Optical rotation computational techniques

Structural information at the molecular level can be extracted using a number of experimental techniques which include, but are not restricted to, optical rotation, infra-red and ultra-violet spectroscopy, nuclear magnetic resonance in the solid state and in solution, diffraction using electrons, neutrons or x-rays. Not all of them, however, are capable of yielding structural details to the same desirable extent. By far, experience shows that x-ray fiber diffraction (2), in conjunction with computer model building, is the most powerful tool which enables to establish the spatial arrangement of atoms in polymer molecules. 

The measurement of the optical rotatory power of chiral substances has been of major importance in the characterization of the enantiomeric purity. A number of computational techniques have been developed in the last year to evaluate this property. A recent review [142] shows in detail the advances in this field. Application of the new implementation of the evaluation of the optical rotatory power has allowed to the study of the conformational [143-146] and solvent effects [147,148] on the magnitude and sign of the optical rotation power. 

In addition to spectrophotometric or spectroscopic measurements, there are a number of other optical measurements that can be used to monitor the course of various reactions. Among the optical properties that can be used for these studies are optical rotation, refractive indices, fluorescence, and colorimetry. There also are several electrical measurements that may be used for analysis of solutions under in situ conditions. Among the properties that may be measured are dielectric constants, electrical conductivity or resistivity, and the redox potential of solutions. These properties are easily measured with instrumentation that is readily interfaced to a computer for data acquisition and manipulation for analysis. However, most of these techniques should be used only after careful calibration and do not give better than 1% accuracy without unusual care in the experimental work. 

The potent Sch 54445 (1085, Fig. 13.38, proposed structure shown) was reported by Chu and co-workers in 1997 after being isolated from the fermentation broth of Actinoplanes sp. (692). This is a member of the albofungin family, possessing both a xanthone and an isoquinoline component. The structure was proposed as 1085 based on information derived from a variety of spectroscopic techniques, although a combination of NOES Y data and CD spectra with computational techniques proved not sufficient to elucidate the stereochemistry around the G-ring. However, an anti-configuration was proposed based on the similarity of the optical rotation to albofungin, a compound for which the stereostructure has been established. 

Quantum mechanical computation for electronic circular dichroism and vibrational optical activity With the exception of the exciton chirality method for the development of sector and helicity rules, reference compounds are needed. Their absolute configuration has been usually obtained by X-ray analyses. The progress in numerical techniques of quantum mechanics nowadays allows performing computations of the rotational strength and its sign and, thus, the CD spectra with high reliability for CD... 

---