Sunday, June 03, 2018

A Brief Note: Stereoisomers

Natural products are various in chirality and rich in enantiomer. Enantiomers is one of the isomers types. Isomer is the term for a molecule which has different features because of different molecule structure arrangement even though the number of the atoms is the same. There are so many types of isomers. 

Structural isomers mean a compound with the same formula can have various forms of its structure. This type of isomers includes chain isomerism, position isomerism, and functional group isomerism. While in the stereoisomers, the bond structure is the same, but the position of the atom in spatial is different. This group includes diastereoisomers and enantiomers. What makes diastereoisomers and enantiomers different are, diastereoisomers are the conformational isomers which are not mirror images of one another and are not superimposable, while enantiomers are mirror images of one another and are not superimposable. To make it clear, see the picture below.

Vibrational Circular Dichroism (VCD) Spectroscopy is a spectroscopy that can assign stereoisomers because it can differentiate the absorption of left and right circularly polarized light. Here, we have 2 important keywords, Left Circularly Polarized (LCP) and Right Circularly Polarized (RCP) Light. The unpolarized light can not differentiate stereoisomers, the result of the measurement using samples which contain two enantiomers is identical, so it fails to differentiate it. Circular dichroism appeared as the solution to assign two enantiomers, because the light used is the polarized one. The polarized light wave comes through an optical device and then converts the light into a circularly polarized light: RCP and LCP.

If the sample is the chiral compound, then the spectrum will be different, but no for achiral compound, the spectrum will be identical. Therefore, the principal of this assignment is using CD spectrum. There are so many kinds of instrumentation for the measurement, one that was quite popular was Electronic CD (ECD) spectroscopy. The instrument was always developing, then we know VCD Spectroscopy, which the circular dichroism occurs due to vibrational excitations.

That's all. If there is any mistakes, I beg your pardon. You may help me revise it if you know what is more correct. Anyway, thanks for visiting :)