Vibrational and geometric structures of Nb₃C₂ and Nb₃C₂⁺ from pulsed field ionization-zero electron kinetic energy photoelectron spectra and density functional calculations

Vibrational frequencies of three niobium normal modes of triniobium dicarbide neutral and cation have been determined from pulsed field ionization-zero electron kinetic energy photoelectron spectra. The niobium stretching mode has a frequency of 326 cm^-1 in the neutral and 339 cm^-1 in the ion. The two deformation modes have frequencies of 238 and 82 cm^-1 in the neutral and a degenerate frequency of 258 cm^-1 in the ion. The geometry of the triniobium dicarbide has been established by comparing the experimental spectra with theoretical calculations. The cluster has a trigonal bipyramid geometry with carbon atoms capping on each face of the metal frame. The cation cluster has D_3h symmetry whereas the neutral cluster has lower symmetry resulting from a Jahn-Teller distortion. A second low-lying structure with doubly bridging carbon atoms has been identified by the calculations but has not yet been observed.