19] Raman optical activity

Abstract

Raman optical activity (ROA) is the difference in the intensity of Raman scattered light with respect to right and left circularly polarized incident or scattered radiation, or both. The form of ROA defined with circular polarization modulation of the incident laser was first described theoretically in 1971 and was first measured in 1973. As with other forms of natural optical activity, a molecular sample must be chiral to possess a nonzero ROA spectrum. The measurement of an ROA spectrum requires particular care to avoid the presence of optical artifacts since ROA intensities are small, being typically three to four orders of magnitude below those of the parent Raman spectrum. The value of ROA as a spectroscopic technique lies in its particular sensitivity to chiral molecular environments. After nearly two decades of steady development, ROA is now emerging as an important new spectroscopic tool in the study of the stereo conformations of optically active molecules, including biological macromolecules and metal complexes. Advances in three principal areas have revolutionized the measurement of ROA spectra. These are (1) the use of alternative scattering geometries including backward scattering, (2) the use of charge-coupled device (CCD) detectors as nearly ideal, low-background, large area, multichannel detectors, and (3) the introduction of new polarization modulation techniques involving the measurement of circular polarization intensity differences associated with the scattered beam.