95Mo and 17O NMR studies of aqueous molybdate solutions

Aqueous molybdate solutions with molybdenum concentrations of [Mo] = 1, 0.4, and 0.2 M have been studied by NMR at pH 7-1 and in 0.3-6 M HClO4. The Mo-95 NMR spectrum of isopolyanion (IPA) Mo7O246- (I) at pH = 5 consists of a signal at 210 ppm and two overlapping peaks at 32 and similar to 15 ppm with the intensity ratio similar to 1:4:2, and that of beta-Mo8O264- (II) consists of two signals at similar to 100 and 10 ppm with the intensity ratio similar to 1:3. A broad Mo-95 NMR line at around 0 ppm was observed in the pH range of IPA Mo36O1128- (III), and a signal of cationic oxospecies including MoO22+ (IV) was observed from -62 to -69 ppm. Two protonation sites of IPA I have been identified from O-17 NMR spectra, which suggests binding of up to two protons. The distribution diagram, derived from the Mo-95 NMR spectra, is given for [Mo] = 0.4 M. The Mo-95 NMR signals shift to lower frequencies with increasing number and strength of the Mo-O terminal bonds.