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Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: xshadow on October 09, 2020, 07:39:50 AM

Title: NMR- H cis/ trans cinnamic acid - difference
Post by: xshadow on October 09, 2020, 07:39:50 AM
I have this question:

"What is the difference in NMR 1H  spectrum between trans 4-methoxycinnamic acid and the cis one. ""


The only difference I can see is the different coupling of the olefinic hydrogen

Jcis < jtrane

(https://i.imgur.com/Dl9aY3k.jpg)

Am I missing something?!
Thanks
Title: Re: NMR- H cis/ trans cinnamic acid - difference
Post by: Babcock_Hall on October 09, 2020, 08:23:54 AM
I would imagine that there are small differences in chemical shifts, but the difference in coupling constants is what comes to mind.  Do you know about the Karplus relationship?
Title: Re: NMR- H cis/ trans cinnamic acid - difference
Post by: xshadow on October 09, 2020, 12:03:52 PM
Quote from: Babcock_Hall on October 09, 2020, 08:23:54 AM
I would imagine that there are small differences in chemical shifts, but the difference in coupling constants is what comes to mind.  Do you know about the Karplus relationship?

We only studied  the different coupling constant for  cis trans  or geminal  hydrogens
Title: Re: NMR- H cis/ trans cinnamic acid - difference
Post by: Babcock_Hall on October 09, 2020, 12:15:19 PM
The cis and trans hydrogens show a type of vicinal (3-bond) coupling.  The magnitude of three-bond coupling constants depends on the torsional (dihedral) angle.  This is the Karplus relationship. 
Title: Re: NMR- H cis/ trans cinnamic acid - difference
Post by: AWK on October 09, 2020, 12:20:24 PM
Differences in coupling constants are relatively small (about 3 Hz), but the are also quite significant differences in chemical shits of these protons.
Title: Re: NMR- H cis/ trans cinnamic acid - difference
Post by: xshadow on October 10, 2020, 04:17:32 AM
So  main  differenze:

coupling  due to karplus relationship

and chemical shieft (hydrogen in "cis" to the C=O should resent of a bigger field effect than the one in trans to C=O )