Complex rotatory power

Comparing this expression with Eq. (4.17), one finds for the complex rotatory power... 

Finally, we may simultaneously express the birefringence and the dichroism by defining the complex rotatory power < > ... 

The chiral discrimination in the self-association of chiral l,3a,4,6a-tetrahydroi-midazo[4,5-d]imidazoles 3 has been studied using density functional theory methods [37], (Scheme 3.20). Clusters from dimers to heptamers have been considered. The heterochiral dimers (RR SS or SS RR) are more stable than the homochiral ones (RR RR or SS SS) with energy differences up to 17.5 kJ mol-1. Besides, in larger clusters, the presence of two adjacent homochiral molecules imposes an energetic penalty when compared to alternated chiral systems (RR SS RR SS...). The differences in interaction energy within the dimers of the different derivatives have been analyzed based on the atomic energy partition carried out within the AIM framework. The mechanism of proton transfer in the homo- and heterochiral dimers shows large transition-state barriers, except in those cases where a third additional molecule is involved in the transfer. The optical rotatory power of several clusters of the parent compound has been calculated and rationalized based on the number of homochiral interactions and the number of monomers of each enantiomer within the complexes. 

In modem organic chemistry optical-rotatory dispersion, the variation with wavelength of optical-rotatory power (and certain related properties), is used in molecular-structure investigation as a means of identifying and characterizing chromophore groups. Automatic polarimetric spectrophotometers of high complexity have been developed for this purpose. 

A number of complexes of molybdic acid, particularly those -ith acetylacetone (Mo02[CH(COMe)2]2), with salicylaldehyde, and with organic acids, have been described, and a number of measurements made of rotatory power. ... 

In all the sugars we usually find a carbon united with hydrogen, with hydroxyl, and with two complex radicals if the radicals are different, as very generally happens, the sugar in question should be active. It has been observed, in fact, that most sugars possess rotatory power. 

Complex-formation with a cation does not, in itself, affect the optical rotatory power of a carbohydrate. However, complex-formation is often accompanied by a change of conformation that causes a change in the optical rotation. For example, the rotation of D-glucitol and, to a lesser extent, of D-mannitol is affected by the presence of cations, in the order Na" " < Mg < Zn " " < Ba " < Sr " " < Ca (see Section III,1). The optical rotation of methyl jS-o-ribopyranoside and ) D-lyxopyranoside would, undoubtedly, be substantially changed by complex-formation, but this experiment has, apparently, not been reported. Therefore, a chmige of optic rotation on addition of a salt can be regarded as proof of complex-formation, but lack of such change does not necessarily indicate that no complex is formed. 

When 0 > e, i.e., within the region of total reflexion, p given by (4.1.39) becomes complex, showing that the medium is now circularly dichroic. The real part which represents the rotatory power is... 

When < a K, p becomes a complex quantity. The real part gives the rotatory power and the imaginary part the circular dichroism. Since no dissipative mechanism is built into the model it follows that the imaginary part of p is associated with the reflexion of one of the components. The reflexion band is centred at x = 0, i.e., = q or A, = nP where A, is the... 

THE ELECTRONIC SPECTRA, OPTICAL ROTATORY POWER, AND ABSOLUTE CONFIGURATION OF O-PHENANTHROLINE AND 2,2 -DlPYRIDYL COMPLEXES,... 

Krai, M. Optical rotatory power of complex compounds. Matrix elements of operators Del and R x Del. Collect. Czech. Chem. Commun. 35, 1939-1948 (1970)... 

The optical rotation is complex if 5eft >q, in which case the real part corresponds to the rotatory power and the imaginary part to circular dichroism. As stressed previously, there is no spectral feature contributing to this imaginary or loss term therefore, the imaginary part of j//d has to be associated with the reflection of one of the components in a range bound by (5 e) 9 to > (8 e) and centered atq=0,... 

The optical purity is not known, the rotatory power is lower than for the one reported for the polymer obtained with a i/-camphor complex, nevertheless the reduction is total in our case. 

Optical rotations [a] d of the two polymers were found to be different, although identical chemical structures (except end groups) were expected according to the literature [34]. In order to clarify the origin of these differences, ORD measurements of polymers and of the monomer (VI) taken as a suitable model compound were undertaken. Rotatory powers and ORD were found to be strongly dependent on the polycondensation medium (Figure 6). In a first step it was found that the polymer prepared in alkaUne condition and the model compound showed very similar ORD in dioxane and both obeyed the one-term DRUDE equation. In contrast, ORD of the polymer prepared in acidic condition was complex and obeyed the Moffitt equation withflio> and Xq constants respectively +492,-755 and 212 quite similar to constants found for helical polypeptides [35, 36]. 

A mean value of Kc = 32 Imole" was found, and the molar specific rotatory power of the complex. The contribution of the second phenomenon to the total optical activity was evaluated by difference between experimental data and those calculated for the complex above (Figure 16b). It was attributed to a solvent-like effect [55]. Of course, this solvent-like effect disappears as the salt excess decreases since the concentration of the complexed form also decreases according to the equilibrium equation (1). This explains the vanishing of the effect at low r values (Figure 16b). 

A wellknown example is given by the ionisation of poly L-glutamic acid. Inversion of the size of the rotatory power in the visible region [24], the change from complex ORD to quasi simple ORD [25] and the change from three main dichroic bands to one only [26] result from the conformational transition from the helix-a unionized form to the random coil ionized form. A similar behaviour has been observed for a stereoregular polybase poly L-lysine [26]. 

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