Nuclear spin-spin coupling via nonbonded interactions. 8. The distance dependence of through-space fluorine-fluorine coupling

The (FF)-F-19-F-19 nuclear spin-spin coupling constants J(FF) for a Set Of eighteen compounds related structurally to 1,8-difluoronaphthalene were measured by F-19 NMR spectroscopy. The FF distances d(FF) in these compounds were determined by ab initio 3-21G* molecular orbital calculations. Consistent with the lone-pair overlap theory of the origins of through-space (FF)-F-19-F-19 coupling, an exponential relationship is found between J(FF) and d(FF) (regression coefficient r(2) = 0.991), and a linear relationship is found between JFF and the extent of the overlap interaction between the in-plane fluorine 2p lone-pair orbitals (regression coefficient r(2) = 0.993). The magnitudes of these lone-pair interactions were estimated from molecular orbital energies obtained by ab initio 6-31G* calculations for a model consisting of a pair of HF molecules separated by various distances.