NMR Fluorine-Fluorine Coupling Constants in Saturated Organic Compounds

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Abstract
The spectra of several fluorine-substituted, saturated, organic compounds have been investigated. It has been found that the coupling constants between 1,2 fluorine atoms are usually near zero. The coupling constants between 1,3 fluorine atoms in a free chain are usually between 7 and 10 cps if all of the intermediate skeletal atoms are carbon atoms. If one of the intermediate skeletal atoms is a nitrogen atom, these coupling constants go up to between 10 and 17 cps. The coupling constants between 1,4 fluorine atoms are usually in the range 2–7 cps when a nitrogen atom is in the intermediate skeletal chain. Rings usually reduce all coupling constants below the above stated values. Several exceptions are found to these generalizations. These generalizations with their exceptions lead the authors to believe that in the case of coupling between fluorine atoms, the main effect is not through-the-bonds, but rather a direct-through-space interaction. It is demonstrated that the restricted rotation about the carbon-carbon bonds has little to do with the near-zero coupling constants, but that these are readily explained on the basis of through-space coupling.

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