Cotton effects exciton coupled

Figure 13. Definition of exciton chirality. Summation of the two Cotton effects (broken lines) separated by Davydov splitting A). gives the curves shown in solid lines. Adapted from N. Harada, K. Nakanishi. Circular Dichroic Spectroscopy - Exciton Coupling in Organic Stereochemistry, University Science Books. Mill Valley, California. 1983...

Exciton coupling between the two styryl chromophores in each of the conformers (A)-(C) leads to a negative Cotton effect at around 270 nm, as shown by theoretical analysis using the 71-electron SCF Cl dipole velocity MO method. Therefore, the tram-2,.1-disubstituted spirocyclo-propane-1,9 -fluorene derivative 6, from which the distyryl derivative was obtained, must have accordingly the 2S3S absolute configuration122. 

The application of exciton coupling between the benzoate and the s-trans-diene or -enoate chromophore in axially dissymmetric molecules derived from hydroxy-substituted adaman-tanone allowed determination of not only the absolute but also the relative configuration. All benzoates 2-5 of 4R configuration show negative exciton Cotton effects, with amplitudes lower for (2 )-adamantylidene compounds 2, 4 compared with 2Z-isomers 3, 5, in which the two interacting chromophores are at a closer distance143. 

The absolute configuration of axially chiral (phenylmethylene)cyclobutane derivative 6 is similarly determined from the exciton Cotton effect due to the coupling of the styrene and l,f-biphenyl chromophore transition dipoles144. 

Figure 17. Negative benzoate Cotton effect due to exciton coupling with the n-n transition of the olefinic chromophore.

Exciton coupling is also observed in methacrylate derivatives of allylic alcohols151 and in a benzoate derivative of a homoallylic alcohol152. For benzoates of propargylic alcohols of S configuration 11 in which the O-Bz bond is assumed anti to the C-R1 bond, a Dc Voe calculation predicts a positive benzoate Cotton effect, in accordance with experimental153. 

The pairwise additivity principle when applied to bichromophoric exciton-coupled systems, such as those composed of 4-bromobenzoate (BB) and 4-methoxycinnamate (MC) derivatives of cyclic polyols, yields more information from the CD spectra of complex molecules173. In such systems both degenerate ( homo") interactions, i.e., BB/BB and MC/MC, and nondegenerate ("hetero") interactions, i.e., BB/MC, contribute to the exciton CD spectra. As the homo exciton Cotton effects arc well separated (BB 236/253 nm, MC 287/322 nm) a unique pattern of CD curves is obtained for each derivative of cyclic tri- or tetraol. This method is particularly useful in identification of glycopyranosides174, a procedure essential for microscale structure determination of oligosaccharides by the exciton chirality method175. 

Accordingly, this rotamer provides a positive rotatory contribution due to exciton coupling, although the amplitude of the exciton Cotton effect is smaller than that in the cyclic system, due to conformational averaging . 

The chirality of [2.2]paracyclophane derivatives has been deduced as being (—)(R) on the basis of the exciton theory of coupled oscillators 67) and confirmed by experimental results (see 2.9.1 and 2.9.3). In these compounds a negative Cotton effect at 270 nm (corresponding to the p-band) seems to be specific for the (R)-chirality 54). 

The pure contribution from exciton coupling in testosterone 17/Mp-chlorobcnzoatc) (152) was estimated by subtracting the CD spectra of exciton interaction-free 17/J-hydroxy-4-en-3-one (153) and 4-en-3/3-ol-17/Mp-chlorobenzoate) (154) from the experimental CD spectrum of 152. The exciton CD curve (+16.2 (247), —12.8 (230)) obtained is much more symmetrical, as required by the theory349. Several more examples of benzoates and sorbates of steroidal 4-en-3-ones were treated in similar way, thus smoothing the imbalance of the exciton Cotton effects due to contributions of component chromophores. 

No exciton coupling was observed for the dialdehyde 165. The CD of (+)-166 also shows a simple pattern. The small amplitude of those Cotton effects can be attributed to the complicated polarization spectra of benzophenone chromophore and to the conformational flexibility of the 2-tolyl group. In contrast to (+)-165 and (+)-166, the quinone (+)-167 exhibits relatively strong Cotton effects ascribed to exciton interaction between favorably oriented transition moments in the 9,10-anthraquinone chromophore359. 

Rigorously, ORD and CD spectra are related through the Kronig-Kramers theorem, a well-known general relationship between refraction and absorption, i.e. nL - nR is determined by eL - % for A from zero to infinity [128], (The analogous relationship between refraction and reflection applies to cholesteric liquid crystals.) Hence, in order to maximize ORD in the transparent region, Cotton effects, associated with exciton coupling (both intramolecular and intermolecular), have... 

Figure 35. (a) Stable chair conformation of methyl a-D-mannopyranoside and (b) exciton coupling CD Cotton effects of its 2,3,4-tris-p-chlorobenzoate. (c) Pairwise couplings of the 2,3,4-tri-p-chlorobenzoate with (-) chiralities noted for each. 

In contrast to the trans-disubstituted phenylcyclopropanes cis-disubstituted phenylcyclopropanes, such as 23 and 30, do not show any detectable CD signals a. This is reasonable, if the optical activity of the two lowest energy electronic bands of phenylcyclopropanes are generated essentially by the coupled-oscillator mechanism (exciton mechanism for the degenerate case, such as 19). In cis-phenylcyclopropanes the interacting transition moments will lie (approximately) in the same plane. Hence, in the coupled-oscillator mechanism there will be only a very small Cotton effect (which is zero, if... 

A full report of the c.d. of steroidal diol bis-(p-dimethylaminobenzoates) confirms the reliability of theoretical calculations of the coupled Cotton effects of the remote ester groups (exciton chirality method). The c.d. curves show two maxima of opposite signs, separated by some 27 nm, and with intensity inversely proportional to the square of the interchromophore distance. The profile of the observed c.d. curve results from the superimposition of two component curves, each of asymmetric shape.A review of the uses of chiroptical techniques for structural and conformational studies includes examples of the assignment of stereochemistry to steroids and terpenoids, among a wide variety of natural products. The Octant Rule for carbonyl compounds and the rules applicable to other chromophoric systems are discussed critically. 

The qualitative definition of exciton chirality is very simple (1) if two transition moments constitute a clockwise screw sense, CD shows positive first and negative second Cotton effects. On the other hand, (2) if they describe a counterclockwise screw sense, negative first and positive second Cotton effects are observed. In most cases, intense exciton-coupled CD Cotton effects are observed at the long-axis-polarized transition, and therefore the above results are rephrased as follows ... 

Figure 10 Typical pattern of exciton-coupled CD Cotton effects and UV absorption band.

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