Rotation observables

Specific rotation is the number of degrees of rotation observed if a 1-dm tube is used and the compound being examined is present to the extent of 1 g per 100 mL. The density for a pure liquid replaces the solution concentration. 

The amount of rotation observed in a polarimetry experiment depends on the number of optically active molecules encountered by the light beam. This number, in turn, depends on sample concentration and sample pathlength. If the concentration of sample is doubled, the observed rotation doubles. If the concentration is kept constant but the length of the sample tube is doubled, the observed rotation is doubled. It also happens that the amount of rotation depends on the wavelength of the light used. 

Mutarotation (Section 25.5) The change in optical rotation observed when a pure anomer of a sugar is dissolved in water. Mutarotation is caused by the reversible opening and closing of the acetal linkage, which yields an equilibrium mixture of anomcrs. 

Let us now apply the technique in some specific cases. The existence of optical activity of several compounds in solution has been explained due to the presence of several active forms of the compound in equilibrium with each other and various assumptions about the forms were also put forward. The equilibrium between the different forms depended on external conditions. But a definite explanation was put forward in 1930 about tartaric acid and it was said that the molecule exists in the following three conformations and each of which makes a certain contribution to the rotation observed. 

Optical activity is the ability of a compound to rotate the plane of polarized light. This property arises from an interaction of the electromagnetic radiation of polarized light with the unsymmetric electric fields generated by the electrons in a chiral molecule. The rotation observed will clearly depend on the number of molecules exerting their effect, i.e. it depends upon the concentration. Observed rotations are thus converted into specific rotations that are a characteristic of the compound according to the formula below. 

The decrease in rotation observed on changing the solvent to dimethyl sulfoxide is probably reflecting a decreased population of 5 c. The molecular rotation of 42° for 5 in 1,2-dichloroethane suggests a change in the equilibrium toward a situation such as,... 

Since the specific rotation of starch saccharified under these conditions has been found by experiment to be [a]D — + 20 2 °, the percentage of starch, a, in the flour is given by a == 9 9 P, P being the rotation observed. 

Polis DL et al (1999) Shear-induced lamellar rotation observed in a diblock copolymer by in situ small-angle X-ray scattering. Macromolecules 32(14) 4668—4676... 

To be effective, the light sources used for chiroptical detection systems must have an intensity that is much greater than those ordinarily used in polarimetry. This comes about because the angular rotations observed for the very low analyte concentrations and very short sample pathlengths typical of an analytical liquid chromatograph, are extremely small (mdeg. and less). Conventional light sources have been replaced with laser illumination but these are not without problems, a major one of which is the instability in the emission [23],... 

Instead, suppose the x and y axes were themselves precess-ing clockwise (when viewed from above) around the z axis at the same frequency the nuclear spins are precessing. Further suppose we, the observers, were precessing around the z axis at the same frequency. To differentiate this rotating coordinate system from the fixed (i.e., laboratory frame) system, we will use labels and z to represent the three rotating axes (the z axis is coincident on and equivalent to the z axis). To us rotating observers, the rotating axes and B, appear stationary, and M will rotate in the plane perpendicular to B. These relationships are shown in Figure 2.9,... 

Fig. 2 (a) 120° unidirectional chemically driven rotation of a triptycene-helicene molecular motor (b) stylized representation of the process from (a) and representation of the sense of triptycene rotation observed on the side opposite to helicene... 

Fio. 3.—Correlation Between Specific Rotation and Content of (1 —> 3)-like Linkages in Dextran. (Key 9, rotation observed in formamide , rotation observed in N potassium hydroxide and calculated to formamide by assuming that all dextrans show the same relative difference in rotation in these solvents as do dextrans B-512 and B-1356 S. The numbers designate different strains of micro-organisms in the Northern Utilization Research Development Laboratories at Peoria, Illinois.)... 

Studies of arm-off (COD)Rh[2,2 -C6H3(CH2NMe2)2] (68) have also suggested interconversion proceeds via a T-shaped three-coordinate intermediate. Other apparent rotations observed in [(COD)Rh(bidentate)]+ complexes appear to via breaking of Pd-N or Pd-P bonds. ... 

Finally, the optical rotations observed during hydrolysis of the fully methylated a- and -dextrins in 51 % sulfuric acid approached the rotation of 2,3,6-tri-0-methyl-T)-gluco8e in the same solvent,... 

A value E 12.4 1.5kcal mole" was obtained. Since this value Is comparable with the values of energy barriers to rotation observed for aromatic ketones (measured In a polystyrene matrix) which also show loss maxima around SxlO Hz, It Is... 

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