Solution structure of molybdic acid from Raman spectroscopy and DFT analysis

Protonation of MoO 4 2 - produces the well-characterized polymolybdates, but at concentrations below 10−3 M the dominant species is monomeric molybdic acid, H2MoO4. It is likely to be the species adsorbed on manganese oxide, a process thought to control MoO 4 2 - levels in the ocean, because of the strong proton dependence of MoO 4 2 - adsorption. The molecular structure of H2MoO4 is elusive, since it occurs only in dilute solutions. Using 244 nm laser excitation, near resonance with O → Mo charge-transfer electronic transitions of H2MoO4, we have detected a 919 cm−1 Raman band assignable to νsMoO. Using DFT, we have computed geometries and vibrational modes for the various structures consistent with the H2MoO4 formula. We tested the computations on a series of Mo(VI) oxo complexes with known vibrational frequencies, at several levels of theory. Best agreement with experimental values, at reasonable computational cost, was obtained with the B3LYP functional, employing a LANL2DZ ECP basis set for Mo and the 6-311+G(2df,p) basis set for O and H. Among the possible H2MoO4 structures only those based on the MoO3 unit, with one, two or three coordinated water molecules, gave a scaled frequency for νsMoO that was within two standard deviations of 919 cm−1. Best agreement was obtained for MoO3(H2O)3. The MoO2 and MoO structures gave frequencies that were too high. The Mo(OH)6 structure could be excluded, because its vibrational frequencies shift down strongly upon H/D exchange, whereas the 919 cm−1H2MoO4 band shifts up 1 cm−1 in D2O.