⁹⁵Mo Magic Angle Spinning NMR at High Field:  Improved Measurements and Structural Analysis of the Quadrupole Interaction in Monomolybdates and Isopolymolybdates

Abstract

In this study, 95Mo quadrupole couplings in various molydbates were measured easily and accurately with magic angle spinning (MAS) NMR under a directing field of 19.6 T. The resonance frequency of 54 MHz was sufficiently high to remove acoustic ringing artifacts, and the spectra could be analyzed in the usual terms of chemical shift and quadrupolar line shapes. For monomolybdates and molybdite, the quadrupole coupling dominated the NMR response, and the quadrupole parameters could be measured with better accuracy than in previous lower field studies. Moreover, despite the low symmetry of the molybdenum coordination, the usefulness of such measurements to probe molybdenum environments was established by ab initio density functional theory (DFT) calculations of the electric field gradient from known structures. The experimental NMR data correlated perfectly with the refined structures. In isopolymolybdates, the resonances were shapeless and DFT calculations were impossible because of the large and low symmetry unit cells. Nevertheless, empirical but clear NMR signatures were obtained from the spinning sidebands analysis or the MQMAS spectra. This was possible for the first time thanks to the improved baseline and sensitivity at high fields. With the generalization of NMR spectrometers operating above 17 T, it was predicted that 95Mo MAS NMR could evolve as a routine characterization tool for ill-defined structures such as supported molybdates in catalysis.

Density Functional Theory Calculations of 95Mo NMR Parameters in Solid‐State Compounds

“…[98,99] For all compounds, C Q values computed with PBE are in better agreement with experiments than those from LDA; the difference with PBE values is equal to approximately 0.15 MHz. This systematic MoO 4 0.00 0.00 0.00 0.00 0.00 [7] 0.00 0.00 0.00 0.00 0.00 mP60 overestimation is of the order of the error between PBE and the experimental results. This certainly comes from the fact that GGA functionals described better than LDA the electronic density near the nuclei, because strong density gradients are present in these regions.…”

“…A linear fit of the data was performed and yields a slope of À0.48 (R 2 and standard deviation are equal to 0.03 and 213 ppm, respectively). Computed and experimental values are rather poorly correlated since perfect agreement would result in a line with a slope of À1 as seen from Equation (7). To identify the origin of this discordance, the influence of different parameters of the calculation has been studied.…”

“…This latter should correspond to the calculated isotropic chemical shielding of a 1.5 m Na 2 MoO 4 aqueous solution which was used as reference. [7] As the theoretical study of this compound would require the study of dynamic and solvent effects that are beyond the scope of this article, we used the signal of the Mo atom of Na 2 MoO 4 as a new reference. In this case, the intercept of the linear fit becomes À793 ppm (shifted by + 33 ppm).…”

“…All solid-state 95 Mo NMR experiments have recently been performed using highfield spectrometers. [7,8,10] The experimental error on the CS was equal to or lower than 2 ppm for all compounds APW + lo calculations were carried out using the WIEN2k code, [111,112] in the PBE GGA. [113] For all compounds, sphere sizes were determined automatically.…”

NMR Spectroscopy of Intermetallic Compounds: An Experimental and Theoretical Approach to Local Atomic Arrangements in Binary Gallides

“…On the other hand, these quantities can be calculated by quantum mechanical methods giving insight into the chemical bonding. [4][5][6][7][8][9][10][11][12][13][14] Due to the metallic conductivity in the majority of intermetallic compounds, the interactions of the nuclear spins and the conduction electrons have to be considered in addition. [15][16][17] This accounts for an evaluation of the NMR spectroscopy method to specify reliable parameters for the investigation of atomic interactions in intermetallic compounds.…”