Oxygen-isotope exchange rates for three isostructural polyoxometalate ions

Eric Villa, C. Andreì Ohlin, William H. Casey

Research output: Contribution to journalArticle

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Abstract

We compare oxygen-isotope exchange rates for all structural oxygens in three polyoxoniobate ions that differ by systematic metal substitutions of Ti(IV) → Nb(V). The [H xNb 10O 28] (6-x)-, [H xTiNb 9O 28] (7-x)-, and [H xTi 2Nb 8O 28] (8-x)- ions are all isostructural yet have different Brãnsted properties. Rates for sites within a particular molecule in the series differ by at least ∼10 4, but the relative reactivities of the oxygen sites rank in nearly the same relative order for all ions in the series. Within a single ion, most structural oxygens exhibit rates of isotopic exchange that vary similarly with pH, indicating that each structure responds as a whole to changes in pH. Across the series of molecules, however, the pH dependencies for isotope exchanges and dissociation are distinctly different, reflecting different contributions from proton- or base-enhanced pathways. The proton-enhanced pathway for isotope exchange dominates at most pH conditions for the [H xTi 2Nb 8O 28] (8-x)- ion, but the base-enhanced pathways are increasingly important for the [H xTiNb 9O 28] (7-x)- and [H xNb 10O 28] (6-x)- structures at higher pH. The local effect of Ti(IV) substitution could be assessed by comparing rates for structurally similar oxygens on each side of the [H xTiNb 9O 28] (7-x)- ion and is surprisingly small. Interestingly, these nanometer-size structures seem to manifest the same general averaged amphoteric chemistry that is familiar for other reactions affecting oxides in water, including interface dissolution by proton- and hydroxyl-enhanced pathways.

Original languageEnglish
Pages (from-to)5264-5272
Number of pages9
JournalJournal of the American Chemical Society
Volume132
Issue number14
DOIs
StatePublished - Apr 14 2010
Externally publishedYes

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Oxygen Isotopes
Isotopes
Ion exchange
Ions
Oxygen
Protons
Substitution reactions
Molecules
Hydroxyl Radical
Oxides
polyoxometalate I
Dissolution
Metals
Water

All Science Journal Classification (ASJC) codes

  • Chemistry(all)
  • Catalysis
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Oxygen-isotope exchange rates for three isostructural polyoxometalate ions. / Villa, Eric; Ohlin, C. Andreì; Casey, William H.

In: Journal of the American Chemical Society, Vol. 132, No. 14, 14.04.2010, p. 5264-5272.

Research output: Contribution to journalArticle

Villa, Eric ; Ohlin, C. Andreì ; Casey, William H. / Oxygen-isotope exchange rates for three isostructural polyoxometalate ions. In: Journal of the American Chemical Society. 2010 ; Vol. 132, No. 14. pp. 5264-5272.
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abstract = "We compare oxygen-isotope exchange rates for all structural oxygens in three polyoxoniobate ions that differ by systematic metal substitutions of Ti(IV) → Nb(V). The [H xNb 10O 28] (6-x)-, [H xTiNb 9O 28] (7-x)-, and [H xTi 2Nb 8O 28] (8-x)- ions are all isostructural yet have different Br{\~a}nsted properties. Rates for sites within a particular molecule in the series differ by at least ∼10 4, but the relative reactivities of the oxygen sites rank in nearly the same relative order for all ions in the series. Within a single ion, most structural oxygens exhibit rates of isotopic exchange that vary similarly with pH, indicating that each structure responds as a whole to changes in pH. Across the series of molecules, however, the pH dependencies for isotope exchanges and dissociation are distinctly different, reflecting different contributions from proton- or base-enhanced pathways. The proton-enhanced pathway for isotope exchange dominates at most pH conditions for the [H xTi 2Nb 8O 28] (8-x)- ion, but the base-enhanced pathways are increasingly important for the [H xTiNb 9O 28] (7-x)- and [H xNb 10O 28] (6-x)- structures at higher pH. The local effect of Ti(IV) substitution could be assessed by comparing rates for structurally similar oxygens on each side of the [H xTiNb 9O 28] (7-x)- ion and is surprisingly small. Interestingly, these nanometer-size structures seem to manifest the same general averaged amphoteric chemistry that is familiar for other reactions affecting oxides in water, including interface dissolution by proton- and hydroxyl-enhanced pathways.",
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