Raman optical activity measurement

Chiral objects absorb left and right circularly polarized light to slightly different extents. Tltis phenomenon of circular dichroism [1] became the basis of the most widespread practical chiroptical method in the past few decades. There are some other chiroptical methods based on the interaction of chiral matter with circularly polarized light Optical rotatory dispersion is based on the analogous difference in refraction. Raman optical activity measures differences in scattered light, and circularly polarized luminescence deals with the difference in emission. 

CD spectra cannot be obtained from spectral responses to left and right circularly polarized light measured in series and subtraction ofthe two corresponding signals. However, this concept is implemented in the Raman optical activity measurement. 

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). 

Abstract Now an incisive probe of biomolecular structure, Raman optical activity (ROA) measures a small difference in Raman scattering from chiral molecules in right- and left-circularly polarized light. As ROA spectra measure vibrational optical activity, they contain highly informative band structures sensitive to the secondary and tertiary structures of proteins, nucleic acids, viruses and carbohydrates as well as the absolute configurations of small molecules. In this review we present a survey of recent studies on biomolecular structure and dynamics using ROA and also a discussion of future applications of this powerful new technique in biomedical research. 

L. A. Nafie and D. Che, Theory and Measurement of Raman Optical Activity, in Modern Nonlinear Optics, Part 3 . M. Evans and S. Kielich (eds), John Wiley Sons, Ltd, Chichester, 1994 p. 105. 

J. Haesler, Construction of a new forward and backward scattering Raman and Raman optical activity spectrometer and graphical analysis of measured and calculated spectra for (R)-[2H, 2H2, 2H3]-neopentane, PhD Thesis, University of Fribourg, 2006. 

We present the basic concepts and methods for the measurement of infrared and Raman vibrational optical activity (VOA). These two forms of VOA are referred to as infrared vibrational circular dichroism (VCD) and Raman optical activity (ROA), respectively The principal aim of the article is to provide detailed descriptions of the instrumentation and measurement methods associated with VCD and ROA in general, and Fourier transform VCD and multichannel CCD ROA, in particular. Although VCD and ROA are closely related spectroscopic techniques, the instrumentation and measurement techniques differ markedly. These two forms of VOA will be compared and the reasons behinds their differences, now and in the future, will be explored. 

Vibrational optical activity (VOA) is a relatively new area of natural optical activity. It consists of the measurement of optical activity in the spectral regions associated with vibrational transitions in chiral molecules. There are two basic manifestations of VOA. The first is simply the extension of electronic circular dichroism (CD) into the infrared region where fundamental one-photon vibrational transitions are located. This form of VOA is referred to as vibrational circular dichroism (VCD). It was first measured as a property of individual molecules in 1974 [1], and was independently confirmed in 1975 [2]. Within the past twelve years, VCD has been reviewed on a number of occasions from a variety of perspectives [3-15], and two more reviews are currently in press [16,17], The second form of VOA has no direct analog in classical forms of optical activity. Optical activity in Raman scattering, known simply as Raman optical activity (ROA), was measured successfully for the first time in 1973 [18], and confirmed independently in 1975 [19], ROA has been described in detail and reviewed several times in the past decade from several points of view [20-24], and two additional reviews [25,26], one with a view toward biological applications [25] and the other from a theoretical perspective [26], are currently in press. In addition, two articles of a pedagogical nature are in press that have been written for a general audience, one on infrared CD [27] and the other on ROA [28],... 

An efficient method to visualize the properties of the normal coordinates is to calculate activity measurements, AM. They show which components of the dipole moment vector and the polarizabilty tensor are modulated by the vibration, and the relative sign of the infrared and Raman optical activity (Schrader et al., 1984 Schrader, 1988). The necessary transformation of the eigenvectors (Eq. 5.2-13) needs only seconds of computer time. The AMs are useful to assign vibrations to symmetry species and to check the input of the frequency calculation the symmetry of the Cartesian coordinates of the atoms as well as of the force constant matrix. This program is part of the SPSIM program package (Fischer et al., 1989). 

The first genuine ROA effects were measured by Barron et al. in 1973, following some earlier claims which proved to be artifacts. Two years later, the first review of Raman optical activity (and on Rayleigh OA) by Barron and Buckingham (1975) appeared. The theory of ROA is explained in the book of Barron (1982), which also detailes other types of optical activity. 

