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

42 Citations (Scopus)

Abstract

Protonation of MoO4 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 MoO4 2 - levels in the ocean, because of the strong proton dependence of MoO4 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 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

Fingerprint

Raman Spectrum Analysis
Vibrational spectra
Discrete Fourier transforms
Raman spectroscopy
acids
Laser excitation
Manganese oxide
Acids
Protonation
Level control
Molecular Structure
Oceans and Seas
Molecular structure
Adsorption
Protons
Charge transfer
Lasers
manganese oxides
Costs and Cost Analysis
frequency shift

Keywords

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

ASJC Scopus subject areas

  • Biochemistry
  • Inorganic Chemistry
  • Physical and Theoretical Chemistry
  • Materials Chemistry

Cite this

Solution structure of molybdic acid from Raman spectroscopy and DFT analysis. / Oyerinde, Oyeyemi F.; Weeks, Colin L.; Anbar, Ariel; Spiro, Thomas G.

In: Inorganica Chimica Acta, Vol. 361, No. 4, 03.03.2008, p. 1000-1007.

Research output: Contribution to journalArticle

Oyerinde, Oyeyemi F. ; Weeks, Colin L. ; Anbar, Ariel ; Spiro, Thomas G. / Solution structure of molybdic acid from Raman spectroscopy and DFT analysis. In: Inorganica Chimica Acta. 2008 ; Vol. 361, No. 4. pp. 1000-1007.
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T1 - Solution structure of molybdic acid from Raman spectroscopy and DFT analysis

AU - Oyerinde, Oyeyemi F.

AU - Weeks, Colin L.

AU - Anbar, Ariel

AU - Spiro, Thomas G.

PY - 2008/3/3

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N2 - Protonation of MoO4 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 MoO4 2 - levels in the ocean, because of the strong proton dependence of MoO4 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 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.

AB - Protonation of MoO4 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 MoO4 2 - levels in the ocean, because of the strong proton dependence of MoO4 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 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.

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KW - Molybdate

KW - Molybdenum

KW - Molybdic acid

KW - Raman spectroscopy

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