Bonding and structure of Y3C2 and Y3C2+

Vibrational frequencies of yttrium normal modes of triyttrium dicarbide neutral and cation have been determined from pulsed field ionization-zero electron kinetic energy photoelectron spectra. The yttrium stretching mode has a frequency of $\text{228\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ in the ion. The two deformation modes have frequencies of 82 and $\text{24\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ in the neutral and a degenerate frequency of $\text{86\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ in the ion. Geometries and ground electronic states have been determined by the comparison of the experiments and theoretical calculations. The cluster has a trigonal bipyramid structure with the ion symmetry of ${\mathrm{D}}_{\text{3h}}$ and the neutral ${\mathrm{C}}_{\mathrm{2\nu }}.$ The ground state is identified as ${}^2{\mathrm{B}}_{\mathrm{1}}$ for the neutral and ${}^1{\mathrm{A}}_{\mathrm{1}}^{\mathrm{\prime }}$ for the ion. While the Y–Y bonding has a covalent nature, the Y–C bonding has considerable ionic character. Both the Y–Y and Y–C bonds are stronger in the ion than in the neutral. The enhancement of the Y–Y bonding upon ionization is largely due to the removal of the antibonding $\text{5s}$ character from the yttrium atoms and the Y–C due to the increase of the electrostatic attractions between the yttrium and carbon atoms.