Circular Dichroism for Determining Absolute Configuration

The absolute configuration is a way of describing stereochemical molecules, defined according to the spatial configuration of stereoisomers. This is a method of categorizing based on the actual three-dimensional distribution of the structure, unrelated to optical activity or flocculation activity. The most commonly used description method is the Cahn-Ingold-Prelog (CIP) rules. In spectroscopy, Circular Dichroism (CD) is a powerful tool that can be used to study the structure and conformation of molecules, especially large biomolecules. CD spectra are based on the dependency of molecules on the differential absorption of left and right circularly polarized light.

 

Here's how you can use circular dichroism spectroscopy to determine the absolute configuration of a molecule:

 

1. Record the Circular Dichroism Spectrum

First, you need to perform a circular dichroism spectroscopic analysis on your sample. In this process, you will record the difference in absorption of left and right circularly polarized light by your sample.

 

2. Analyze the Circular Dichroism Spectrum

Next, you need to analyze your CD spectrum. This includes determining the major absorption bands and their intensities, which can provide information about the structure of your sample.

 

3. Compare with the Spectra of Known Configurations

Then, you need to compare your sample's CD spectrum with the CD spectra of samples with known configurations. This is often achieved by looking up relevant information in the literature. You need to find molecules with structures similar to your sample, and look at their CD spectra.

 

4. Determine the Absolute Configuration

Finally, you can judge the absolute configuration of your sample based on the similarity of your sample's CD spectrum to the CD spectra of samples with known configurations. If your sample's CD spectrum is similar to the CD spectrum of a sample with a known R configuration, then your sample likely has an R configuration. Similarly, if it's similar to the CD spectrum of a sample with a known S configuration, then it likely has an S configuration.

 

Please note that this method does not provide 100% certainty, as the CD spectrum may be affected by many factors, including the effect of the solvent, and the concentration and temperature of the sample. Nevertheless, it is still a very useful tool that can provide strong evidence for the determination of a molecule's absolute configuration. That's the step-by-step process of using circular dichroism spectroscopy to determine the absolute configuration of a molecule.