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Topic: Absolute referencing of NMR spectra  (Read 14770 times)

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Offline kriggy

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Absolute referencing of NMR spectra
« on: September 11, 2017, 09:00:25 AM »
Im just in the middle of some NMR work and found this text:
https://www.chem.wisc.edu/~cic/nmr/Guides/Other/Xi_chem_shift_scale.pdf

Its basicaly about referencing non hydrogen NMR spectra, while it seems like a recommended method by IUPAC I havent realy seen it in the literature. But since Im measuring some 19F NMR and I dont have any 19F reference, I would like to use it as 19F referencing method. But I wonder  if its OK to reference 13C to residual CDCl3 and 19F using this method or if I should reference both spectra in a same way. Also, Im not entirely sure if I understand the method how it is supposed to be done: do I have to actualy measure the sample with TMS or is it enough to reference the 1H spectra to residual solvent signal and continue from there?

Is Anyone is using this method anyway ?

Thanks

Offline Flatbutterfly

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Re: Absolute referencing of NMR spectra
« Reply #1 on: September 11, 2017, 08:12:04 PM »
The reference for H-1, C-13 and Si-29 NMR spectroscopy is TMS (SiMe4) that is taken as δ = 0.00 with downfield from the reference taken as positive.  No one adds TMS for H-1 and C-13 these days; you just punch in what solvent you are using and machine does the rest (e.g., residual CHCl3 in deuterochloroform for H-1 NMR; C-13 of say CD2Cl2).  You must use a deuterated solvent so the machine can lock on.  I have not done any F-19 NMR but CFCl3 is used as the standard.  What you could do is to use a smidgen of C6F6 (it is just like benzene) and use that as an internal reference and refer it back to CFCl3.  Once again downfield (more +ve shifts) is positive.

Offline wildfyr

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Re: Absolute referencing of NMR spectra
« Reply #2 on: September 11, 2017, 11:29:31 PM »
Seems like you could use just about any fluorinated compound to set your shifts as long as its spectrum in that solvent is known and its not in such a high concentration that you start getting shifts due to intramolecular interactions.

No editor would be mad if you used trifluorotoluene instead of CFCl3, just like no one cares if you use CHCl3 instead of TMS in a 1H.

Offline kriggy

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Re: Absolute referencing of NMR spectra
« Reply #3 on: September 12, 2017, 03:02:25 AM »
Thanks guys.

The thing is tha the method in the paper describes referencing spectra without adding any reference standart so I was thinking about using it for the 19F (so I dont have to run 20 samples again). ofc, another option seems to be just report the 19F spectra without referencing at all.

Offline wildfyr

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Re: Absolute referencing of NMR spectra
« Reply #4 on: September 12, 2017, 09:24:12 AM »
I can't call myself an expert on this, but I have reported 19F with no reference, and didnt hear any complaints. I've also not encountered it in the modern literature when I've looked at other groups' spectra. This was usually pretty straightforward 19F NMR spectra where they were just confirming a product, not using the spectra to deeply explore properties.

Offline kriggy

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Re: Absolute referencing of NMR spectra
« Reply #5 on: September 12, 2017, 11:10:38 AM »
I can't call myself an expert on this, but I have reported 19F with no reference, and didnt hear any complaints. I've also not encountered it in the modern literature when I've looked at other groups' spectra. This was usually pretty straightforward 19F NMR spectra where they were just confirming a product, not using the spectra to deeply explore properties.

Bolded the important part. I didnt encounter it either which realy surprised me since it is supported by IUPAC since early 2000s.

also, when I used the automatic method from mestrenova, my 13C spectra shifter like 80 ppm upfield. So it seems its a no go for now

Offline Flatbutterfly

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Re: Absolute referencing of NMR spectra
« Reply #6 on: September 12, 2017, 11:38:14 AM »
You must reference your spectra to a standard!  Otherwise your chemical shifts will be meaningless.  As I said you can add a trace of hexafluorobenzene to the sample and find the shift in the lit and convert to δ (CFCl3).
If in the unlikely event that C6F6 reacts you use an external standard.
After recording the sample spectrum immediately run a spectrum of your D-solvent with a tiny drop of C6F6 to get its δ.   The modern NMR machines have such stable magnetic fields there is negligible error recording shifts this way.

