For example, in 19F NMR of trifluoro acetic acid, the signal will come at around -76ppm (similary, for hexafluorobenzene -164). Is there any role of electronegativity of fluorine or does it solely depend on electron density at fluorine?
that's strange, I wrote a reply to your question about an hour ago - and it's not here. anyway, the reason is that the 0.0 ppm point in 19F NMR is the signal of CFCl3 (just like TMS in 1H and 13C NMR it is a somewhat arbitrary choice). so when you have something more electron-withdrawing than CCl3 group bonded to fluorine atom, its chemical shift will be positive (e.g., the chemical shift of F2 is ca. +420ppm), but for most organofluorine compounds 19F chemical shifts will be negative.
the chemical shifts in NMR spectroscopy are relative, so it all depends on the primary standard used. in fluorine NMR, the 0.0 chemical shift is given to CFCl3 (trichlorofluoromethane), therefore most organofluorine compounds have negative chemical shifts. but if you connect fluorine to something more electron-withdrawing than CCl3 group, its chemical shift will become positive (e.g., chemical shift of F2 is ca. +420 ppm, chemical shifts of SF5 group are at +60 and +40ppm (I don't remember those precisely). so it is analogous to proton chemical shifts.
that's strange, I wrote a reply to your question about an hour ago - and it's not here. anyway, the reason is that the 0.0 ppm point in 19F NMR is the signal of CFCl3 (just like TMS in 1H and 13C NMR it is a somewhat arbitrary choice). so when you have something more electron-withdrawing than CCl3 group bonded to fluorine atom, its chemical shift will be positive (e.g., the chemical shift of F2 is ca. +420ppm), but for most organofluorine compounds 19F chemical shifts will be negative.
Fluorine shift's from -200 ppm to +600 ppm.
and refernce can be CFCl3 or C6F6, which are different (-162.9 ppm).
So the major signals are in the negative domain.
If you have diffuclties in the 19F NMR spectrum send it. Normally I can help you.
Perhaps the attached comments will further clarify the issue. Best wishes in your research!
www.chem.wisc.edu/areas/reich/handouts/nmr/f-data.htm shows the wide range of 19F chemical shifts.
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