Optical rotation wavelength variation

FIGURE 16.6 Variation of optical rotation with wavelength in the region ofa hypothetical absorption band (shown as the dashed line). 

Variation of Optical Rotation 3.I.3.2.I. With Wavelength, Absorption... 

It has been remarked33 that measurement of the optical rotation is assumed by many chemists to be a trivial experimental procedure because the basic instrument is relatively simple and the process is readily adapted to undergraduate laboratories. The fact is, however, that optical rotations are not necessarily selfconsistent because variations can occur with any of the parameters often assumed to be constants, i.e., temperature, concentration, wavelength, solvent. 

Optical rotatory dispersion involves measuring the variation of optical rotation with wavelength. There is an abrupt reversal of rotation in the vicinity of an absorp- tion band. If the complex is initially levorotatory (Fig. 12.25a), the ORD curve fells to a minimum, rises rapidly to a maximum, and then slowly falls. If the complex was initially dextrorotatory, the effect is reversed with the ORD curve rising first to a ... 

Circular dichroism is closely related to optical rotatory dispersion, the variation of optical rotation with wavelength. Optical rotation depends upon the difference in refractive index (t)l tIr) between left-handed and right-handed polarized light. Rotation a... 

Thus far our discussion has centered upon rotatory dispersion in regions of relative transparency in which measurements are instrumentally feasible and theory reasonably secure. However, the featureless monotonic variation of optical rotation with decreasing wavelength changes radically at optically... 

In the case of o-glucose, there are practically only pyranoses in solution. Their interconversion can be formulated as a reversible reaction with a first-order law (1.3), Ca and Cj8 being concentrations (activities) of each anomer. The rate of disappearance is given by equation (1.4) which is integrated in the usual manner. A practical approach is to convert the concentration variations into optical rotation variations with a set wavelength, giving equation (1.5) where r and r represent the rotations measured for f = 0 and t = . 

Abstract The modified equation-of-motion coupled cluster approach of Sekino and Bartlett is extended to computations of the mixed electric-dipole/magnetic-dipole polarizability tensor associated with optical rotation in chiral systems. The approach - referred to here as a linearized equation-of-motion coupled cluster (EOM-CCl) method - is a compromise between the standard EOM method and its linear response counterpart, which avoids the evaluation of computationally expensive terms that are quadratic in the field-perturbed wave functions, but still yields properties that are size-extensive/intensive. Benchmark computations on five representative chiral molecules, including (P)-hydrogen peroxide, (5)-methyloxirane, (5 )-2-chloropropioniuile, (/ )-epichlorohydrin, and (75,45)-norbornenone, demonstrate typically small deviations between the EOM-CCl results and those from coupled cluster linear response theory, and no variation in the signs of the predicted rotations. In addition, the EOM-CCl approach is found to reduce the overall computing time for multi-wavelength-specific rotation computations by up to 34%. 

The optical activity of bile acids and their sodium salts depends on their purity, the wavelength of light employed, the solvent, and the concentration. The values given in standard references (27, 74) are often not comparable. Josephson (75) showed that increased concentration of bile salts in water substantially decreases the optical activity. The concentration, however, made little difference if the salts were studied in alcohol. Since the bile salts do not form micelles in alcohol (unpublished observations by the author), the variations noted in water may be related to the formation of micelles. The optical rotation of the acids in other organic solvents is not affected appreciably by concentration. The reader is referred to the earlier work of Sobotka (28) and Josephson (75). 

This equation predicts that the optical rotation diverges at a critical wavelength X =np. The variation of p with wavelength is illustrated schematically in Fig. 10. 

The relaxation time did not depend on the parameters used to follow the kinetics, but only on the final temperature. It was found to be identical using optical rotation, absorption spectroscopy at different wavelengths, and fluorescence. Furthermore, when an extrinsic probe was used (fluorescence or absorption variations of the anthranyloyl group in anthranyloyl chymotrypsin) the same relation time was observed. 

Many substances can rotate the plane of polarization of a ray of plane polarized light. These substances are said to be optically active. The first detailed analysis of this phenomenon was made by Biot, who found not only the rotation of the plane of polarization by various materials (rotatory polarization) but also the variation of the rotation with wavelength (rotatory dispersion). This work was followed up by Pasteur, Biot s student, who separated an optically inactive crystalline material (sodium ammonium tartrate) into two species which were of different crystalline form and were separately optically active. These two species rotated the plane of polarized light equally but in opposite directions and Pasteur recognized that the only difference between them was that the crystal form of one was the mirror image of the other. We know to-day, in molecular terms, that the one necessary and sufficient condition for a substance to exhibit optical activity is that its molecular structure be such that it cannot be superimposed on its image obtained by reflection in a mirror. When this condition is satisfied the molecule exists in two forms, showing equal but opposite optical properties and the two forms are called enantiomers. 

All of the studies discussed above for silver have been done with an incident beam of 1064 nm. These studies have proven that the anisotropy in the nonlinear polarizability from the silver surface is not purely free-electron-like at these wavelengths, that the anisotropy can be correlated with surface symmetry, and that the SH response measured in situ is nearly identical to that measured in UHV. The issue of the sensitivity of the rotational anisotropy to surface electronic properties has been the topic of very recent work which has been conducted by variation of the incident wavelengths to where optical resonances in the bulk or surface electronic structure can be accessed. 

In the exciton-photon interaction, the translational molecular motions have negligible effects owing to the small amplitude of the translation compared to the optical wavelength. In contrast, the molecular rotations may cause an important variation of the transition dipole the librations may be strongly coupled to the incident photon via its coupling to the exciton. If DX(R) is the transition dipole of an a molecule in a unit cell, the first-order expansion in the libration coordinate 8 around the u axis will give... 

Most TTSs are cool stars. Therefore, their radiation is sensitive to temperature variations on their surface, especially at short wavelengths. The existence of surface inhomogeneities (cool and hot spots) of rotating stars leads to a periodic modulation of the flux illuminating the CS dust and, as a result, to the periodic modulation of the intrinsic polarization of stars. Its amplitude depends on the amplitude of the light modulation as well as on the CS disk parameters including its inclination to the line-of-sight and the optical parameters of CS dust. 

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