Abstract: Niobium is an important chemical element having numerous practical applications in electronics and catalysis. In heterogeneous catalysis, niobium has been used since the early 1970s. Hydrated niobium oxides (Nb2O5·xH2O) have exhibited favorable properties such as catalytic activity, selectivity and stability, and to date many catalytic applications of niobium oxide have been identified and adapted in the chemical industry [1]. Growing interest in niobium chemistry, both in practical applications and in fundamental sciences, stimulates the development of new research spectroscopic tools. Solid-state 93Nb NMR is one of the most promising techniques to study the local environment of niobium in solids, particularly when diffraction methods may fail due to disorder or in amorphous phases. On the one hand, 93Nb, the only NMR active isotope of niobium, has several advantageous NMR properties, such as the high gyromagnetic ratio and the 100% natural abundance that makes it one of the most NMR receptive isotopes. On the other hand, its high nuclear spin (I = 9/2), the large quadrupole moment (Q = 0.32 barn) which introduces a second-order quadrupolar broadening to the central transition (-1/2―+1/2) spectra, often hamper 93Nb NMR researches, both in solutions and in solid state. In addition, 93Nb exhibits a rather large chemical shift range, which can cause significant broadening of static and MAS 93Nb NMR spectra of disordered systems. Computational chemistry has made good progress in calculations of NMR parameters of solids. Gauge Including Projector Augmented Wave method (GIPAW) introduced in 2001 has shown astonishing accuracy in calculating NMR parameters of periodic systems [2]. In this presentation the application of GIPAW method to niobium oxide compounds will be demonstrated. 93Nb NMR spectra are broadened by large quadrupole constant and by the distribution of spectral parameters. Sometimes that makes it impossible to determine NMR parameters of niobium spectra. Quantum chemical calculations made retrieval of NMR parameters easier and helped eliminate ambiguity in NMR experiments. Accuracy of the GIPAW method has been successfully tested; it allowed us to extend correlations between NMR parameters and structure of niobium oxide compounds [3]. Computation of NMR parameters provides a better understanding of complex experimental 93Nb NMR spectra, for example in the case of VNb9O25, a compound containing three different crystallographic niobium sites. The GIPAW calculations of NMR parameters of this compound allowed us to successfully explain its experimental NMR spectra [3]. Theoretical data may provide a helpful tool for interpretation of experimental results, for example, in case of excessively broadened spectra. Using the GIPAW method the distribution of NMR parameters of AlNbO4 was investigated. This compound has two sites; however, it possesses occupation disorder broadening X-ray reflections and peaks in 27Al and 93Nb NMR spectra. Theoretical computations allowed us to identify different sites and to propose some conclusions on the structure of this compound. Quantum chemical methods can provide structural insights in the case of complex NMR spectra and serve as a provision against incorrect identification of experimental NMR signals. K8Nb6O19, a compound with Lindqvist ion Nb6O198-, which has clear, but unfittable 93Nb NMR spectra, was studied. Initially, utilizing periodic DFT approach and using known X-ray data the crystallographic cell was simulated. The subsequent calculation of theoretical NMR parameters allowed us to fit the experimental spectrum. In addition, computational chemistry can investigate atomic mobility and help to identify experimentally observed sites, like in case of niobium oxalate, in which water loss was found to affect magnitude of nuclear quadrupole constant. The niobium site is seven-coordinated, two oxalate groups and water molecules are bonded to it. A small change in atomic coordinates strongly influences the NMR parameters of niobium site. Theoretical study showed that the chelate ligands are very mobile and the energy barriers between different states are small. These results can be used as a useful guide for understanding the mechanism of other processes. Authors thank funding provided via RFBR(project No 17-03-00531) and budget project No. 0303-2016-0002 for Boreskov Institute of Catalysis. ¹ K. Tanabe. Catalysis Today, 78: 65–77, 2003. ² C. J. Pickard and F. Mauri. Phys. Rev. B: Condens. Matter Mater. Phys., 63: 245101, 2001. ³ Evgeniy Papulovskiy, Alexandre A. Shubin, Victor V. Terskikh, Chris J. Pickard, Olga B. Lapina. Phys. Chem. Chem. Phys., 15: 5115-5131, 2013.

Cite: Papulovskiy E. , Shubin A.A. , Lapina O.B.
First-Principles Calculations Applied to Niobium Oxide Compounds
EUROMAR 2017 02-06 Jul 2017