Combined photoelectron spectroscopy and ab initio study of the hypermetallic Al3C molecule

The chemical structure and bonding of the hypermetallic Al3C and Al3C- species have been studied by photoelectron spectroscopy and ab initio calculations. Al3C- is found to have a planar triangular (D-3h, (1)A(1)') structure (when averaged over zero-point vibrational modes) and Al3C is found to have a triangular distorted planar structure (C-2 upsilon, B-2(2)) with one elongated Al-C bond. Four peaks in the photoelectron spectra of Al3C- were identified at 2.56, 2.69, 3.23, and 4.08 eV. Assignment of the observed features was made on the basis of the ab initio calculations. The experimental adiabatic electron affinity of Al3C was measured to be 2.56 +/- 0.06 eV, compared to 2.47 eV calculated at the CCSD(T)OVGF/6-311 + G(2df) level of theory. The excellent agreement between the calculated and experimental electron affinity, vibrational frequencies, and excitation energies allowed us to completely elucidate the geometrical and electronic structure of the Al3C molecule and its anion. (C) 1999 American Institute of Physics. [S0021-9606(99)01018-1].