January 18, 2021, 02:19:52 AM
Forum Rules: Read This Before Posting


Topic: NMR- H cis/ trans cinnamic acid - difference  (Read 274 times)

0 Members and 1 Guest are viewing this topic.

Offline xshadow

  • Full Member
  • ****
  • Posts: 312
  • Mole Snacks: +1/-0
NMR- H cis/ trans cinnamic acid - difference
« 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



Am I missing something?!
Thanks

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4462
  • Mole Snacks: +272/-19
Re: NMR- H cis/ trans cinnamic acid - difference
« Reply #1 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?

Offline xshadow

  • Full Member
  • ****
  • Posts: 312
  • Mole Snacks: +1/-0
Re: NMR- H cis/ trans cinnamic acid - difference
« Reply #2 on: October 09, 2020, 12:03:52 PM »
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

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4462
  • Mole Snacks: +272/-19
Re: NMR- H cis/ trans cinnamic acid - difference
« Reply #3 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. 

Offline AWK

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7853
  • Mole Snacks: +545/-92
  • Gender: Male
Re: NMR- H cis/ trans cinnamic acid - difference
« Reply #4 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.
AWK

Offline xshadow

  • Full Member
  • ****
  • Posts: 312
  • Mole Snacks: +1/-0
Re: NMR- H cis/ trans cinnamic acid - difference
« Reply #5 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 )

Sponsored Links