Shapes of cagelike metal carbide clusters: First-principles calculations

The infrared spectra of the metcars X8C12 with X=V, Nb, Zr, in the gas phase, recently determined experimentally using the infrared resonance enhanced multiphoton ionization technique, are computed from first-principles for several of the lowest-energy local minima found for these clusters. By comparing the experimental spectra and the theoretical results, we assign the dominant shape of each cluster species present in the molecular beams. It is found that the most abundant species corresponds to a T-d-like shape which also coincides, in all cases, with the equilibrium structure found via first-principles unconstrained optimization. The occurrence of absorption strength in regions not accountable for theoretically indicates that other structures, associated with low-lying isomers, coexist with the ground-state conformation in the beams.