Tetracoordinated Planar Carbon in the Al4C Anion. A Combined Photoelectron Spectroscopy and ab Initio Study

The chemical structure and bonding of Al4C and Al4C have been studied by photoelectron spectroscopy and ab initio calculations. While Al4C is known to be a tetrahedral molecule, the data reported here suggest that Al4C has D4h symmetry (when averaged over zero-point vibrational motions) and thus is the smallest species identified to date that contains a tetracoordinated planar carbon atom. The experimental vertical electron detachment energy of Al4C (2.65 ( 0.06 eV) compares well to 2.71 eV calculated at the CCSD(T)/6-311+G(2df) level of theory. The excellent agreement between the calculated and experimental electron detachment energies, excitation energies, and other spectral features allows us to elucidate the structure of the tetracoordinated planar-carbon-containing Al4C anion.