Circularly polarized infrared light

It was observed that the irradiation with circularly polarized infrared light of the columnar hexagonal phase yields uniform homeotropic ally aligned LC domains. 

For the measurement of differential absorption spectra in the visible and ultraviolet spectral region, a light modulation technique was introduced over two decades ago [7]. The principle of this measurement also governs the measurement of infrared VCD. This technique involves the high frequency modulation of the exciting beam between left and right circular polarization states via a photoelastic modulator, and measuring the differential absorption with a lock-in amplifier tuned to the modulation frequency. 

Fig. 7.5 Alignment of DLCs by infrared irradiation, a Infrared irradiation with linearly polarized light and b irradiation with circular polarized light. Reproduced with permission from [122]. Copyright 2006 Wiley-VCH...

FIGURE 3.13 Schematic representations of alignment change of columnar phase of triph-enylene derivative (a) infrared irradiation with linearly polarized light and (b) irradiated with circularly polarized light. Reprinted with permission from Reference 111. Copyright 2006 John Wiley Sons, Inc. 

Circular dichroism (CD) is a chiroptical spectroscopy that measures the differential absorption of left versus right circularly polarized light. Its higher sensitivity to molecular conformation and configuration has made CD spectroscopy a more powerful tool in the structural analysis of various chiral supramolecular systems than its parent achiral absorption spectroscopies such as ultraviolet (UV), visible (vis), and infrared (IR) spectra (Figure 1). CD measurements in the UV and vis regions are the most widely... 

There is a totally different manner in which infrared spectroscopy is applied to study macromolecular conformations. Here, researchers make use of what is known as infrared dlchroism. In the next chapter we will be discussing the phenomenon called circular dichroism. Circular dichroism means the difference in the absorption of right- and left-handed circularly polarized light by an optically active compoimd. The phenomenon used in infrared dichroism is not circular dichroism. One can refer to this as linear dichroism. This phenomenon means that molecules oriented in one direction will absorb plane polarized light differently when the electric vector of the light is parallel to the orientation of the molecule and when it is perpendicular to this orientation. Whether the compound is optically active or not is of no consequence to this phenomenon even optically inactive compounds will show linear dichroism. 

We have been discussing electronic transitions and ultraviolet or visible circular dichroism. However an optically active molecule will also have infrared CD due to its vibrational transitions. The measurement of infrared CD is very difficult, but some data exist [29]. Another related measurement is the Raman circular intensity differential [30]. It is the difference in Raman scattered intensity when right and left circularly polarized light is... 

We have seen that the enantiomers of a molecule have the same properties (melting and boiling points, refractive index, etc.) and that spectroscopy that does not involve polarized light is incapable of distinguishing between them, so that their infrared, NMR, ultraviolet-visible and Raman spectra are identical. On the other hand, the optical rotation, optical dispersion and circular dichroism give results that are opposite in sign for the two enantiomers. We have also seen that in certain cases it is possible to determine the absolute configuration of a molecule in the crystal by X-ray diffraction. 

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