L. Ernst et al., SYN-AR,AR'-DIFLUOROMETACYCLOPHANES - STRONG F-19,F-19 SPIN-SPIN INTERACTIONS TRANSMITTED THROUGH-SPACE, Chemische Berichte, 127(6), 1994, pp. 1119-1124
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 compu
tations, those of 1 and 3 were also determined by X-ray structural ana
lyses. 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 assum
ed and even larger with respect to earlier theoretical calculations. O
nly weak F,F spin interaction occurs in 8, the anti isomer of 4.