Vibrational optical activities

Flood, Thomas C., Stereochemistry of Reactions of Transition Metal-Carbon Sigma Bonds, 12, 37. Floss, Heinz G., Stereochemistry of Biological Reactions at Proprochiral Centers, 15, 253. Freedman, T. B., Stereochemical Aspects of Vibrational Optical Activity, 17, 113. 

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). 

Both VCD and ROA have been extensively developed along both experimental and theoretical lines (15). In the process, two sensitive new approaches to the direct investigation of molecular stereochemistry have become available. While other forms of vibrational optical activity (VOA) may yet be developed, the present two appear to be the most fundamental and the easiest to measure. In addition, they represent complementary rather than redundant approaches to the same stereochemical structural information. 

The primary motivation for the development and application of vibrational optical activity lies in the enhanced stereochemical sensitivity that it provides in relation to its two parent spectroscopies, electronic optical activity and ordinary vibrational spectroscopy. Over the past 25 years, optical rotatory dispersion and more recently electronic circular dichroism have provided useful stereochemical information regarding the structure of chiral molecules and polymers in solution however, the detail provided by these spectra has been limited by the broad and diffuse nature of the spectral bands and the difficulty of accurately modeling the spectra theoretically. 

The measurement of vibrational optical activity requires the optimization of signal quality, since the experimental intensities are between three and six orders of magnitude smaller than the parent IR absorption or Raman scattering intensities. To date all successful measurements have employed the principles of modulation spectroscopy so as to overcome short-term instabilities and noise and thereby to measure VOA intensities accurately. In this approach, the polarization of the incident radiation is modulated between left and tight circular states and the difference intensity, averaged over many modulation cycles, is retained. In spite of this common basis, there are major differences in measurement technique and instrumentation between VCD and ROA consequently, the basic experimental methodology of these two techniques will be described separately. 

The stereospecificity of vibrational optical activity, on the other hand, arises from the fact that the vibrations take place in a chiral framework and VOA intensities depend in large part on the extent and phasing of coupling or mixing of vibrational motion in different parts of the molecule. In addition, many local symmetry restrictions are lifted in chiral molecules, for example, the local de-... 

The authors wish to acknowledge support from the National Science Foundation and the National Institutes of Health for research in vibrational optical activity. 

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