Relativistic DFT-NMR of 19F in Tantalum(V) and Niobium(V) Fluorohalides

Abstract

The ¹⁹F NMR chemical shift in tantalum(V) and niobium(V) fluorohalides of general formula MF n X m Y l ⁻ (M = Ta, Nb; X, Y = Cl, Br; n + m + l = 6, n 24/12 1) is investigated by means of relativistic DFT calculations. A systematic benchmark of 21 computational protocols is presented, varying the density functional (BLYP, B3LYP, BH & HLYP;, PBE0), the basis set (DZ to QZ4P), the geometry optimization scheme, the inclusion of solvent effects via PCM or COSMO continuum models, and the type of Hamiltonian: non-relativistic, two-component ZORA scalar, twocomponent ZORA spin-orbit, and relativistic 4-component. The spin-orbit contribution to the fluorine shielding constant is found to be substantial, ranging from approximately 8 to 19 ppm across the series, confirming the importance of properly accounting for relativistic effects in these heavy-metal systems. The best agreement with experimental data is achieved at the ZSO-B3LYP(COSMO)/QZ4P level of theory, using geometries optimized at the ω-B97XD(PCM)/def2-TZVPP level, with correlation coefficients close to unity and mean absolute errors below 5 ppm for the tantalum series. Calculated chemical shifts are also reported for all 46 possible fluorohalide structures (containing at least one fluorine atom) of tantalum(V) and niobium(V), including those not experimentally detected. Finally, a re-examination of the experimental ¹⁹F NMR data reveals a likely misassignment in the niobium(V) series: the resonance originally attributed to the fluorine atom trans to bromine in cis-NbF 4 Br 2 ⁻ deviates by more than 50 ppm from the computed value and from the expected linear correlation between isostructural Ta(V) and Nb(V) complexes, suggesting an incorrect structural assignment in the original work.