In the first section of this tutorial, we stated that circular dichroism is the differential absorbance of left-hand circularly polarised and right-hand circularly polarised light, and that chiral molecules exhibit circular dichroism. But what are chiral molecules?
Chiral molecules exist as pairs of mirror-image isomers. These mirror image isomers are not super-imposable and are known as enantiomers. The physical and chemical properties of a pair of enantiomers are identical with two exceptions: the way that they interact with polarised light and the way that they interact with other chiral molecules.
Circular birefringence and optical rotation
Chiral molecules exhibit circular birefringence, which means that a solution of a chiral substance presents an anisotropic medium through which left circularly polarised (L-CPL) and right circularly polarised (R-CPL) propagate at different speeds. A linearly polarised wave can be thought of as the resultant of the superposition of two circularly polarised waves, one left-circularly polarised, the other right-circularly polarised. On traversing the circularly birefringent medium, the phase relationship between the circularly polarised waves changes and the resultant linearly polarised wave rotates. This is the origin of the phenomenon known as optical rotation, which is measured using a polarimeter. Measuring optical rotation as a function of wavelength is termed optical rotatory dispersion (ORD) spectroscopy.
Circular birefringence - the orange cuboid represents the sample
Unlike optical rotation, circular dichroism only occurs at wavelengths of light that can be absorbed by a chiral molecule. At these wavelengths Left-and right-circularly polarised light will be absorbed to different extents. For instance, a chiral chromophore may absorb 90% of R-CPL and 88% of L-CPL. This effect is called circular dichroism and is the difference in absorption of L-CPL and R-CPL. Circular dichroism measured as a function of wavelength is termed circular dichroism (CD) spectroscopy and is the primary spectroscopic property measured by a circular dichroism spectrometer such as the Chirascan.
Circular dichroism - the orange cuboid represents the sample
Optical rotation and circular dichroism stem from the same quantum mechanical phenomena and one can be derived mathematically from the other if all spectral information is provided. The relationship between optical rotatory dispersion, circular dichroism, absorption spectra and chirality are shown below, with a comparison of the two enantiomers of camphor sulphonic acid.
CD, ORD and Absorbance spectra of R and S forms of camphor sulphonic acid
Although ORD spectra and CD spectra can theoretically provide equivalent information, each technique has been used for very distinct applications. Optical rotation at a single wavelength is used as a general measurement tool for chiral molecules, to determine concentration and as a determinant of chiral purity compared to a known standard. The simplicity and low-cost of the experiment and instrumentation makes it ideal for this application. Circular dichroism spectra on the other hand are better spectrally resolved than ORD spectra, and consequently more suitable for advanced spectral analysis.