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 language | English |
|---|---|
| Pages (from-to) | 5264-5272 |
| Number of pages | 9 |
| Journal | Journal of the American Chemical Society |
| Volume | 132 |
| Issue number | 14 |
| DOIs | |
| State | Published - Apr 14 2010 |
| Externally published | Yes |
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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 journal › Article
}
TY - JOUR
T1 - Oxygen-isotope exchange rates for three isostructural polyoxometalate ions
AU - Villa, Eric
AU - Ohlin, C. Andreì
AU - Casey, William H.
PY - 2010/4/14
Y1 - 2010/4/14
N2 - 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.
AB - 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.
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U2 - 10.1021/ja100490n
DO - 10.1021/ja100490n
M3 - Article
VL - 132
SP - 5264
EP - 5272
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 14
ER -