Abstract

We have recently developed a comprehensive picture of the difference spectra expected from the use of Raman difference spectroscopy (RDS). Both frequency shifts and bandwidth changes contribute to the nature of such spectra. We have also described a number of experimental applications where the RDS technique is used to measure frequency shifts and bandwidth changes with great precision. This work as well as that from other laboratories has recently been reviewed. In these publications, it has been mentioned that the RDS theory is also immediately applicable to analyzing infrared absorption bands; however, no experimental results were presented. One reason for this is that it is inherently more difficult to obtain useful information on frequency shifts and bandwidth changes from infrared difference spectra due to the limitations on infrared absorbances. That is, it is generally impractical to work with infrared bands whose absorbances considerably exceed about 1.0 absorbance unit (less than 10% transmittance), whereas for RDS large intensities are desirable for RDS measurements. Since many strong infrared bands go "off-scale" for pure liquids in even short path length cells, it is not practical to use infrared difference techniques to measure solvent shifts relative to the pure liquid for many samples. However, frequency differences between dilute solutions may be measured.

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