Solution structure of molybdic acid from Raman spectroscopy and DFT analysis

Oyeyemi F. Oyerinde, Colin L. Weeks, Ariel Anbar, Thomas G. Spiro

Research output: Contribution to journalArticle

47 Scopus citations

Abstract

Protonation of MoO42 - 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 MoO42 - levels in the ocean, because of the strong proton dependence of MoO42 - 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 νsMo{double bond, long}O. 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 νsMo{double bond, long}O 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.

Original languageEnglish (US)
Pages (from-to)1000-1007
Number of pages8
JournalInorganica Chimica Acta
Volume361
Issue number4
DOIs
StatePublished - Mar 3 2008

Keywords

  • Density functional theory
  • Molybdate
  • Molybdenum
  • Molybdic acid
  • Raman spectroscopy

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry
  • Materials Chemistry

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