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.