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Topic: NMR  (Read 5517 times)

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Offline rleung

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NMR
« on: November 26, 2005, 02:35:25 PM »
Hi,

I need to state the signals that I receive in an H NMR for (Z)-1-chloro-1-propene.  The book says that the three hydrogens on the methyl group that is cis to Cl produces a doublet of doublets, but I don't get why.  Shouldn't it only produce a doublet since there is only one H on its adjacent carbon?  Thanks.

Ryan

Offline Hima Potturi

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Re:NMR
« Reply #1 on: November 26, 2005, 02:56:50 PM »
It couples with vinylic proton as well.  :)

Offline Albert

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Re:NMR
« Reply #2 on: November 26, 2005, 03:39:20 PM »
The two H are slightly different, so you can see two different kinds of coupling.
Moreover, you should be able to appreciate even a heteronuclear coupling (between CH3- and Cl-).
« Last Edit: November 26, 2005, 03:43:06 PM by Albert »

Offline rleung

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Re:NMR
« Reply #3 on: November 26, 2005, 07:23:53 PM »
Thanks so much.  I am still confused about one other thing though.  If a vinylic carbon is able to split a signal like that, then how come it can't do the same in the following molecule.  My book says that hydrogen C is a doublet, but according to what you guys have told me about my first problem of this thread, shouldn't hydrogen c be split into a multiplet since it will be split by the vinylic hydrogens a and b as well?  Thanks.

Ryan

Offline Winga

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Re:NMR
« Reply #4 on: November 26, 2005, 08:48:14 PM »
Normally, you cannot observe J-J splitting when the intervening bond is equal to or more than 4 between 2 types of protons.
However, you can still observe that splitting in aromatic and vinylic molecules.

For your case, theoretically, the proton C is multiplet, but there is one more thing.
The proton C is aldehydic, the J-J splitting is relative very small, you can only observe the splitting by expanding the signal (except for very high frequency NMR spectrometer). Why it is small? I can only tell you that it is affected by the carbonyl group.

Therefore, you can observe the J-J splitting of proton C mainly by the adjacent proton (it is very small also), and sometimes, you can only observe 1 peak for aldehyde molecules.

Offline Albert

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Re:NMR
« Reply #5 on: November 27, 2005, 03:55:03 AM »
For your case, theoretically, the proton C is multiplet, but there is one more thing.
The proton C is aldehydic, the J-J splitting is relative very small, you can only observe the splitting by expanding the signal (except for very high frequency NMR spectrometer). Why it is small? I can only tell you that it is affected by the carbonyl group.
I just want to add to this magnificent explaination that you have always to remember that elements like O and N play an important role in H-NMR spectra. In this case, O provides a sort of barrier for the aldehydic proton, making coupling almost invisible.

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