Spin–Spin Interactions Across the “Cove” in the (Z) and (E) Isomers of 1,1′‐Difluoro‐9,9′‐bifluorenylidene

Abstract

AbstractIn the (Z) isomer, but not in the (E) isomer of title compound 1, moderate 19F,19F spin–spin coupling across the “cove” is observed [J(F,F) = 11.0 Hz], which is deemed to obey a through‐space mechanism. The coupling between the isochronous fluorine nuclei of 1Z causes second‐order effects in the 1H, 13C and 19F NMR spectra of this compound and allows its easy distinction from the (E) isomer. A B3LYP/6‐31+G(d) calculation of 1Z as an isolated molecule was carried out which gave a non‐bonded F,F distance d(F,F) of 2.711 Å and an angle between the two C–F bond vectors of 34.1°. The computed F···F compliance constant of 6.96 Å/mdyn points to a very weak interaction. According to our previous correlation between d(F,F) and through‐space J(F,F), the F,F distance in 1Z would be expected to go along with a coupling constant of ca. 46 Hz. This discrepancy between prediction and experiment gives support to our earlier presumption that a simple d(F,F)/J(F,F) correlation may not sufficiently describe through‐space couplings and that an angular factor should also be taken into account. An X‐ray diffraction study of 1Z resulted in an F,F distance of 2.638 Å and an angle between the C–F bonds of 36.4°, in fair agreement with the MO calculation. The rigidity of the (Z) isomer was studied using theoretical compliance constants. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)