MadSci Network: Chemistry

Re: Does the refractive index vary in a pair of enantiomers?

Date: Wed Apr 15 20:37:32 2009
Posted By: Jerry Franzen, Chemistry Teacher
Area of science: Chemistry
ID: 1239549637.Ch

You ask a very interesting question.  The answer is "No, but..."  For 
ordinary unpolarized light, the index of refraction for a pair of 
enantioners (no matter how many chiral centers as long as they are truly 
enantiomers) is the same.  Although the index of refraction does change 
with wavelength, it is the same for the enantiomers for each wavelength.

However, it is known that each member of a pair of enantiomers rotates the 
plane of polarization of plane polarized light, and that they each rotate 
the plane of polarization in opposite directions to the same amount.  The 
prevailing explanation of this factor is that the plane polarized light 
can be resolved into equally intense "left" and "right" circularly 
polarized light components.

Plane polarized light can be described as having the electric vector 
vibrating like a sine wave in only one plane. The magnetic vector comes 
along as polarized in a plane perpendicular to the plane of the electric 
vector. One enantiomer interacts both in absorption and refractivity 
(index of refraction) differently with the two circularly polarized 
components, and this difference in interaction with the two components is 
responsible for the optical rotation.

Please see;

So the bottom line is one enantiomer does have different indices of 
refraction for the left" and "right" circularly polarized light 
components.  And the two enantiomers do have different refractive indices 
for either "right" or "left" circularly polarized light.

Dr. Jerry Franzen    

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