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 ⁹⁵Mo NMR spectrum of isopolyanion (IPA) Mo₇O₂₄^6- (I) at pH = 5 consists of a signal at 210 ppm and two overlapping peaks at 32 and ∼15 ppm with the intensity ratio ∼1:4:2, and that of β-Mo₈O₂₆^4- (II) consists of two signals at ∼100 and 10 ppm with the intensity ratio ∼1:3. A broad ⁹⁵Mo NMR line at around 0 ppm was observed in the pH range of IPA Mo₃₆O₁₁₂^8- (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 ¹⁷O NMR spectra, which suggests binding of up to two protons. The distribution diagram, derived from the ⁹⁵Mo NMR spectra, is given for [Mo] = 0.4 M. The ⁹⁵Mo NMR signals shift to lower frequencies with increasing number and strength of the Mo−O terminal bonds.