[xshadow]

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

J_cis < j_trane



Am I missing something?!
Thanks

[Babcock_Hall]

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?

[xshadow]

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

[Babcock_Hall]

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. 

[AWK]

Differences in coupling constants are relatively small (about 3 Hz), but the are also quite significant differences in chemical shits of these protons.

[xshadow]

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 )