On the structure of the perfluorinated F-4-ethyl-3,4-dimethyl-hexan-3-yl carbanion: a structural dynamic 19F-DNMR study

The perfluoro-4-ethyl-3,4-dimethylhexan-3-yl carbanion was prepared by reaction of tetramethylammonium fluoride with the parent C2F4-pentamer olefin in acetonitrile as a stable but moisture sensitive white salt. 13C- and 19F-NMR spectra of the carbanion were recorded, whereby the latter is first order with exception of the AB-type spectrum of the prochiral CF2(5)-groups. mperature NMR spectra down to −90 °C show dynamic behavior, and a step-wise freezing out of molecular rotations occurs. Activation energies of such barriers of interconversion and rate constants were determined from spectral data. Likewise, the carbanion site structure is described. In addition, the molecular structure and its rotational isomers are calculated by the quantum chemical AM 1 method. As the major structural result, the carbanion site is flat and sp2 hybridized in contrast to an inverting pyramidal carbon atom. This planar arrangement is confirmed by the occurrence of diastereotopic fluorines of the CF2(2) group in the molecule, where the equatorial fluorine of that group is locked in the plane of the perpendicular pz-orbital. ne containing groups in the neighborhood of the lone electron pair of the anion suffer strong electron deshielding and a remarkable 19F-NMR downfield shift takes place as a consequence of a chemical shift anisotropy.