Contributions of Nearly-Degenerate States to the Photoelectron Spectra of the Vanadium Dicarbide Anion.

A theoretical study by using a wide variety of quantum chemical methods has been carried out to investigate the nature of the ionization processes that are responsible for the experimental observed photoelectron spectra of the anionic VC2- stoichiometry. In agreement with previous studies, the most stable structures for the anionic and neutral vanadium dicarbide species were unambiguously found to be cyclic isomers. However, concerning the nature of the ground state of the anionic cluster there appear to be two candidates that are nearly degenerate. Only by considering both these anionic states as initial states could a substantial novel and complete assignment for the observed anion photoelectron spectra be proposed. A thorough analysis of the electronic structures not only allows us to distinguish the one-electron processes but also enables to disclose their natures. All the lower binding energy bands involve ionizations out of a dominant V+ orbital. Opposed, the higher positioned bands are the outcome of an electron detachment out of the C22- ligand 3σg orbital. Finally, the experimentally observed vibrational progressions in the photoelectron spectra of VC2- were simulated on the basis of harmonic frequency analyses at the B3LYP level and the derived Franck-Condon factors.