syn‐ar,ar ′‐Difluorometacyclophanes: Strong 19 F, 19 F 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 19 F, 19 F 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 Å) 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 .