Oxygen and vanadium exchange processes in linear vanadate oligomers

The existence of the linear tri- and tetra-vanadate anions in aqueous solutionhas been confirmed by V-51 and O-17 NMR and potentiometry, yielding the formation constants. Their resonances are mostly broadened by an exchange process which is shown by the linewidths and by magnetisation-transfer experiments to be independent of the monomeric or the dimeric vanadates also present, and also of the solvent oxygens. The broadenings are not consistent with a simple process of exchange, but instead reveal the presence of an intermediate, probably cyclic, having a short but not insignificant lifetime. Also, simple proton transfer between the oxygens in the aqueous monoprotonated monovanadate anion is sufficiently slow to be detectable by NMR spectroscopy.