The planar carbon story

The events leading up to the design and characterization, through ab initio molecular orbital calculations, of dimethanospiro[2.2]octaplane, the first neutral saturated hydrocarbon containing a planar tetracoordinate carbon atom, are described. Dimethanospiro[2.2]octaplane is predicted to have bond lengths that are all less than 1.60 Angstrom, and a strain energy per carbon (46 kT mol(-1)) that is considerably less than that of cubane. These are both pointers to its likely stability. The calculated ionization energies of octaplane, spiro[2.2]octaplane and dimethanospiro[2.2]octaplane (4-5 eV) are comparable to those of the alkali metals, lithium and sodium, and the cations of these molecules are all found to contain planar tetracoordinate carbon atoms. Hemispiro[2.2]octaplane is found to have a proton affinity (1213 kT mol(-1)) greater than that of "proton sponge". Several of these species are suggested as attractive synthetic targets.