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Why do enantiomers rotate polarized light?

  1. Jun 9, 2015 #1
    We are currently learning about enantiomers in organic chemistry class. So far, we've covered what makes an enantiomer, the concept of chirality, optical isomer naming systems, and the physical and chemical properties of enantiomers.

    One of the physical properties listed is that enantiomers rotate polarized light. An enantiomer of type R might rotate polarized light in the (+) direction, while its counterpart (S) might rotate light in the (-) direction. The sources I've read don't explain the physical mechanism that produces this rotation.

    My initial thought is that the light will bounce into and off of different enantiomers in different directions, presumably because of the spatial arrangement of the atoms. I'm hoping someone will be able to add more.

    Thank you!
     
  2. jcsd
  3. Jun 10, 2015 #2

    DrDu

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    Yes, this is roughly correct. There are different models to understand optical rotation. One of the simplest is the coupled oscillator model. Consider a molecule with two chromophoric groups spaced by some distance d and rotated to each other by an angle phi. If the angle is not equal to 0 or 180 degrees, the molecule will in general be chiral.
    Now if light is shone onto the molecule it may excite one of the two chromophores. If the two chromophores are coupled, the excitation may wander to the other chromophore and this chromophore may de-excitate emitting light again. But as the orientation of the chromophore is rotated with respect to the first one, the polarisation plane of the emitted light will be rotated, too. You can also convince yourself that a net effect will remain even if the molecules are oriented erratically.
    The effect of consecutive scattering from different molecules is cumulative, so that the plane of polarization will rotate with a constant velocity with the distance the light travels in the medium.
     
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