Circularly polarized incident

Fig. 1. The basic ROA experiment measures a small difference in the intensity of Raman scattering from chiral molecules in right- and left-circularly polarized incident light. Reprinted from Barron et al., 2000, Prog. Biophys. Mol. Biol. 73,1-49, with permission from Elsevier Science.

Over the past decade two forms of vibrational optical activity have become established. One is called vibrational circular dichroism (VCD), the extension of electronic circular dichroism into the infrared vibrational region of the spec-tram. The first measurements of VCD were reported by George Holzwarth and co-workers at the University of Chicago in 1973 for crystals (3) and 1974 for neat liquids (4). In VCD one measures the small difference in the absorption of a sample for left versus right circularly polarized incident infrared radiation. The early stages of the development of VCD have been reviewed from several perspectives (5-8). 

The other form of optical activity in vibrational transitions is known as Raman optical activity (ROA). Here, also, one measures an intensity difference for left compared to right circularly polarized incident radiation however, optical activity in light scattering has no direct analog in electronic spectroscopy. ROA was first measured by Laurence Barron, A. D. Buckingham, and M. P. Bogaard in 1973 (9) and several reviews of the subject have since appeared (10-14). 

Fig. 7.1. Two equivalent ROA experiments involving Stokes vibrational Raman scattering at angular frequency oj — ojv in incident light of angular frequency oj far from resonance, a The ICP ROA experiment measures JR - JL, where JR and JL are the scattered intensities (shown here as unpolarized) in right- and left-circularly polarized incident light, respectively, b The SCP ROA experiment measures hi-h., where Jr and JL are the intensities of the right- and left-circularly polarized components, respectively, of the scattered light using incident light of fixed polarization (shown here as unpolarized)...

The difference in absorbance of left and right circularly polarized incident radiation (MCD), or the extent of the rotation of a beam of plane polarized light (MORD), with the sample located in a magnetic field aligned parallel to the axis of light propagation is observed. 

CD spectroscopy measures the difference in absorbance between left- and right-circularly polarized incident radiation, in the near ultraviolet to visible and infrared regions of the spectrum. 

This article reviews all the published work concerned with the study of vibrational optical activity in chiral molecules from measurements of a small difference in the intensity of Raman scattering in right and left circularly polarized incident light. The history and basic theory are described briefly, followed by an account of the instrumentation and the precautions that must be observed in order to suppress spurious signals. The various theories that have been proposed in order to relate stereochemical features to the observations are then outlined, this being followed by a survey of all reported Raman optical activity spectra. 

The Raman approach to vibrational optical activity is based on measurement of a small difference in the intensity of Raman-scattered light from chiral molecules in right and left circularly polarized incident light, and several reviews have appeared previously1 -S). However, another review is now timely because important experimental and theoretical developments have since brought Raman optical activity (ROA) to a new level of maturity. 

Hug and Surbeck 35) have proposed the use of Ada. = daL — daR, the difference of the Raman differential scattering cross sections in left and right circularly polarized incident light. This is the Raman equivalent of the circular dichroism Ae = el — eR, where s is the decadic molar extinction coefficient and, unlike the measured (but not the theoretical) IR — IL which depends on both sample and instrumental factors, is solely a molecular parameter. They introduced a chirality number q defined by... 

Several difl ereni types of cllipsometers arc available commercially. The earliest type was the null-type ellipsometer in which a circularly polarized incident beam was reflected off the sample surface onto an analyzer. The incident-beam polarization stale was chosen by a polarizer and compensator so that linearly polarized light was obtained after reflection. 1 he analyzer was then rotated until it was perpendicular to the polarization axis of the light coming from the sample as indicated by a minimum in the light iniciisiiy. Some instruments today still use the null principle, bui they are computer controlled and have charge-coupled-device (C (-I)f cameras as detectors. 

Fig. 6.7 a Molecular structures of the three components for b a 22-pm-thick gradient-pitch glassy liquid crystal film with (27) (28) (29) at 1.00 0.23 0.13 molar ratio through irradiation at 334 nm (140 pW/cm ) and 100 °C for 2 h using circularly polarized incidents for transmission measurement, and c 8-pm-thick constant-pitch films with (27) (28) (29) at 1.00 1.65 0.40 molar ratio through irradiation at 334 mn (70 pW/cm ) and 120 °C for durations as Indicated using unpolarized incidents for transmission measurement. Used with permission [41]... 

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