A New Look at the Structure and Vibrational Spectra of Small Niobium Clusters and Their Ions

Abstract

Geometries, electronic structures, and vibrational spectra of a series of small niobium clusters Nbn (n = 2−6) and their cations and anions are reinvestigated with DFT calculations with cc-pVaZ-PP basis sets. CCSD(T) calculations are also carried out for relative energies. In each cluster, different lower lying states are close in energy or quasidegenerate. Stable Nbn clusters prefer high coordination state and 3D shape. Clusters with an odd number of electrons usually have a doublet ground state (except for Nb2+). Neutral and cationic clusters with an even number of electrons tend to possess a triplet ground state, with an exception for Nb4 (Td, 1A1). For anions, a competition between low and high spin manifolds is observed. Due to the closed electronic shells, the systems possessing 10 (Nb) and 20 valence electrons (Nb4) are observed to be more thermodynamically stable than their neighbors. Electron affinities and ionization energies are reevaluated. In particular, new assignments for the vibrational spectra of the Nb5 and Nb6 systems are proposed on the basis of a comparison of calculated results with available experimental data.