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Topic: Coupling in hydroxyl protons  (Read 3871 times)

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Offline halcyonmind

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Coupling in hydroxyl protons
« on: August 27, 2013, 07:46:49 AM »
Coupling is generally either not seen or very weak when it comes to hydroxyl protons (or indeed any heteroatom-bonded proton) on account of the rapid exchange rate, and thus short period of residency on the heteroatom, shorter than the time required for an NMR transition. This page gives some values to this:

"Under normal conditions for NMR spectra acquisition, no coupling is observed between the hydroxyl hydrogen and hydrogens on the carbon atom to which the hydroxyl group is attached. A typical alcohol undergoes intermolecular proton exchange at a rate of 105 protons per second. This means that the average time of residence of a proton on oxygen is 10-5 seconds. About 10-2 to 10-3 second is required for an NMR transition event to occur and be recorded. As far as the NMR spectrometer is concerned, the hydroxyl proton is unattached more frequently than it is attached to oxygen, and the spin interaction between the hydroxyl proton and any other proton in the molecule is effectively decoupled."

http://www.chemistry.ccsu.edu/glagovich/teaching/316/nmr/couplingoxygen.html

But I'm really struggling to find any published work to back these values up. I don't doubt them, they sound very reasonable, but I'd rather not reference a webpage in a paper! Can anyone think of anything published?

Offline BobfromNC

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Re: Coupling in hydroxyl protons
« Reply #1 on: August 27, 2013, 04:18:31 PM »
There are many reference books for NMR, I would look in them for references, or just use Scifinder or even Google scholar to find real papers.

Offline halcyonmind

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Re: Coupling in hydroxyl protons
« Reply #2 on: August 28, 2013, 12:41:58 PM »
Thanks for your reply. I'd already tried finding papers am drawing a blank as to what search terms to use to find what I'm after. "NMR proton exchance heteroatom" and various variations bring up mostly papers about protein analysis, but nothing I can see relating to the text in the OP. Any ideas?

Offline Archer

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Re: Coupling in hydroxyl protons
« Reply #3 on: August 28, 2013, 12:50:26 PM »
I'm afraid that this is the best I can come up with, most people who run NMR have a copy of this paper anyway but just in case you haven't got it:

http://pubs.acs.org/doi/abs/10.1021/jo971176v?journalCode=joceah

This paper has a very brief mention on "slow" exchange rates and observed coupling of certain alcohols in specific solvents.

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Offline halcyonmind

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Re: Coupling in hydroxyl protons
« Reply #4 on: September 02, 2013, 04:48:18 PM »
I'm afraid that this is the best I can come up with, most people who run NMR have a copy of this paper anyway but just in case you haven't got it:

http://pubs.acs.org/doi/abs/10.1021/jo971176v?journalCode=joceah

This paper has a very brief mention on "slow" exchange rates and observed coupling of certain alcohols in specific solvents.

Thanks Archer. As it happens (not too surprising I suppose) I've got the second table from that paper stuck on the wall behind my monitor! It's so frustrating when received wisdom in chemistry doesn't appear to have actually been published anywhere. I'm sure it has, in the depths of a horrendous physchem paper somewhere! Thanks again for your help.

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