Structural Evolution, Vibrational Signatures and Energetics of Niobium Clusters from Nb2 to Nb20

A comprehensive review on geometric and electronic structures, spectroscopic and energetic properties of small niobium clusters in the range from two to twenty atoms, Nbn, n = 2–20, in three different charged states is presented including a systematic comparison of quantum chemical results with available experimental data to assign the lowest-lying structures of Nbn clusters and their IR spectra and some basic thermochemical parameters including total atomization (TAE) and dissociation (De) energies based on DFT and CCSD(T) results. Basic energetic properties including electron affinities, ionization energies, binding energies per atom, and stepwise dissociation energies are further discussed. Energetic parameters of small sizes often exhibit odd–even oscillations. Of the clusters considered, Nb2, Nb4, Nb8 and Nb10 were found to be magic as they hold the numbers of valence electrons corresponding to the closed-shell in the electron shells [1S/1P/2S/1D/1F.....]. Nb10 has a spherically aromatic character, high chemical hrT high chemical hardness and large HOMO–LUMO gap. The open-shell Nb15 system is also particularly stable and can form a highly symmetric structure in all charged states. For species with an encapsulated Nb atom, an electron density flow is present from the cage skeleton to the central atom, and the greater the charge involved the more stabilized the cluster is.