DFT-NMR Investigation and V-51 3QMAS Experiments for Probing Surface OH Ligands and the Hydrogen-Bond Network in a Polyoxovanadate Cluster: The Case of Cs-4[H2V10O28]center dot 4H(2)O
Résumé
This work shows that the combination of first-principles calculations and V-51 NMR experiments is a powerful tool to elucidate the location of surface hydroxyl groups and to precisely describe the hydrogen bond network in the complex decavanadate cluster Cs-4[H2V10O28]center dot 4H(2)O, enhancing the strength of NMR crystallography. The detailed characterization of H-bond networks for these kinds of inorganic compounds is of primary importance and should benefit from the DFT-NMR predictions by considering explicitly the periodic boundary conditions. The determination of the CS4[H2V10O28]center dot 4H(2)O structure by single-crystal X-ray diffraction was not sufficiently accurate to provide the location of protons. From available diffraction data, five different protonated model structures have been built and optimized using DFT-based methods. The possible interconversion of two decavanadate isomers through a proton exchange is evaluated by calculating the energy barrier and recording variable-temperature H-1 MAS NMR spectra. First-principles calculations of V-51 NMR parameters clearly indicate that these parameters are very sensitive to the local intermolecular hydrogen-bonding interactions. Considering the OFT error limits, the fairly good agreement between calculated and experimental NMR parameters arising from the statistical modeling of the data allows the unambiguous assignment of the five V-51 NMR signals and, thus, the location of OH surface ligands in the decavanadate cluster. In particular, first-principles calculations accurately reproduce the V-51 quadrupolar parameters. These results are fully consistent with V-51 3QMAS NMR spectra recorded with and without H-1 decoupling. Finally, correlations are established between local octahedral VO6 deformations and V-51 NMR parameters (C-q and Delta delta), which will be useful for the characterization of a wide range of chemical species containing vanadium(V).