Exact planarity at the central carbon atom is achieved, according to molecular orbital calculations, in the strained polycyclic cage hydrocarbon dimethanospiro[2.2]octaplane (see structure). There are no glaringly long C−C bonds, which might have reflected inherent instability in this molecule that is yet to be synthesized.
A new class of saturated hydrocarbons, in which a spiropentane-type unit is bound by a cyclic hydrocarbon, has been investigated by using ab initio molecular orbital calculations at the B3-LYP and MP2 levels. These molecules have been given the trivial name hemispiroalkaplanes. Hemialkaplanes, which are analogous molecules built-up from a neopentane-type unit and a cyclic hydrocarbon, have also been examined. The hemispiroalkaplanes are predicted to contain a pyramidal-tetracoordinate carbon atom that possesses a lone pair of electrons. Protonation at this apical carbon atom is found to be highly favourable, resulting in a remarkably high basicity for a saturated hydrocarbon. The proton affinities of the hemispiroalkaplanes are calculated to be more than 1170 kJmol(-1), even greater than that of the diamine "proton sponges". Structural parameters, heats of formation and strain energies for the novel hydrocarbons are detailed.
A planar tetracoordinate carbon atom in a singly‐charged saturated hydrocarbon is found for the first time. Ab initio calculations at the HF/6‐31G* level predict a planar geometry for the central five‐carbon moiety in the octaplane radical cation (1•+). The ionization energy of octaplane (5 eV) is found to be remarkably low for a saturated hydrocarbon; indeed, it is comparable to that of the alkali metals lithium and sodium.
So leicht ionisierbar wie Lithium der Natrium ist Octaplan 1, wie nun erstmals durch ab‐initio‐Rechnungen bestimmt wurde. Drüber hinaus ergab sich, daß das Radikalkation 1·+ eine höhere Symmetrie (C4h) als 1 (S4) hat und das zentrale C‐Atom in 1·+ planar tetrakoordiniert ist.
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