Another stereochemical nomenclature for the classification of chiral molecules uses the optical activity of a chiral compound at a given wavelength to denominate (+)-and (-)-enantiomers. Enantiomers that rotate linearly polarized light at 589.3 nm to the right as seen towards the light-source show positive rotation. They are called (+)-enantiomers. Enantiomers that rotate linearly-polarized light at 589.3 nm to the left are called (-)-enantiomers.
However, the R or S-configuration of a given enantiomer does not necessarily determine the direction of optical rotation (+) or (-)! The relationship between absolute configuration and the sense of optical rotation is subtle and has exercised theoreticians for many years (Barron 2004). Chiroptical measurements showed that the enantiomer of S-a-alanine given in Figs. 2.1 and 2.6 on the right is (+)-S-a-alanine. As a consequence the enantiomer given on the left is (-)-R-a-alanine.
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