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Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: jamesrb on May 03, 2016, 09:08:11 PM

Title: CNMR: proton coupled vs. decoupled question
Post by: jamesrb 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).
Title: Re: CNMR: proton coupled vs. decoupled question
Post by: Babcock_Hall 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.
Title: Re: CNMR: proton coupled vs. decoupled question
Post by: jamesrb on May 03, 2016, 10:04:03 PM: 1 pentet?

H₁=α
H₂=α
F₁=α
F₂=α

H₁=α
H₂=β
F₁=α
F₂=α

etc. etc. etc.
Title: Re: CNMR: proton coupled vs. decoupled question
Post by: sjb on May 04, 2016, 04:52:41 AM: Would you expect ²J_CH and ²J_CF couplings to be the same?
Title: Re: CNMR: proton coupled vs. decoupled question
Post by: Babcock_Hall on May 04, 2016, 08:52:47 AM: @OP, I am not following what you wrote.
Title: Re: CNMR: proton coupled vs. decoupled question
Post by: jamesrb on May 04, 2016, 03:53:52 PM: Quote from: sjb on May 04, 2016, 04:52:41 AM
Would you expect ²J_CH and ²J_CF couplings to be the same?

I would not expect ²J_CH and ²J_CF to be the same.
Title: Re: CNMR: proton coupled vs. decoupled question
Post by: jamesrb on May 04, 2016, 04:06:22 PM: Quote from: Babcock_Hall on May 04, 2016, 08:52:47 AM
@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:

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fs32.postimg.org%2Fu95my7tup%2FCH2_F2.jpg&hash=541fc288296a84d6a8c4bb39f81bfe354277b209) (http://postimg.org/image/u95my7tup/)
Title: Re: CNMR: proton coupled vs. decoupled question
Post by: Babcock_Hall 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.
Title: Re: CNMR: proton coupled vs. decoupled question
Post by: Irlanur 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.