Many NMR users (of the organic variety) come to believe that all 13C resonances are singlets as the most common way to collect the data is with proton decoupling, which indeed reduces most 13C resonances to singlets. It is a surprise to some to find multiplicity in the 13C resonances when a nucleus such as 31P or 19F is present in the molecule. Below is the aromatic region of the proton decoupled 13C NMR spectrum of alpha, alpha, alpha-trifluoro-
p-tolualdehyde (the aldehyde peak is not shown). The assignments are given by the colors in the figure. Note the multiplicities in the 13C resonances due to the 1, 2 and 3 bond J coupling to 19F.
Some people find it surprising that the trifluorinated methyl group comes in the aromatic region of the spectrum. Fluorinated carbons are often difficult to find in 13C spectra with low signal-to-noise ratios as the signal is spread over multiple lines and can be buried in the noise.
9 comments:
I recently made a couple of molecules containing a perfluorinated octyl group. The C-F coupling was extensive and I became curious about the option of obtaining 19F-decoupled 13C NMR spectra. The NMR staff at my institute had no experience with this but told me to go ahead and just try it. The result wasn't pretty and I'm wondering if the process could somehow be improved.
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The problem occurs when you have to decouple both the protons and the 19F at the same time. To do this you will need a triple rssonance probe set up for 13C, 19F and 1H. If the 1H chennel of your double resonance tunes down to 19F than you should be able to get rid of the 19F couplings however the 13C-1H couplings will be present and will complicate the spectrum. One interesting option may be to do a 19F-13C HMQC with 19F decoupling.
Well, I am not sure I necessarily need to decouple 1H in this case. However, what I found was that although the carbons with protons attached were now coupled they were not very well-resolved - and the fluorinated carbons lost a lot of fine-structure without becoming a set of signals that correlated nicely with the number of fluorinated carbons I know to be present.
The approach that was suggested me was to simply set dn=19f (Varian unity inova 400 MHz) and change no other parameters.
Second Repy to comment:
It sounds to me as if you are getting only partial 19F decoupling. If you just changed "dn" to 19F then the only way you would get efficient 19F decoupling is if the proton channel of the probe was retuned to 19F, the power level ("dpwr"?) was set correctly for 19F waltz decoupling and the decoupler offset ("dof") was set near the 19F resonances.
Broadband 19F decoupling presents a challenge owing to the wide range of 19F chemical shifts and 13C-19F couplings which can exceed 300 Hz. Adiabatic decoupling schemes (e.g., STUD and WURST) allow the generation of very wide decoupler bandwidths without increasing the power dissipation in the sample and damaging the probe. Spectrometers equipped with a waveform generator and a switchable probe can acquire not only broadband 19F-decoupled but even simultaneously broadband 19F and 1H decoupled 13C spectra.
See,for example, Figure 5 in "13C NMR spectra of halocarbons" by Anthony Foris, Magn. Reson. Chem. 2001;39:386-398.The figure shows all single 13C signals for CF3CCl2CH2CHClF.
Question: would a fluorine coupled DEPT experiment look the same but with splitting peaks down or up?
Kristine,
I believe so.
Glenn
Hello Sir,
My skeleton has 3 methoxy group attached to ester group and one fluorine atom to other aryl ring of same skeleton. I need to know whether methoxy peaks in carbon-13 will get splitted by the fluorine atom.
Nitika,
13C-19F coupling can often be seen over 4 bonds in aliphatic compounds and over 8 bonds in conjugated systems.
Glenn
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