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

The chemical structure and bonding of the hypermetallic Al3C and Al3C- spec ies have been studied by photoelectron spectroscopy and ab initio calculati ons. 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 elo ngated Al-C bond. Four peaks in the photoelectron spectra of Al3C- were ide ntified at 2.56, 2.69, 3.23, and 4.08 eV. Assignment of the observed featur es was made on the basis of the ab initio calculations. The experimental ad iabatic electron affinity of Al3C was measured to be 2.56 +/- 0.06 eV, comp ared to 2.47 eV calculated at the CCSD(T)OVGF/6-311 + G(2df) level of theor y. The excellent agreement between the calculated and experimental electron affinity, vibrational frequencies, and excitation energies allowed us to c ompletely elucidate the geometrical and electronic structure of the Al3C mo lecule and its anion. (C) 1999 American Institute of Physics. [S0021-9606(9 9)01018-1].