Kendrekar,

"Coupling over one (1J), two (2J) and three (3J) bonds usually dominates the fine structure of NMR spectra, but coupling across four (4J) and five (5J) bonds is often seen, especially through π bonds (double and triple bonds, aromatic carbons)". See

---http://www.chem.wisc.edu/areas/reich/nmr/05-hmr-03-jcoupl.htm

Good luck

Pravinkumar,

As you probably know, F-19 is a highly abundant, NMR active isotope of fluorine.  Thus it can "couple" with carbon nuclei in a similar manner to that seen between two neighboring hydrogen nuclei.  C-H couplings are not seen in carbon NMR because the standard C NMR pulse sequence involves C-H decoupling, to prevent peak splitting due to attached hydrogens.  However, common CNMR pulse sequences lack any kind of C-F decoupling, thus the C-F splitting is observed.  If you talk to an NMR specialist, he/she can probably help you to adapt a C NMR pulse sequence to incorporate C-F decoupling, if you wish to eliminate this from your spectrum.  Hope this helps.

Large 1J, 2J values characteristic of C-F help in ease of carbon identification in 13C of a fluorinated compound but getting a nice "q" takes large no of scans

Thanks

Prof.Adel Amer and William M. Hewitt for valuable Suggestions.

Regard

Pravin

HI Pravin

In normal 1D 13C spectra, the NMR pulse sequences are designed in such a way that during the acquisition 1H decoupling is applied in order the suppress larger the 13C-1H couplings. Other wise spectra would be more complex to analyze. If you have acquired a 1H decoupled 13C NMR spectra still you will see the 13C coupling to 19F. The resonating frequency for eg. 1H=300MHz and 19F=  272 MHz. Thereforem their value is very clode. In such experiments 100 % 1H decoupling is possible but not 100% 19F. If you want to see 19F free 13C NMR spectra you need to decouple 19F.

For spitting that you have observed is because of  n+1 rule. If you observe 13C, then n=3 for 19F this leads to quartet in 13C nmr spectra. Similarly 19F 1D NMR you will see doublet.

The CF splitting observed in 13C NMR spectra simply because the 13C NMR experiments are usually carried out with H-C decoupling. Thus the CF splitting observed while the CH do not. One can switch the case and decouple the CF while obtain the CH coupling by adjusting the decoupling to CF coupling value (about 272 Hz)

CF couplings in proton-decoupled 13C NMR spectra are fully effective and quite large (even 6J(CF) couplings are sometimes resolvable). To decouple the fluorines as well as protons you would need a a triple-resonance probe (minimum 30 k$ investment) and a separate power amplifier to supply the RF tuned to the fluorine frequency (add some 5 k$ for that). Even so, broad-band decoupling the fluorines is not easy because fluorine spectra are spread over a much larger frequency interval than proton spectra (consequently, several times more RF power is needed to do the trick).

Fortunately, there is normally no need to do that. C-F couplings are of course strictly first order and quite well known, so it is easy to take them into account in the interpretation of the spectra. In fact, they are very very helpful, so I can hardly imagine that any structural chemist might wish to eliminate them, except in very special cases. Triple resonance probes are commercially available, but you do not see many of them around.

you can read Journal of Molecular Structure 1121 (2016) 167-179