SYN-AR,AR'-DIFLUOROMETACYCLOPHANES - STRONG F-19,F-19 SPIN-SPIN INTERACTIONS TRANSMITTED THROUGH-SPACE

The close approach of the fluorine nuclei in the difluorinated syn-[3.3]metacyclophanes 1, 2 and their [2.2] analogues 3, 4 leads to strong F-19,F-19 spin-spin interactions which obey a through-space mechanism. The geometries of 1-4 were estimated by MM2 molecular mechanics computations, those of 1 and 3 were also determined by X-ray structural analyses. The decrease of the F,F internuclear distance in the series 1, 2, 3, 4 (2.78-2.48 angstrom) correlates with an increase of J(FF) from +42.1 to +99.2 Hz. It follows that, for a given distance d(FF), J(FF) is up to twice as large as a previous empirical correlation has assumed and even larger with respect to earlier theoretical calculations. Only weak F,F spin interaction occurs in 8, the anti isomer of 4.