Diene chromophore

Dieser Kohlenwasserstoff wurde daher von Bohlmann u. Mitarb. (47) naher untersucht. Nach FeinchrOmatographie und anschlieBender Tieftemperatur-Kristallisation konnte die Substanz als bei 18° schmel-zende Nadeln rein erhalten werden. Sie zeigte ein UV-Spektrum mit Maxima bei 347 335,5 324,5 305,5 288 269 258,5 mp (s = 35100, 12200, 40300, 25100, 12700,101000, 49700) (in Petrolather), das einem En-triin-en- bzw. Triin-dien-Chromophor zuzuordnen war. Durch IR-... 

We give here only a brief summary of the electronic absorption spectrum of the diene chromophore which has been extensively treated by Gross and Schnepp5 with reference to a- and /1-phellandrene. 

In molecules like 2, or 3, the 1,3-diene chromophore is planar. Formally, the presence of R differentiates the two vertical halves of the molecule, which becomes chiral10. The physical meaning of this differentiation is that it induces dissymmetric vibrations, which determine, in turn, a dynamic twist of the chromophore. 

We shall consider here in more detail two models first a dynamic coupling approach, due to Weigang33, who considered optical activity deriving from the coupling of electric dipoles (the diene chromophore and the polarizable bonds around it) and second, a localized orbital investigation, which permits one to separate the contributions from the intrinsic diene optical activity and from the axial substituents. 

Furthermore, for both 28 and for 26, MM calculations indicate that even the individual double bonds are highly twisted (15-20°). This could be (vide infra) a further mechanism giving rise to optical activity of the tt-tt transition of the diene chromophore. 

Mason and coworkers10 studied the chiral bicyclic derivatives 2, 3, 33 and 34, having known absolute configuration. These molecules possess a planar s-cis diene chromophore and formally their chirality is due to the presence of the D or CH3 substituents, which rule out all the symmetry planes. However, it is interesting to point out a peculiar structural... 

Here ei and e2 are the unit vectors of oscillator 1 (the diene chromophore) and 2 (the C-CH3 dipole), respectively R12 is the vector joining their midpoints, G12 is the point-dipole interaction term and N is the Avogadro number all quantities are expressed in c.g.s. units. In the absorption region of the chromophore, where the perturber does not show a significant absorption, the above formula reduces to the simpler equation410... 

The first report of the chiroptical properties of a planar s-fraws-diene chromophore is due to Di Corato12, who described the CD data of (+)-(S)-6, which shows a positive weak (Ae +0.2) Cotton effect at about 220 nm, in both the E and Z isomers. Lardicci and coworkers13 described in 1978 the absorption and CD spectra of the planar s-fraws-diene derivatives 7 and 41. 

Christl and Lang (390) noticed an upheld signal shift of about 10 ppm when they compared the methine carbon atoms in bicyclo[1.1.0]butane (287) (391) and octavalene (288). On the other hand, in benzvalene (289), the corresponding shielding is +48.3 (203), because in this molecule back-donation from the Walsh orbitals to the ir -orbital of the olefinic group (290) is conceivable. Such an interaction is not possible in 288, because the ir -orbital of the diene chromophore is of different symmetry (390). 

The stereochemistry and the presence of substituents also influence UV absorption by the diene chromophore. For example ... 

The UV data indicate nonplanar geometry in the diene chromophore. Chernyshev et al. found an unsaturated C-containing ring ... 

CXXXI. Its properties are those to be expected of such a structure for example, the UV-spectrum shows absorption at 224 m/u. (e = 14000) corresponding to one diene chromophore in the C4o-molecule, and oxidation by osmium tetroxide-sodium chlorate followed by periodate fission gives both acetaldehyde and formaldehyde. The Hofmann degradation can be completed by treatment of descurarine with alkali to yield the ditertiary ether base CXXVIII (129). 

As a model compound for the theoretical calculation of CD spectra, we adopted the molecule (12bS)-26 having the essential part of the Jt-electron system contained in the naphthalene-diene compounds 22-25.1 Namely, in addition to the naphthalene and conjugated diene chromophores, the lone-pair electrons of the two methyl ether and furan ring oxygens are also included. The absolute configuration of 26 was arbitrarily chosen to be 12b5 for the calculation. The molecular geo-... 

B. Chemistry and Physical Properties.—The Pyro Chromophore. The apparent diene chromophore in the pyro compounds derived from the aconitine family of alkaloids was first observed in 1952. It was not until 1960, when the structure of the pyro-compounds was secure, that an explanation for this was proposed. For the case of pyrodelphonine the proposed excitation is illustrated in (24 R = OMe) (25). Since then, other examples of this type of chromophore have... 

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