Nuclear Spin−Spin Coupling via Nonbonded Interactions. 8.1 The Distance Dependence of Through-Space Fluorine−Fluorine Coupling

The 19 F 19 F nuclear spin−spin coupling constants J FF for a set of eighteen compounds related structurally to 1,8-difluoronaphthalene were measured by 19 F 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 19 F 19 F 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 J FF and the extent of the overlap interaction between the in-plane fluorine 2 plone-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.