Chiroptical properties temperature effects

The problems of conformational averaging and of considering solvent effects are interconnected. For instance, one of the more obvious effects when going from the gas phase to solution is the resulting change of mole fractions for solutes with multiple conformers [115, 159]. One example of this is epichlorohydrin, which is discussed elsewhere in this chapter. A conformer distribution with several thermally accessible conformers that have considerably different ORs and ECD spectra might also result in a marked temperature dependence of measured and simulated chiroptical properties. 

A very detailed description of solvent effects, along with temperature effects from nuclear motion (typically with classical nuclei), is obtained from average computed chiroptical response properties computed along the trajectory of a... 

Helical Poly(phenylacetylenes). The most widely studied helical-substituted polyacetylenes are based on poly(phenylacetylene) with chiral ring substituents. Pol5unerization of chiral phenylacetylenes was first reported in 1995 (72). 4-(-)-Menthoxycarbonylphenylacetylene (41) was subjected to the polymerization with several transition metal catalysts such as [(nbd)RhCl]2 and WCle. The resultant Rh-based polymer shows a large optical rotation and intense CD effects in the electric absorption region of the main chain. The polymer, thus, exists in a helical conformation with an excess of one-handed screw-sense. Poly(phenylacetylene) with small chiral pendants, poly(42), in contrast, displays poorer chiroptical properties. Interestingly, an increase in temperature steeply increases the optical rotation of poly(41) if the polymer is produced with a W catalyst. Such a drastic enhancement of chiroptical properties is not observed in the case of Rh-based poly(41). 

Marchessault, Sarko etal. [10, 11] have investigated the chiroptical properties of amylose xanthates extensively. The CD of amylose triacetate in trifluoroethanol shows, apart from the expected acetyl z -> tt transition at about 230 nm, an additional shortwave peak. This is again explained by assuming a helical conformation of the amylose chain. Dextran acetates and mycodextran acetates, on the other hand, do not show conformational effects. The considerable changes in the CD spectra of amylose triacetate films when the temperature increases from 30 to 190°C are particularly remarkable. As X-ray diffraction analysis shows, these effects are connected with crystallisation processes and transformation during annealing. 

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