Nafie (1992) has given a review about the latest VOA instrumentation. Until 1988, the only measured form of ROA was incident circular polarisation (ICP) ROA, but as the process observed in Raman spectroscopy is a two-photon process, there are four possibilities for measuring Raman optical activity. ICP ROA is the unpolarized measurement of the Raman radiation emitted upon excitation with alternating right and left circularly polarized light. It is shown in Fig. 6.3-12, following the sketches of Nafie. As the first of the other possibilities scattered circular polarisation (SCP) ROA was measured. This... 

Ah initio and experimental Raman optical activity in (+)-(/ )-Methyloxirane were reported by Bose et al. (1990). The measured spectra include depolarized, polarized, and magic angle Raman and ROA spectra. Magic angle (i.e. transmission axis of the analyser set at 35.26 ° to the vertical) ROA spectra contain only contributions arizing from the electric dipole magnetic dipole polarizability. All three kinds of spectra were calculated with two basis sets and compared to experiments. All spectral features could be reproduced correctly in sign but only moderately in intensity. 

Raman optical activity can not only be measured in liquids or in solution, but also in crystals. As an example, the Raman CID spectra of both enantiomorphic forms of sodium chlorate single crystals (Lindner, 1994) together with their sum spectra are shown in Fig. 6.3-18. Here a broken line represents the (-)-form. The transitions examined belong to the triply degenerate polar F phonons split into transverse and longitudinal modes. 

The measurement of Raman optical activity of achiral molecules in magnetic fields is also possible. An example of this technique is the magnetic ROA spectrum of the iridium (IV) hexachloride dianion published by Barron et al. (1987). The researchers used a magnetic field of 1 Tesla. 

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. 

Optical activity is an old subject dating back to the early years of the last century. But it is far from exhausted. Recent developments in optical and electronic technology have led to large increases in the sensitivity of conventional optical activity measurements, and have enabled completely new optical activity phenomena to be observed. Optical activity has been traditionally associated almost exclusively with electronic transitions, but one important advance over the past decade has been the extension of optical activity measurements into the vibrational spectrum using both infrared and Raman techniques. It is now apparent that the advent of vibrational optical activity has opened up a new world of fundamental studies and practical applications. 

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. 

Raman optical activity has only been measured so far in pure liquids and strong solutions. Crystals and powders are harder to study crystals must be polished and oriented carefully to eliminate artefacts, whereas multiple scattering in powders depolarizes the incident light. It would be of great interest to measure pure rotational, and rotational-vibrational, ROA in gases, but insufficient scattered intensity has so far prevented this. An additional complication in resonance scattering is that circular dichroism of the incident beam can contribute to the measured circular intensity difference. 

An optical multichannel Raman instrument functions as a spectrograph rather than as a monochromator due to the absence of an exit slit. The Raman light at the output of a grating instrument is dispersed across a detector consisting of an electronic image sensor that functions as an electronic photographic plate . For optical activity measurements, the components between the laser and the sample are essentially the same as described in the previous section for the scanning instrument. 

Some of these less used systems have limited applications in specific areas and combine HPLC with, for instance, chemiluminescence techniques [48], viscometry [49], optical activity measurement [50], piezoelectric crystals for mass scanning [51], atomic absorption and emission spectrometry [52-54], photoacoustic monitors [55], nuclear magnetic resonance [56], electron spin resonance [57], Raman [58] and photoconductivity measurement [59]. Details on these and other innovative detection systems are presented in the review by Bruckner [60]. 

In addition to a review of the recent developments in the preparation of chiral amino compounds, developments concerning the interpretation of their ORD and CD in the visible and ultraviolet spectral regions will be reviewed, together with the emerging impact of vibrational (infrared) optical activity (VGA) observations, including vibrational circular dichroism (VCD) and Raman optical activity (ROA) measurements, on important stereochemical problems concerning chiral amino compounds. 

In addition to the magnetic Raman circular intensity difference, outlined above, that all molecules can show in a magnetic field, chiral molecules can show a circular intensity difference without a magnetic field. This natural Raman optical activity was first observed by Barron, Bogaard and Buckingham in 1973 Z27j, and provides detailed stereochemical information by measuring vibrational optical activity. 

LA Nafie, D Che. Theory and measurement of Raman optical activity. In M Evans, S Kielich, eds. Modern Nonlinear Optics, Part 3. New York Wiley, 1994, vol 85, pp 105-149. 

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