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Topic: CNMR: proton coupled vs. decoupled question  (Read 5176 times)

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Offline jamesrb

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CNMR: proton coupled vs. decoupled question
« on: May 03, 2016, 09:08:11 PM »
Am I correct in assume that in 13CNMR spectroscopy proton decoupled spectrums still show 13C-19F coupling? And that a proton coupled spectrum would show both 13C-19F and 13C-1H coupling? So in CH2F2 for the proton decoupled spectrum we would see 3 lines (1 triplet). And in the proton coupled spectrum we would see 6 lines, 2 triplets (as the coupling constant for C-H and C-F are different).
« Last Edit: May 04, 2016, 04:56:54 AM by sjb »

Offline Babcock_Hall

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Re: CNMR: proton coupled vs. decoupled question
« Reply #1 on: May 03, 2016, 09:25:04 PM »
That looks mostly correct, except for your last sentence.  Make a coupling tree and see what happens.

Offline jamesrb

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Re: CNMR: proton coupled vs. decoupled question
« Reply #2 on: May 03, 2016, 10:04:03 PM »
1 pentet?

H1
H2
F1
F2

H1
H2
F1
F2

etc. etc. etc.

Offline sjb

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Re: CNMR: proton coupled vs. decoupled question
« Reply #3 on: May 04, 2016, 04:52:41 AM »
Would you expect 2JCH and 2JCF couplings to be the same?

Offline Babcock_Hall

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Re: CNMR: proton coupled vs. decoupled question
« Reply #4 on: May 04, 2016, 08:52:47 AM »
@OP, I am not following what you wrote.

Offline jamesrb

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Re: CNMR: proton coupled vs. decoupled question
« Reply #5 on: May 04, 2016, 03:53:52 PM »
Would you expect 2JCH and 2JCF couplings to be the same?

I would not expect 2JCH and 2JCF to be the same.

Offline jamesrb

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Re: CNMR: proton coupled vs. decoupled question
« Reply #6 on: May 04, 2016, 04:06:22 PM »
@OP, I am not following what you wrote.

Instead of scanning my drawing of the coupling tree I just summarized what I thinking regarding the spins of the four atoms attached to the carbon. There are two hydrogen atoms: H1 and H2 and two fluorine atoms: F1 and F2. When I drew the tree I came up with five lines. Now I am thinking there should be 8. I have never drawn a coupling tree before and this may not be the best molecule to start with but none the less I tried:



Offline Babcock_Hall

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Re: CNMR: proton coupled vs. decoupled question
« Reply #7 on: May 04, 2016, 08:48:43 PM »
The H-1 decoupled spectrum is a 1:2:1 triplet, due to coupling from fluorine.  If one mentally turns off the H-1 decoupler, each triplet should split into a triplet.  I believe that there will be 9 lines.

Offline Irlanur

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Re: CNMR: proton coupled vs. decoupled question
« Reply #8 on: May 06, 2016, 04:30:04 AM »
It should be a triplet of a triplet (=9 lines). the number of lines will depend on the magnitude of the couplings and the resolution.

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