Offline wildfyr

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Re: Absolute referencing of NMR spectra
« Reply #7 on: September 12, 2017, 02:02:14 PM »
Flatbutterfly,
I understand that it is obviously good practice to include a standard, and will hardly hurt anything, but why do you think it is uncommon in the modern literature? If it was so critical, don't you think reviewers and editors would be asking authors to include it?

kriggy,
do you have a convenient organosoluble fluoride source around you feel comfortable adding? hexafluorobenzene does sound like a good, common one. Many labs have tosyl fluoride (63 ppm or so) or just fluorobenzene (-112 ppm), which probably work just as well if they don't interfere in the relevant part of the spectrum.

Offline Babcock_Hall

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Re: Absolute referencing of NMR spectra
« Reply #8 on: September 12, 2017, 03:09:42 PM »
I am far from an expert, but I have occasionally used a shift standard in an interior coaxial NMR tube.  This can be helpful in P-31 NMR.

Offline Flatbutterfly

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Re: Absolute referencing of NMR spectra
« Reply #9 on: September 12, 2017, 11:00:28 PM »
You don't mention the reference used because it is understood that you are using the conventional (IUPAC) reference.

Offline Flatbutterfly

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Re: Absolute referencing of NMR spectra
« Reply #10 on: September 12, 2017, 11:05:18 PM »
Yes we originally used 85% H3PO4 as an external standard in a coaxial tube (10 mm tubes).  More recently we just added a tiny drop of P(OMe)3
(cheap, air-stable) and used that as the reference relative to 85% phosphoric acid.

Offline wildfyr

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Re: Absolute referencing of NMR spectra
« Reply #11 on: September 12, 2017, 11:06:54 PM »
I'm not talking about the peak list, I look at the raw spectra. Threre's no reference compound. Heres a perfect example, a seminal paper on SuFEx, the newest click reaction, out of the Nobel prize organic lab of Barry Sharpless (http://onlinelibrary.wiley.com/doi/10.1002/anie.201309399/abstract). Scroll down to the bottom of the supplementary for the fluorine NMRs. Nothing but sulfonyl fluoride peaks around 60 ppm.

Offline Flatbutterfly

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Re: Absolute referencing of NMR spectra
« Reply #12 on: September 12, 2017, 11:30:13 PM »
So take a fluoro cmpd and run it on your NMR machine and look up the lit reference for the cmpd and use that as a reference.  If you are using a modern machine >400 MHz the field is remarkably stable so there will be little if any loss of accuracy.  If your C-13 shifts are off by ~80 ppm then you have made a serious error.  C-13 shifts should agree to ~0.2 ppm (same solvent, same T) from different labs.

Offline Irlanur

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Re: Absolute referencing of NMR spectra
« Reply #13 on: September 13, 2017, 06:44:09 AM »
I recently had a Discussion about 19F NMR referencing with a fellow PhD student. Apparantly, it's a pain if you want to be able to compare different measurements or use it in a similar way to chemical shifts in 1H or 13C NMR. As far as I know, the shifts are also extremly solvent dependent. Bottom line what I would do: just describe what you have done and how or if you referenced your spectra. Then wait until someone figures out something meaningful and practically doable.

Offline kriggy

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Re: Absolute referencing of NMR spectra
« Reply #14 on: September 13, 2017, 07:12:29 AM »
So take a fluoro cmpd and run it on your NMR machine and look up the lit reference for the cmpd and use that as a reference.  If you are using a modern machine >400 MHz the field is remarkably stable so there will be little if any loss of accuracy.  If your C-13 shifts are off by ~80 ppm then you have made a serious error.  C-13 shifts should agree to ~0.2 ppm (same solvent, same T) from different labs.

Yes there needs to be some problem witth the absolute referencing function in MNova.

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