Circular dichroism planar conformation

As chiral molecules, CD hosts are one of the best chiral selectors [23]. They can also induce circular dichroism signal in an achiral guest. As shown by Zhang and Nau [24] for the complexes of p-CDwith bicyclic azaalkanes 290 or 291, this effect may allow one to determine the orientation of the latter molecule in the CD cavity. An interesting example of the influence of 11 on the guest conformation was reported by Brett and coworkers [25]. They have shown that p-amino-/ -nitrobiphenyl 292, which is planar in the solid state, becomes nonplanar in the solid state complex with 3-CD. 

Computational efforts to describe the conformational preferences of (R,R)-tartaric acid and its derivatives - mainly for isolated molecules - were made recently [18-25]. The conformations of these molecules also attracted attention from experimental chemists [22-40]. (/ ,/ [-tartaric acid and its dimethyl diester were observed in crystals, in conformations with extended carbon chain and planar a-hydroxy-carboxylic moieties (T.v.v and Tas for the acid and the ester, respectively) [25-28] (see Figure 2). The predominance ofthe T-structure was also shown by studies of optical rotation [31], vibrational circular dichroism (VCD) [23], Raman optical activity [32, 35], and nuclear magnetic resonance (NMR) [22, 33, 34]. The results of ab-initio and semiempirical calculations indicated that for the isolated molecules the Tsv and T as conformers were those of lowest energy [22, 21, 23, 25]. It should be noted, however, that early interpretations of NMR and VCD studies indicated that for the dimethyl diester of (/ ,/ [-tartaric acid the G+ conformation is favored [36-38]. 

The colour of ICPs can also be sensitive to temperature. Substituted polythiophenes, in particular, show a marked blue shift of the highest wavelength absorption band when films or solutions are heated [7-9]. These reversible colour changes have been attributed to a twisting of the polymer backbone to a less ordered non-planar conformation. Less dramatic thermochromic effects have also been reported for polyanilines [3,10,11] where circular dichroism studies of... 

Experimental data show that the rings of oxazole and 4,5-dihydrooxazole are planar. Oxazo-lidines, on the other hand, are not flat, and their conformation depends on the nature and position of substituents. The conformations of 4- and 5-oxazolidinones were studied using circular dichroism <83X3139,85T603>. 4-Oxazolidinones exist predominantly in the envelope conformer (29). As the size of the C-2 substituents increase, this conformer is destabilized. 4-Substituted-3-alkylsulfonyl-5-oxazolidinones exist mostly in conformer (30), in which the C-4 substituent is in the axial position, but conformer (31) also contributes. The equilibrium shifts towards the diaxial conformer as the size of the C-4 substituent or the alkylsulfonyl group increases. In contrast to these results, an x-ray analysis of crystalline (32) revealed an almost planar structure <92HCA913). 

We want to be able to deduce the conformation of a polymer in solution from its measured circular dichroism spectrum. For a linear polymer each ordered conformation must be a helix. The limiting cases for a helix are a straight line and a circle. A helix of identical residues is characterized by very few parameters. These are the radius of the helix, the rise per residue (the distance along the helix axis) and the number of residues per turn. A few examples will make this clear. A planar, all trans polyene will have a radius of zero (the helix is a straight line), a rise of 1.44 A and one residue per turn. A polypeptide a-helix has a radius of 2.28 A at the a-carbon, a rise per residue of 1.50 A and 3.6 residues per turn. One strand of a polynucleotide helix in the Watson-Crick B-form of DNA has a radius of 5.72 A at Cr of deoxyribose, a rise per residue of 3.4 A and 10 residues per turn. 

Hjertberg et al. ° have reported the circular dichroism (CD) spectrum of the related chiral polythiophene 30, which contains a chiral amino acid group attached to the thiophene units by an ether linkage. In water solvent, a pair of exciton-coupled CD bands are observed in the visible region associated with the k-k transition band in the UV-visible spectrum at ca. 430 nm. These chiroptical features are attributed to a helical conformation adopted by the polymer backbone under the influence of the chiral side chain. However, in methanol solvent no CD bands are observed and the tt-tt absorption band is blue shifted. This suggests a change in polymer chain conformation in methanol solvent to a more planar arrangement. 

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