Electronic structures, vibrational and thermochemical properties of neutral and charged niobium clusters Nb(n), n = 7-12

Exploring the Structural and Electronic Properties of Niobium Carbide Clusters: A Density Functional Theory Study

This paper systematically investigates the structure, stability, and electronic properties of niobium carbide clusters, NbmCn (m = 5, 6; n = 1-7), using density functional theory. Nb5C2 and Nb5C6 possess higher dissociation energies and second-order difference energies, indicating that they have higher thermodynamic stability. Moreover, ab initio molecular dynamics (AIMD) simulations are used to demonstrate the thermal stability of these structures. The analysis of the density of states indicates that the molecular orbitals of NbmCn (m = 5, 6; n = 1-7) are primarily contributed by niobium atoms, with carbon atoms having a smaller contribution. The composition of the frontier molecular orbitals reveals that niobium atoms contribute approximately 73.1% to 99.8% to NbmCn clusters, while carbon atoms contribute about 0.2% to 26.9%.

What Determines the Drastic Reactivity of Nbn+ Clusters with Nitric Oxide under Thermalized Conditions?

We report an in-depth study of the adsorption and reaction of NO with cationic Nb_n⁺ (n = 1-20) clusters under thermalized conditions in a laminar flow tube reactor in tandem with a customized triple quadrupole mass spectrometer (FT-TQMS). It is found that the small-sized Nb_n⁺ clusters (2 ≤ n ≤ 7) readily react with NO giving rise to dominant fragmentation products pertaining to the loss of a stable diatomic molecule NbO or NbN. In contrast, the reaction products of larger-sized clusters (n ≥ 10) proceed through diverse channels, including NO adsorption, N₂/N₂O release, and even NO₂ formation. These experimental observations provided the incentive for us to dig deep into the reaction mechanism with the help of DFT calculations. In contrast to the NO-donation coordination in transition metal complexes, here the cationic Nb_n⁺ clusters exhibit dominant electronic donation in initiating the reactions with NO molecules. We fully demonstrated the reaction rate constants, compared the reaction energy diagram of typical Nb_n⁺ clusters, and unveiled the distinct interaction mechanism of niobium clusters available for NO activation and conversion.

Effects of All-Electron Basis Sets and the Scalar Relativistic Corrections in the Structure and Electronic Properties of Niobium Clusters

In this paper, an augmented all-electron double-ζ basis set is used in calculations of the structure and electronic properties of small niobium clusters. The B3PW91 and M06 DFT functionals with and without second order Douglas-Kroll-Hess (DKH) scalar relativistic corrections are also utilized. Furthermore, an additional d Gaussian type function is introduced in the standard basis sets in order to improve the description of the clusters orbitals in the valence band. Our findings show that the extra d function is important to yield accurate results of electronic properties and, in addition, the DKH corrections can be relevant when the all-electron basis sets are used in the calculations. Our best results are obtained with the M06 functional together with the DKH second order corrections and with the extra d function added to the all-electron basis set.

Structures, spectra, and energies of niobium clusters from Nb13 to Nb20

A comprehensive theoretical investigation on structures and properties of niobium clusters in the range from 13 to 20 atoms, in three different charged states, is performed by using the BPW91 and M06 functionals and the cc-pVDZ-PP basis set. These species are predicted to prefer low spin ground state, i.e., singlet (for even electron) and doublet (for odd electron) systems. In terms of growth mechanism, a compact structure with one Nb encapsulated by a cage formed from five and six triangles is found to be favored over an icosahedral evolution. Unlike many 3d metals, whose volumes are much smaller, 13 and 19 Nb atoms clusters do not exist as icosahedra and double-icosahedra. A distinct case is Nb(15) as it bears a slightly distorted bcc structure. For some systems, several lower lying isomers are computed to be so close in energy that DFT computations cannot clearly establish their ground electronic states. The existence of structural isomers with comparable energy content is established for Nb(n) species with n = 13, 18, 19, and 20 in both neutral and charged states. The vibrational (IR) spectra are also calculated. While the spectra of smaller systems are strongly dependent on addition or removal of an electron from the neutral, the spectra of the larger size clusters are mostly independent of the charged state. The neutrals and their corresponding ions usually have a quite similar IR pattern. Electron affinities (EA), ionization energies (IE), average binding energies, dissociation energies, and frontier orbital energy gaps are evaluated. The computed EAs and IEs are generally in fair agreement with experiment. The Nb(15) system is observed to be stable and it can form a highly symmetric structure in all charged states with both open and closed electron shells.