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Adam R. Johnson, Harvey Mudd College
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Joined: 11/17/2007 - 10:55am

13C NMR of fluorinated aromatics

We are having some problems with interpreting our 13C NMR spectra of some fluorinated sulfonamides.  Both p-CF3 and 3,5-di-CF3-aryl.  We are seeing too many peaks and are wondering if we should see 2, 3 and 4 bond coupling to F in our 13C resonances.  We see a quartet for the CF3, and then doublets (or doubled peaks).  Sorry this is a vague description.  Any thoughts or advise?  

Adam 

Chip Nataro, Lafayette College
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I would say it is very likely that you can see at least 3 bond if not 4 bond coupling.  And there is likely all kinds of lovely magnetic inequivalencies.  I don't have a ton of experience with this sort of stuff, but I do remember N3P3F6 from my post doc days.  hexachlorocyclotriphosphazene gives a singlet in the 31P.  The fluoro analog is a complete and utter mess due to the magnetic inequivlancy.  That is 3 bond coupling through the ring, so I would expect you to be seeing at least that.
Nancy Scott Burke Williams, Scripps College, Pitzer College, Claremont McKenna College
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Joined: 11/17/2007 - 10:57am
I'd echo what Chip said. You should be getting quartets, and quartets that are signal-to-noise challenged look like doublets. 4-bond C-H couplings can easily be in the 6-10 Hz range, and 19F doesn't have that much different a gamma than a proton.
Adam R. Johnson, Harvey Mudd College
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Joined: 11/17/2007 - 10:55am
What I thought was a big F-C 1 bond coupling constant of 13 Hz was actually a big F-C 2 bond coupling constant of 13 Hz.  The F-C 1 bond coupling constant is 180 Hz, and we saw the tops of the quartet and thought that one was an extra peak.  Now, I am going to type F-C 1 bond coupling constant one more time, and submit.  F-C 1 bond coupling constant.