Multiple isomers in the photoelectron spectra of small mono-niobium carbide clusters

Abstract

We calculate the photoelectron spectrum of small mono-niobium carbide clusters (NbC(n)) using density functional theory for clusters with n = 2-7 and the symmetry adapted cluster configuration interaction method for the smallest clusters (n = 2-4). Theoretical spectra of a single structure cannot explain all peaks present in the spectrum measured by Zhai et al. [J. Chem. Phys. 115, 5170 (2001)]. However, we can match all peaks in the experimental spectra if we assume that the beam contains a combination of cyclic and linear structures. This finding is even more surprising given the fact that some of the excited metastable geometries have energies as large as 0.5 eV above the ground state. Our result is confirmed by both theoretical approaches. We suggest further experiments, using additional beam cooling, to corroborate this observation.

Ground and Low-Lying Excited States of NbC3–/0 Clusters: Assignment of the Anion Photoelectron Spectra from Multiconfigurational Calculations

“…6 Computational attempts have been made for the NbC n −/0 (n = 2−7) clusters to interpret these spectra. 14,15,19,22 For the NbC 3 − cluster, the photoelectron spectra show a total of four bands at 1.69, 2.54, 2.89, and 3.25 eV that were, respectively, labeled as X, A, B, and C. 6 The geometric and electronic structures of NbC 3 clusters were investigated with the density functional theory (DFT) and multireference singles and doubles configuration interaction (MRSDCI) methods. 15 The ground state of the neutral cluster was calculated to be the 1 2 A 1 of the cyclic-NbC 3 isomer.…”

“…6 Recently, DFT and (symmetry adapted cluster-configuration interaction) SAC−CI calculations predicted a singlet of the cyclic isomer as the anionic ground state, with the triplet of the same isomer less stable for 0.08 and 0.12 eV. 22 Furthermore, depending on the computational level, the lowest energy state of linear-NbC 3 − isomer was calculated to be a singlet (SAC−CI) or a triplet (DFT), but either two states were at least 0.69 eV above the cyclic singlet ground state. Only by comparing the simulated photoelectron spectrum at the SAC−CI level with all experimentally observed bands in the anion photoelectron spectra, it was concluded that both cyclic and linear isomers are likely to contribute to the spectra.…”

“…Only by comparing the simulated photoelectron spectrum at the SAC−CI level with all experimentally observed bands in the anion photoelectron spectra, it was concluded that both cyclic and linear isomers are likely to contribute to the spectra. 22 − isomer were, respectively assigned to the X and A bands. 23 However, in this case, a nearly equal stability of the tetrahedral-like FeO 4 − and η 2 -(O 2 )FeO 2 − isomers was obtained at the high-accurate RCCSD(T) level.…”

“…For NbC 3 − cluster on the contrary, the linear-NbC 3 − isomer is positioned by SAC−CI to be as much as 0.89 eV less stable than the cyclic-NbC 3 − isomer. 22 With such a large difference in stability, it is hard to imagine any involvement of the linear-NbC 3 − isomer in the experimental anion photoelectron spectra of the NbC 3 − stoichiometry. Obviously, more sophisticated quantum chemical calculations are indispensable in an effort to resolve the matter.…”

“…In previous work, these spectra were interpreted by employing the DFT and SAC−CI computational results. 22 For the anionic cluster, a singlet state of cyclic-NbC 3 − isomer was determined to be the anionic ground state, with the singlet of linear-NbC 3 − estimated to be 0.69 and 0.89 eV less stable. On the basis of these computational results, the X band was assigned to an electron detachment between the cyclic-NbC 3 −/0 structures.…”