# Why and how chiral molecules rotate plane of polarisation?

1. Apr 15, 2012

### jd12345

I have been studying stereochemistry and it says that optically active molecules(chiral molecules) rotate plane of polarisation. I suppose thats because of the electrons in the molecule.
I know that all molecules can rotate the plane of polarisation but when achiral moelcules are present in bulk they do not. And when chiral molecules are present in bulk they do. So its a macroscopic property. But why does this happens - whats special in chiral molecules that when they are present in bulk they rotate the plane?
I would like to understand please . Thank You

2. Apr 15, 2012

### Mike H

3. Apr 15, 2012

### Staff: Mentor

It is a microscopic property - single molecule does change the photon polarization.

From what I understand there is no really simple explanation, which is why you may have problems finding one.

There is lecture from Yale university here: http://www.cosmolearning.com/video-...s-and-the-mechanism-of-optical-rotation-6675/ - prepare to get lost at around 23rd minute.

4. Apr 15, 2012

### epenguin

Yes, as long as you're not into the detailed mechanism and prediction, it's simple.

A transparent material has a refractive index which means that it retards the light. I think of it as molecules get excited by the light but then after a bit they let it go again.

Think of plane polarized light an up down electric vibration and that as an equal amount of clockwise and anticlockwise vibration. The two cancel each other out $\leftrightarrow$ in one direction but combine or reinforce $\updownarrow$ in the direction perpendicular to that so you have vibration in a plane. So you have two components with a handedness. These will interact with molecules that have a handedness differently. One of light helices will be delayed more than the other. (Helices = screws. The analogy of the two kinds of screws and of bolts that was made in the vid may be helpful.) Work it out that if one of the helices is delayed, the orientation of where they reinforce is rotated $\searrow$ (best I could do).

This is an argument of symmetry - it works even without knowing what the mechanism of the delay interaction is. And that is all you, or 98% of people studying chemistry, are required to understand. For the detail of what the interaction is the lecturer has summarised what the state of the art is.

This is for transparent substances. But likewise different enantiomers of light-absorbing substances can absorb the different circularly polarised components of light differently, giving rise to circular dichroism and 'elliptically polarised' light. An effect much exploited in molecular biophysics, protein physical chemistry etc. It can be used empirically for many purposes without always needing any deep understanding of its exact mechanism. http://en.wikipedia.org/wiki/Circular_dichroism

5. Apr 15, 2012

### DrDu

It is not so complicated as it may seem from what was said in this discussion.
First some facts. In a linearly polarized electromagnetic field, the electric and magnetic field are perpendicular to the direction of propagation and to each other. Now consider a metallic helix in the electromagnetic field of this plane wave (assume that the wavelength is much larger than the dimension of the helix). If the electric field points into the direction of the axis of the helix, the field will induce a current. But a current in a coil will also generate a magnetic field along that axis. A changing magnetic dipole will also act as a radiating antenna. But as the magnetic field is parallel to the electric field which induced the current, and the magnetic field has to be perpendicular to the electric field, the wave emitted has to have a polarization orthogonal to the original wave.

See e.g. the article by Tinoco and Woody:
http://jcp.aip.org/resource/1/jcpsa6/v38/i6/p1317_s1?isAuthorized=no [Broken]

There are other models which are explicitly solvable. The first goes back to Paul Drude in the 1890's:
http://www.sciencedirect.com/science/article/pii/S0301010406004915

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