Medium effects on 51V NMR chemical shifts: a density functional study.

Car-Parrinello molecular dynamics simulations were performed for [H2VO4], [VO2(OH2)4]+, and [VO(O2)2(OH2)]- in periodic boxes with 30, 28, and 29 water molecules, respectively, employing the BLYP density functional. On the timescale of the simulations, up to 2 ps, well-structured first solvation spheres are discernible for [H2VO4]- and [VO(O2)2(OH2)]- containing, on average, eight and ten water molecules, respectively. One of the four water molecules directly attached to the metal in [VO2(OH2)4]+ is only loosely bound, and the average coordination number of vanadium in aqueous VO2+ is between five and six. 51V chemical shifts were evaluated at the B3LYP level for representative snapshots along the trajectories, including the water molecules of the solvent by means of point charges. The resulting averaged delta(51V) values are proposed to model the combined effects of temperature (dynamic averaging) and solvent (charge polarization). Both effects are shown to be rather small, of the order of a few dozen ppm. The observed shielding of 51V in the bis(peroxo) complex with respect to the vanadate species is not reproduced computationally.