Rotatory strengths

A particularly useful probe of remote-substituent influences is provided by optical rotatory dispersion (ORD),106 the frequency-dependent optical activity of chiral molecules. The quantum-mechanical theory of optical activity, as developed by Rosenfeld,107 establishes that the rotatory strength R0k ol a o —> k spectroscopic transition is proportional to the scalar product of electric dipole (/lei) and magnetic dipole (m,rag) transition amplitudes,... 

When the cyclohexanone conformation deviates from the chair, the substituent contribution changes for the a-methy group the rotatory strength varies in a regular way with the... 

Many chromophores are suitable for use in the exciton chirality method. One of the features required for such a chromophore is its planarity or near-planarity. Nonplanar (inherently dissymmetric) chromophores would contribute to the CD spectra by other mechanisms. The other limiting factor is the position of the transition in the spectral region studied. For example, the 1B transition in the alkyl-substituted benzene chromophore appears near the short-wavelength recording limit around 200 nm, making its use in the exciton chirality method less attractive. Furthermore, the direction of polarization of the lB transition in alkyl-substituted benzene derivatives is not readily determined. In such cases calculation of the rotatory strength is more reliable than qualitative analysis. 

These rotatory strengths, when combined with the reported optical purities, give [a]aba 426° and 412° for [22.10] and [26.10] betweenanenes, respectively. Since these values automatically indicate their ridiculously high [M],bs ca. 2000, the [a]D apparently are to be reduced to 1/10 th of the reported values. 

The induced magnetic dipole moment has transformation properties similar to rotations Rx, Rt, and Rz about the coordinate axes. These transformations are important in deducing the intensity of electronic transitions (selection rules) and the optical rotatory strength of electronic transitions respectively. If P and /fare the probabilities of electric and magnetic transitions respectively, then... 

In methanol, the extinction of the absorption at 241 nm is less than 40 % of the value in isopentane with little change on lowering the temperature. In isopentane, the extinction drops significantly to the value found in methanol when the temperature is lowered. The data on the temperature-dependent CD in methanol can be interpreted on the basis of a temperature-dependent equilibrium between two chiral species. The change of the rotatory strength appears to be AG° = —2.0 KJ mol-1. This phenomenon is interpreted by the assumption that the cis-conformer about the C(6)-C(7) bond is favored in methanol and, at lower temperatures, in isopentane. 

TABLE 3. Reduced rotatory strengths [R] and D—C —C=G torsion angles for the n — n transition of vi dcutcnokctoncs... 

Experimentally, it has been customary to calculate the so-called rotatory strength, R, of a transition. The integrated rotatory strength of a transition b is related to As as follows ... 

Another open area is the assignment of the various electronic transitions observable in the absorption, CD, and ORD spectra. Owing to the fact that bands in chiroptical spectroscopy have opposite signs, maxima can be more easily identified in CD and ORD spectra than in absorption spectra. Also, such information as the rotatory strength can be used for band assignment, a necessary condition for a widespread use of chiroptical spectroscopy for correlation of configurations. 

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