Evaluation of ⁹⁵Mo Nuclear Shielding and Chemical Shift of [Mo₆X₁₄]^2– Clusters in the Liquid Phase

[Mo₆X₁₄]^2– octahedral molybdenum clusters are the main building blocks of a large range of materials. Although ⁹⁵Mo nuclear magnetic resonance was proposed to be a powerful tool to characterize their structural and dynamical properties in solution, these measurements have never been complemented by theoretical studies which can limit their interpretation for complex systems. In this Article, we use quantum chemical calculations to evaluate the ⁹⁵Mo chemical shift of three clusters: [Mo₆Cl₁₄]^2–, [Mo₆Br₁₄]^2–, and [Mo₆I₁₄]^2–. In particular, we test various computational parameters influencing the quality of the results: size of the basis set, treatment of relativistic and solvent effects. Furthermore, to provide quantum chemical calculations that are directly comparable with experimental data, we evaluate for the first time the ⁹⁵Mo nuclear magnetic shielding of the experimental reference, namely, MoO₄^2– in aqueous solution. This is achieved by combining ab initio molecular dynamics simulations with a periodic approach to evaluate the ⁹⁵Mo nuclear shieldings. The results demonstrate that, despite the difficulty to obtain accurate ⁹⁵Mo chemical shifts, relative values for a cluster series can be fairly well-reproduced by DFT calculations. We also show that performing an explicit solvent treatment for the reference compound improves by ∼50 ppm the agreement between theory and experiment. Finally, the standard deviation of ∼70 ppm that we calculate for the ⁹⁵Mo nuclear shielding of the reference provides an estimation of the accuracy we can achieve for the calculation of the ⁹⁵Mo chemical shifts using a static approach. These results demonstrate the growing ability of quantum chemical calculations to complement and interpret complex experimental measurements.