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vedmecum

confirmation of molecule and optical rotation

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good evening ......

 

yesterday when i was i reading a book of stereochemistry by Eliel few ques. ( may be foolish ) came to my mind ......... is their any relation between degree of optical rotation of a molecule and its various unstable conformations ( not configuration) ?????........how can we say that these particular molecule rotate plane polarized light to any specific degrees ?????

 

we normally calculate optical rotation of a solution i.e a collection of molecules ...... why don't we look for optical rotation of a single molecule ?????? if the degree of optical rotation is related with conformations of molecule then there may be possibility that in a single solution we have different species which can have different optical rotation ........

 

is there any mathematical equation which can relate degree of optical rotation with dihedral angle of molecule ?????

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You seem a little confused between conformation and configuration.

 

Conformation is any possible 3D structure resulting from rotation around bound (e.g. eclipsed and anti-periplanar)

 

Configurations are when you need to break a bond in order to make a molecule look like another (e.g. cis and trans alkenes)

 

Optical rotation is only for chiral molecules. The two different enatiomers can not be rotated around any bond so that they are they look the same (if they could then they would not be chiral). As far as I'm aware, there is no relationship between structure of a chiral molecule and the handedness is rotates light or by which degree...the value obtained is just what it is.

 

In principle, yes you could measure the optical rotation of a single molecule but that would be extremely difficult to do in practise. That is why a solution is used

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thanks for your concern .........

you said that we can calculate optical rotation of single molecule but its a very hard job ....... can you please suggest me any method 4 finding optical rotation of single molecule ????

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Well in priciple you could do exactly the same as you would for a solution...take a single molecules and shine plane polarise light onto it and see what happens...there inlies the difficulty. Getting a single molecule in a certain position and then trying to measure the effect it has will be almost impossible.

 

If you wanted to know which enatiomer you had, you could also try X-Ray diffraction (although ud need a crystal not a molecule). That would give you the exact 3D relationship of the groups and would therefore tell you the enatiomer....

 

Sorry if thats not really what you asked

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hello......if anybody how molecule rotate plane of polarised light then please let me know also ........ upto my knowledge there are certain kind of vibrations in a molecule which cancel the vibrations of light in all direction leaving one ........ but i wanna know how these vibrations get cancel out.???? i need any sort of hint or ans. (which is banned in this forum).......

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