Long-range 1H, 19F spin–spin coupling constants in benzenesulfonyl fluoride and some of its derivatives. Insensitivity of 19F shifts to ortho substituents

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Abstract
The proton and 19F high resolution nuclear magnetic resonance spectra of benzenesulfonyl fluoride and of some derivatives are analyzed to yield spin–spin coupling constants between 19F and the ring protons. These parameters display a marked dependence on the ring substituents and it is not clear how their conformational behaviour can be distinguished from intrinsic substituent perturbations. The large magnitudes of the couplings over six bonds to the para protons, relative to those in benzoyl fluoride, strongly indicate the importance of conformations in which the S—F bond does not lie in the benzene plane. The methyl group replacement technique suggests that the six-bond coupling contains contributions in addition to that from a σ–π mechanism. The chemical shift of the para proton yields a π electron density at the contiguous carbon atom which is considerably smaller than in nitrobenzene, for example. The 19F shifts are much less sensitive to ortho substituents than are the 19F shifts in benzoyl fluoride derivatives, implying much smaller intramolecular van der Waals interactions in the sulfonyl compounds.

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