Geminal Fluoro, Hetero Alkenes

Vicinal Difluoroalkenes. Each of the two vicinal fluorine substituents is significantly shielded by the presence of the other 

The observed trans F-F coupling constants are very large ( 130 Hz), whereas the analogous cis couplings are much smaller ( 15Hz). Both 

Comparing geometric isomers of the type CHF=CFR, a proton cis to fluorine is more deshielded than one that is trans. 

Trifluorovinyl Groups. Trifluorovinyl groups have characteristic chemical shifts and coupling constants that are exemplified with 

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