A Theoretical Approach to Molecular Design: Planar-Tetracoordinate Carbon

A number of novel hydrocarbon cage systems have been designed and characterized using ab initio molecular orbital calculations at the MP2 and B3-LYP levels. In particular, equilibrium structures for five families of molecules, hemialkaplanes, hemispiroalkaplanes, alkaplanes, spiroalkaplanes and dimethanospiroalkaplanes, have been examined in detail with the aim of designing a saturated hydrocarbon with a planar-tetracoordinate carbon atom and with a view to identifying appropriate synthetic targets. The hemialkaplanes and hemispiroalkaplanes are constructed from a spiropentane or neopentane subunit, respectively, which is capped by a cyclic hydrocarbon. The hemispiroalkaplanes are predicted to contain a pyramidal-tetracoordinate carbon atom possessing a lone pair of electrons. Protonation at this apical carbon atom is found to be highly favorable, resulting in a remarkably high basicity for a saturated hydrocarbon. The proton affinities of the hemispiroalkaplanes are calculated to be more than 1170 kJ mol, even greater than those for the diamine “proton sponges”. The alkaplanes and the spiroalkaplanes, which are constructed by bicapping a neopentane or spiropentane subunit, respectively, with a pair of cyclic hydrocarbons, show unprecedented flattening of a tetracoordinate carbon atom. Linking the spiroalkaplane caps with methano bridges gives the dimethanospiroalkaplanes, two of which, dimethanospirooctaplane and dimethanospirobinonaplane, achieve exact planarity at the central carbon atom. They are the first neutral saturated hydrocarbons predicted to contain an exactly planartetracoordinate carbon atom. This has been achieved through structural constraints alone. The electronic structure at the central carbon atom results in a highest occupied molecular orbital corresponding to a p-type lone pair. Consequently, the adiabatic ionization energies for octaplane, spirooctaplane and dimethanospirooctaplane (approximately 5 eV) are predicted to be similar to those of lithium and sodium — incredibly low for a saturated hydrocarbon. Some consideration has been given to likely pathways for unimolecular decomposition for all species. Predicted structures, heats of formation and strain energies for all the novel hydrocarbons are also detailed. Tetramethylhemispirooctaplane and dimethanospirobinonaplane are identified as the preferred synthetic targets.