title: Three-membered ring or open chain molecule - (F3C)F2SiONMe2 a model for the alpha-effect in silicon chemistry.
abstract: (F3C)F2SiONMe2 was prepared from LiONMe2 and F3CSiF3. It was characterized by gas IR and multinuclear solution NMR spectroscopy and by mass spectrometry. Its structure was elucidated by single crystal X-ray crystallography and by gas electron diffraction. (It exists as a conformer mixture.) Important findings were extremely acute SiON angles [solid 74.1(1) degrees , gas anti 84.4(32) degrees and gauche 87.8(20) degrees] and short Si...N distances [solid 1.904(2) A]. The bending potential of the SiON unit was calculated at the MP2/6-311++G(3df,2dp) level of theory and appears very flat and highly asymmetric. The calculated atomic charges (NPA) are counterintuitive to the expected behavior for a classical Si-N dative bond, as upon formation of the Si...N bond electron density is transferred mainly from oxygen to nitrogen, while the silicon charge is almost unaffected. Despite the molecular topology of a three-membered ring, the topology of the electron density shows neither a bond critical point between Si and N atoms nor a ring critical point, but the electron density and Laplacian values are related to other hypercoordinate Si compounds. The electronic properties of (F3C)F2SiONMe2 were compared to those of the adduct (F3C)F2(MeO)Si-NMe3, whose properties and structure were also calculated. The charge distribution and Laplacian values along the Si-N vectors in both molecules are similar but not equivalent. (F3C)F2SiONMe2 contains thus a nonclassical Si...N bond, and its properties can be regarded as a new model for the explanation of the old postulate of an alpha-effect in silicon chemistry, explaining the behavior of compounds with geminal Si and N atoms.
authors.affiliation: Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstr. 30, D-48149 Münster, Germany. mitzel@uni-muenster.de
authors.forename: Norbert W
authors.initials: NW
authors.lastname: Mitzel
authors.forename: Krunoslav
authors.initials: K
authors.lastname: Vojinović
authors.forename: Roland
authors.initials: R
authors.lastname: Fröhlich
authors.forename: Thomas
authors.initials: T
authors.lastname: Foerster
authors.forename: Heather E
authors.initials: HE
authors.lastname: Robertson
authors.forename: Konstantin B
authors.initials: KB
authors.lastname: Borisenko
authors.forename: David W H
authors.initials: DW
authors.lastname: Rankin