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Topic: coupling constant value of cis > trans?  (Read 2591 times)

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Offline pallidol

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coupling constant value of cis > trans?
« on: July 11, 2013, 12:46:25 AM »
dear friends,

I've obtained isomers of 4-hydroxy-5-(4-hydroxyphenyl)pyrrolidin-2-one. It is 5-membered ring compound. The structures were confirmed using single x-ray.I found that J cis is > J trans. From what i've read it is possible to relate it with dihedral angle factor. Any opinion from your guys besides dihedral angle?
 
tq


Offline Archer

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Re: coupling constant value of cis > trans?
« Reply #1 on: July 11, 2013, 02:03:21 AM »
The coupling constant J is directly related to the dihedral angle. See Karplus equation.

X-ray structure is not always the same as the structure in solution though.

In your case you have a 5-membered ring which can't move too much so it will be pretty close. There are a few modifications to Karplus which you may wish to investigate.
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Offline Archer

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Re: coupling constant value of cis > trans?
« Reply #2 on: July 11, 2013, 02:18:12 AM »
This page has a good graph representing the relationship between coupling constants and dihedral angle.

 http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/nmr/nmr2.htm

With cis your angle is close to zero so J is around 10Hz but with trans it is closer to 120° so J is around 5Hz.
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