyclic conjugated polyene is 1,3,5,7-cyclooctatetraene, which was first synthesized in 1911. Cyclooctatetraene is not much different in reactivity and stability from noncyclic conjugated polyenes. It has no aromatic characteristics. Structural studies determined that cyclooctatetraene is nonplanar, and its most stable structure is mb-shaped. This reduces the overlap between the rr bonds, and since the molecule is not planar, the HMO orbital pattern does not apply. Cyclooctatetraene is neither aromatic nor antiaromatic. 

Among the conjugated polyenes or phenyl-substituted olefins, the all-trans isomers can most readily adopt a planar configuration, and planarity becomes progressively more difficult to attain as the number of c double bonds in the molecule increases. This is due to greater intramolecular steric hindrance in the planar configuration of a cis olefin substituted by ethylenic (9) or phenyl (10) groups. 

A few non-conjugated polyenes have been studied by the force-field method. It was calculated that 1,4-cyclohcxadiene is planar (D2 ), although one electron diffraction study (Oberhammer and Bauer, 1969) on the molecule indicated a boat form (C2 ). An independent electron diffraction work indeed gave a planar structure (Dallingaand Toneman, 1967). The problem with the former electron diffraction study seems to have been a misinterpretation of the observed 3,6-distance, a problem related to what is sometimes referred to as shrinkage (see Bartell and Kohl, 1963 and references therein). This effect occurs because the atomic nuclei are undergoing vibrational motion. Thus, one mode of vibration of 1,4-cyclo-hexadiene is as shown in eqn (11). 

The intensity of the DFWM signal for the poly(l,6-heptadiyne)s increased linearly with intensity of the incident beam, as shown in Figure 10. From the direct comparison between the intercepts of the V axis of the polymers and the carbon disulfide reference. the values of the polymers were obtained. And also, as the concentration of the polymer solution was increased, the values of the polymers were increased (Figure 11). The third-order nonlinear optical properties were enhanced with an increase in the bulk of the substituents at the 4-position, which is in accord with the bathochromic shift of Amax in the UV—vis spectra. From these results, it was concluded that the incorporation of bulky substituents into the 4-position of 1,6-heptadiynes enforces the TT-conjugated polyene into the more planar conformation. resulting in a red shift and the increment of the values of the polymers. Recently, Schrock... 

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