The influence of substitution on CH.CF and CF.CF coupling constants

Abstract

The dependence of the coupling constants in CH.CF and CF.CF fragments on the electronegativities of the substituents are investigated. J ^HI _AV obeys the equation J ^HF _AV=55·03 - 3·38ΣE, where ΣE is the sum of the electronegativities of the first atom of the remaining substituents, very closely. J ^FF _AV follows the equation J ^FF _AV=91·4 - 6·15ΣE but only approximately. Consideration of these equations with that previously obtained for J ^HH _AV shows that for highly electronegative substituents all three couplings decrease to zero. Thus although this limit cannot be reached for those couplings involving hydrogen, due to the low electronegativity of hydrogen, the ‘peculiarly small’ CF.CF couplings are shown to be the result of merely the high electronegativity of fluorine plus the relative ease of obtaining perfluorinated ethanes. Furthermore, the temperature dependence of the observed F-F couplings in some unsymmetrically substituted ethanes is shown to be not primarily due to the changing populations of the rotational isomers but due to the change in J ^FF with temperature. This is not the case for the analogous H-H couplings, but may be so for the H-F couplings.