Theoretical Studies of Structures of Vanadate Complexes in Aqueous Solution

Effects of thermal motion and interaction with a solvent on NMR chemical shifts of transition-metal complexes are studied with an approach based on molecular dynamics simulations and averaging of magnetic shieldings computed for snapshots along the trajectory. In some cases, for instance for iron cyanide complexes, large effects on the metal shielding are revealed, which can be traced back to changes in mean bond distances upon dynamical averaging in aqueous solution. For vanadate complexes in water, the effects are less pronounced, and amount to a few dozen ppm, irrespective of the density functional employed. The shielding of the 51V resonance on going from the parent vanadate to peroxovanadate complexes is not reproduced in the simulations. The dynamics of a peroxovanadium-imidazole complex, a model compound for biomimetic oxidants, is studied in the gas phase and in water. In the pristine complex, rapid rotation of the imidazole ligand about the V-N bond occurs on the picosecond timescale. This complex does not coordinate hydrogen peroxide at vanadium. The implications for a possible mechanism of the oxidation reactions are discussed.