Jignesh - 29-1-2003 at 08:49
Why do optical isomers rotate plane polarised monochromatic light?
Good question
Polverone - 30-1-2003 at 01:59
Until you asked I'd never really stopped to consider the mechanism behind optical activity. It's just introduced without explanation in many organic
and general chemistry books. I tried to find web pages to point you to but I wasn't able to find any good ones. I had to look in a physical chemistry
text before I found an explanation.
The explanation I found is more lengthy and uses a lot of mathematical symbols that I can't properly represent here. I will try to give an "executive
summary" without mucking things up:
"Plane-polarized light may be regarded as a superposition of two oppositely rotating circularly polarized components," to quote Atkins directly.
Normally these components travel at the same speed, so no rotation is observed. But chiral molecules pass one rotation of light faster than the other,
so that the two components go out of phase and the resulting beam appears rotated if one enantiomer predominates. It is the interaction of rotated
light and the electronic structure of "handed" (chiral) molecules that slows one rotational variety down more than the other. A racemic mixture slows
both rotational varieties down the same amount, since it contains equal amounts of the optical isomers, and again no rotation can be observed.
That's as clearly as I can put it. If you would like a longer version, including some math, try looking in Physical Chemistry by Peter
Atkins. Even this book admits that "the full theory of optical rotation is complicated," and does not describe it in full detail. So I suspect that it
is a fairly deep topic.
Jignesh - 31-1-2003 at 00:33
Thanks you