Monday, April 26, 2010

J. Am. Chem. Soc., 2010, 132 (13), pp 4653–4668

DFT-NMR Investigation and 51V 3QMAS Experiments for Probing Surface OH Ligands and the Hydrogen-Bond Network in a Polyoxovanadate Cluster: The Case of Cs4[H2V10O28]·4H2O

Lionel A. Truflandier, Florent Boucher, Christophe Payen, Redouane Hajjar, Yannick Millot, Christian Bonhomme and Nathalie Steunou

This work shows that the combination of first-principles calculations and 51V 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 Cs4[H2V10O28].4H2O, 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]·4H2O 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 1H MAS NMR spectra. First-principles calculations of 51V NMR parameters clearly indicate that these parameters are very sensitive to the local intermolecular hydrogen-bonding interactions. Considering the DFT 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 51V NMR signals and, thus, the location of OH surface ligands in the decavanadate cluster. In particular, first-principles calculations accurately reproduce the 51V quadrupolar parameters. These results are fully consistent with 51V 3QMAS NMR spectra recorded with and without 1H decoupling. Finally, correlations are established between local octahedral VO6 deformations and 51V NMR parameters (Cq and Δδ), which will be useful for the characterization of a wide range of chemical species containing vanadium(V).

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