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Optical isomers

  1. Nov 16, 2004 #1
    Optical isomers....

    Optical isomers are so called because they apparently "have differing effects on polarised light" - what exactly happens and why?

    Thanks in advance. :smile:
  2. jcsd
  3. Nov 16, 2004 #2
    Last edited by a moderator: May 1, 2017
  4. Nov 19, 2004 #3
    Ok, so they rotate polarised light - my questions now are:

    a) Why? How do they cause it to rotate?

    b) Why do they turn it the opposite ways?

  5. Nov 19, 2004 #4


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    It has to do with the polarizability of the groups in the molecule. On a molecular level you can think of it as a lightwave "running into" the electron cloud of the molecule and losing a small amount of energy. Groups with more electrons on them tend to "absorb" the light energy a little bit more than groups with fewer electrons. When you change the energy of the light wave then you change the observed rotation because you change the frequency of the light. Essentially, you intercept the wave at a different part along the wave.

    On a single molecule level ALL compounds (even achiral ones) will rotate plane polarized light, but if you have lots of molecules, then essentially all of the possible orientations of those molecules are populated equally. Imagine a molecule in one particular orientation, and then imagine flipping it upside down. The two orientations rotate plane polarized light differently on a single molecule, but in real systems with lots of molecules all of these orientations cancel each other out. If you have a chiral compound, however, some orientations are not accessible, since the chirality can't be changed (that is to say, the molecular orientations with the opposite enantiomer can't be accessed and can't cancel out the rotation caused by the enantiomer you do have).

    As for why you get opposite rotations, it's just physics. If you have one molecule that changes light in one way, its mirror image with do the opposite thing. Imagine holding your left hand out, flat with the thumb facing out. Now suppose that a ball were to hit your hand right at the crook of your thumb so that it was deflected through the angle between your hand and thumb. Now imagine the exact same thing happened with your right hand. The ball would be deflected exactly the same in terms of energy, etc., but the trajectory would be in the opposite direction relative to the initial trajectory. The same thing happens with chiral molecules.
  6. Oct 23, 2005 #5
    Just a few minutes ago I read a sentence "it only allows through phontons whose electric field is oscillating in the same direction as the molecules" when reading about the polarimeter detecting what type of optical isomer a compound consists of.
    Has the elecric field of the photon passing through the orbital/electron cloud something to do with the fact that any group of electrons will have different net magnetic fields associated to them at any given time? My question is really how it is possible for the photon to interact wit the electrons - the photon is infinately small - right? ...and it's not charged either, so you can't talk about a charged particle moving in a magnetic field like you can with electrons and helmholtz coils - so what exactly makes the photon interact with the electrons? (or the other way around)
  7. Feb 15, 2006 #6
    can u please explain this? i got that the frquency of the light will change with change in it's energy ... but why will the plane of polarization rotate ?

    Again is it that the enatiomers will act like a polarizer? - just allowing the light whose electric field (or one of it's components) in parallel to the feild of the molecules ?
  8. Feb 15, 2006 #7


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    Think of light as composed of two perpendicular, two-dimensional waves, one is a sine wave, the other is a cosine wave. Both travel along a third axis. The vector sum of these two gives you the circular polarization that you expect from light. So, if you observe the light at your detector at one particular point (say, where the vertical wave is at the maximum and the horizontal wave is at 0) you can call that zero. If you change the frequency of the incipient light, then there will be a change in the number of wavelengths between the sample and the detector, which would cause the detector to essentially "move" along the axis which the waves are traveling along. If this happens, the vector sum changes and you observe a "rotation" of the light. So, say the frequency changes such that the detector now intersects where the vertical wave is at 0 and the horizontal wave is at 1. That would correspond to a rotation of 90 degrees.

    The molecules aren't really polarizers, it's just that they absorb light differently and with racemic molecules the absorptions are equal in both mirror image possibilities. When there is enantioenriched material, then some of the energy changes can't be cancelled out because the exact opposite arrangement of atoms isn't present to give an equal and opposite energy absorption.

    The plane polarization part just means that you are only exposing light waves that have a particular starting rotation to the sample. If you exposed every possible plane polarization then you couldn't observe a rotation because you would shift everything by the same amount and you'd just get the same mixture that you started with. You couldn't tell which detected wave came from which starting wave.
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