19F-NMR Spectra Recording with Trifluoroacetic Acid Reference

[chem_tr]

Hello,

A friend of mine has synthesized a compound with pentafluorophenyl groups, and is willing to characterize it via an additional ¹⁹F-NMR. However, the person responsible for recording NMR spectra is inexperienced in ¹⁹F-NMR, so we need help on recording the spectra.

We have trifluoroacetic acid, hydrogen tetrafluoroborate, aqueous KF and aqueous HF in our laboratory. In my searches, I have found that trifluoroacetic acid is a good candidate as a reference material. How can we use the compound as a reference? Our NMR spectrometer does not require TMS for ¹H and ¹³C NMR, for example. So the operator knows almost nothing about using a reference material and expects knowledge from us.

In the past, we attempted to record the spectra without a reference material, and the spectra contained peaks in the positive region, but literature suggests those values in the negative region. So we could not rely on the spectrum.

What do you suggest?

Thank you.

 

[movies]

I have always used Varian NMRs, so this is how you would do it on a Varian instrument. I'm not sure how it would be different for Bruker instruments. By the way, it seems that the chemical shift for TFA differs quite a bit from solvent to solvent, so it might be best to use neat TFA for your reference material. More on that http://nmr.chem.indiana.edu/NMRguide/misc/19Fshifts.html .

To reference your spectra, first take a solution of your reference material (TFA is a common one, as you said) and take an F-19 spectrum of it. There should be a single peak at around -78 ppm. Click your cursor so it is close to the peak and then type 'nl' (without the quotes) and hit return. That should put your cursor on the center of the nearest peak. Once you have done that, set the reference by typing 'rl(-78.5p)' and hit return. At this point you are good to go with your other sample. Put the sample of the compound in the NMR and then once you collect the spectrum just report the peaks as they show up. You will need to re-reference your spectra to TFA each time you do this (unless you take multiple F19 spectra in one sitting, then it probably isn't necessary). This technique is called an "external reference" and isn't as accurate as internal referencing, but is safer for your compounds. When you report the data, make sure you state somewhere that the F19 chemical shifts are reported using TFA as an external reference standardized to -78.5 ppm.

A better way to reference your F19 is to use FCCl3, which is a commercial material. It's very inert, so you can add a small amount to your NMR sample and use it as an internal standard. Another convenient feature is that FCCl3 has a chemical shift of 0 in F19 NMR, so it is easy to set the reference line. Plus you only have to take one NMR spectrum. Also, FCCl3 is very low boiling (it's very nearly a gas at room temp.) so it is easy to recover your NMR sample in a pure form afterwards. TFA is not a good internal standard because it is very likely to react with your compound because of its high acidity.

When we ordered some FCCl3 to use as an internal standard we had to sign a couple of forms saying that we would dispose of it properly since it's a nasty green-house gas.

As an aside, all those fluorines will make the C13 spectrum look really wacky because of F19-C13 coupling. It won't look like regular proton-proton coupling either because F19 has a spin of 3/2. The first time I collected a C13 spectrum of a fluorinated compound I thought I had an impurity in there because there were so many peaks. It took me a while to figure out that they were all due to F19-C13 coupling. The coupling constants are usually really big as well (~200-300 Hz).

Hope this helps.

 

[chem_tr]

This helped a lot, more than you'd imagine. Thank you.

 

[Galyna Dubinina]

help to identify peak -157.8 (m) in 19 F NMR, please!

Hello,

A friend of mine has synthesized a compound with pentafluorophenyl groups, and is willing to characterize it via an additional ¹⁹F-NMR. However, the person responsible for recording NMR spectra is inexperienced in ¹⁹F-NMR, so we need help on recording the spectra.

We have trifluoroacetic acid, hydrogen tetrafluoroborate, aqueous KF and aqueous HF in our laboratory. In my searches, I have found that trifluoroacetic acid is a good candidate as a reference material. How can we use the compound as a reference? Our NMR spectrometer does not require TMS for ¹H and ¹³C NMR, for example. So the operator knows almost nothing about using a reference material and expects knowledge from us.

In the past, we attempted to record the spectra without a reference material, and the spectra contained peaks in the positive region, but literature suggests those values in the negative region. So we could not rely on the spectrum.

What do you suggest?

Thank you.

This helped a lot, more than you'd imagine. Thank you.

I have always used Varian NMRs, so this is how you would do it on a Varian instrument. I'm not sure how it would be different for Bruker instruments. By the way, it seems that the chemical shift for TFA differs quite a bit from solvent to solvent, so it might be best to use neat TFA for your reference material. More on that http://nmr.chem.indiana.edu/NMRguide/misc/19Fshifts.html .

To reference your spectra, first take a solution of your reference material (TFA is a common one, as you said) and take an F-19 spectrum of it. There should be a single peak at around -78 ppm. Click your cursor so it is close to the peak and then type 'nl' (without the quotes) and hit return. That should put your cursor on the center of the nearest peak. Once you have done that, set the reference by typing 'rl(-78.5p)' and hit return. At this point you are good to go with your other sample. Put the sample of the compound in the NMR and then once you collect the spectrum just report the peaks as they show up. You will need to re-reference your spectra to TFA each time you do this (unless you take multiple F19 spectra in one sitting, then it probably isn't necessary). This technique is called an "external reference" and isn't as accurate as internal referencing, but is safer for your compounds. When you report the data, make sure you state somewhere that the F19 chemical shifts are reported using TFA as an external reference standardized to -78.5 ppm.

A better way to reference your F19 is to use FCCl3, which is a commercial material. It's very inert, so you can add a small amount to your NMR sample and use it as an internal standard. Another convenient feature is that FCCl3 has a chemical shift of 0 in F19 NMR, so it is easy to set the reference line. Plus you only have to take one NMR spectrum. Also, FCCl3 is very low boiling (it's very nearly a gas at room temp.) so it is easy to recover your NMR sample in a pure form afterwards. TFA is not a good internal standard because it is very likely to react with your compound because of its high acidity.

When we ordered some FCCl3 to use as an internal standard we had to sign a couple of forms saying that we would dispose of it properly since it's a nasty green-house gas.

As an aside, all those fluorines will make the C13 spectrum look really wacky because of F19-C13 coupling. It won't look like regular proton-proton coupling either because F19 has a spin of 3/2. The first time I collected a C13 spectrum of a fluorinated compound I thought I had an impurity in there because there were so many peaks. It took me a while to figure out that they were all due to F19-C13 coupling. The coupling constants are usually really big as well (~200-300 Hz).

Hope this helps.

help to identify peak -157.8 (m) in 19 F NMR, please!

 

[caijie]

Hello,

I am trying to do F-19 NMR. I always find two peaks which is supposed to be one. The proton NMR shows only one conformation. So my question is that what is the temperature you are working with? Is there any possibility that I lower the temperature to try? And also would it come from the coupling with proton?

Thank you very much!

 

[sjb-2812]

As an aside, all those fluorines will make the C13 spectrum look really wacky because of F19-C13 coupling. It won't look like regular proton-proton coupling either because F19 has a spin of 3/2. The first time I collected a C13 spectrum of a fluorinated compound I thought I had an impurity in there because there were so many peaks. It took me a while to figure out that they were all due to F19-C13 coupling. The coupling constants are usually really big as well (~200-300 Hz)

Are you sure? I always thought ¹⁹F had a spin of 1/2, just like ¹H. I agree with the large J values, though.