19F NMR study of the equilibria and dynamics of the Al3+/F- system

A. Bodor, I. Tóth, I. Bányai, Z. Szabó, G. T. Hefter

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Abstract

A careful reinvestigation by high-field 19F NMR (470 MHz) spectroscopy has been made of the Al3+/F- system in aqueous solution under carefully controlled conditions of pH, concentration, ionic strength (I), and temperature. The 19F NMR spectra show five distinct signals at 278 K and I = 0.6 M (TMACl) which have been attributed to the complexes AlF(i)((3-i)+)(aq) with i ≤ 5. There was no need to invoke AlF(i)(OH)(j)((3-i-j)+) mixed complexes in the model under our experimental conditions (pH ≤ 6.5), nor was any evidence obtained for the formation of AlF63-(aq) at very high ratios of F-/Al3+. The stepwise equilibrium constants obtained for the complexes by integration of the 19F signals are in good agreement with literature data given the differences in medium and temperature. In I = 0.6 M TMACl at 278 K and in I = 3 M KCl at 298 K the log K(i) values are 6.42, 5.41, 3.99, 2.50, and 0.84 (for species i = 1 - 5) and 6.35, 5.25, and 4.11 (for species i = 1 - 3), respectively. Disappearance of the 19F NMR signals under certain conditions was shown to be due to precipitation. Certain 19F NMR signals exhibit temperature- and concentration-dependent exchange broadening. Detailed line shape analysis of the spectra and magnetization transfer measurements indicate that the kinetics are dominated by F- exchange rather than complex formation. The detected reactions and their rate constants are AlF22+ + *F- ⇆ AlF*F2+ + F- (k02 = (1.8 ± 0.3) x 106 M-1 s-1), AlF30 + *F- ⇆ AlF2*F0 + F- (k03 = (3.9 ± 0.9) x 106 M-1 s-1), and AlF30 + H*F ⇆ AlF2*F0 + HF (k(H)03 = (6.6 ± 0.5) x 104 M-1 s-1). The rates of these exchange reactions increase markedly with increasing F- substitution. Thus, the reactions of AlF2+(aq) were too inert to be detected even on the T1 NMR time scale, while some of the reactions of AlF30(aq) were fast, causing large line broadening. The ligand exchange appears to follow an associative interchange mechanism. The cis-trans isomerization of AlF2+(aq), consistent with octahedral geometry for that complex, is slowed sufficiently to be observed at temperatures around 270 K. Difference between the Al3+/F- system and the much studied Al3+/OH- system are briefly commented on.

Original languageEnglish
Pages (from-to)2530-2537
Number of pages8
JournalInorganic Chemistry
Volume39
Issue number12
DOIs
Publication statusPublished - Jun 12 2000

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Nuclear magnetic resonance
nuclear magnetic resonance
Temperature
temperature
Equilibrium constants
Interchanges
Isomerization
Ionic strength
isomerization
line shape
Rate constants
Magnetization
Ion exchange
Substitution reactions
Spectroscopy
substitutes
Ligands
aqueous solutions
magnetization
ligands

ASJC Scopus subject areas

  • Inorganic Chemistry

Cite this

19F NMR study of the equilibria and dynamics of the Al3+/F- system. / Bodor, A.; Tóth, I.; Bányai, I.; Szabó, Z.; Hefter, G. T.

In: Inorganic Chemistry, Vol. 39, No. 12, 12.06.2000, p. 2530-2537.

Research output: Contribution to journalArticle

Bodor, A. ; Tóth, I. ; Bányai, I. ; Szabó, Z. ; Hefter, G. T. / 19F NMR study of the equilibria and dynamics of the Al3+/F- system. In: Inorganic Chemistry. 2000 ; Vol. 39, No. 12. pp. 2530-2537.
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abstract = "A careful reinvestigation by high-field 19F NMR (470 MHz) spectroscopy has been made of the Al3+/F- system in aqueous solution under carefully controlled conditions of pH, concentration, ionic strength (I), and temperature. The 19F NMR spectra show five distinct signals at 278 K and I = 0.6 M (TMACl) which have been attributed to the complexes AlF(i)((3-i)+)(aq) with i ≤ 5. There was no need to invoke AlF(i)(OH)(j)((3-i-j)+) mixed complexes in the model under our experimental conditions (pH ≤ 6.5), nor was any evidence obtained for the formation of AlF63-(aq) at very high ratios of F-/Al3+. The stepwise equilibrium constants obtained for the complexes by integration of the 19F signals are in good agreement with literature data given the differences in medium and temperature. In I = 0.6 M TMACl at 278 K and in I = 3 M KCl at 298 K the log K(i) values are 6.42, 5.41, 3.99, 2.50, and 0.84 (for species i = 1 - 5) and 6.35, 5.25, and 4.11 (for species i = 1 - 3), respectively. Disappearance of the 19F NMR signals under certain conditions was shown to be due to precipitation. Certain 19F NMR signals exhibit temperature- and concentration-dependent exchange broadening. Detailed line shape analysis of the spectra and magnetization transfer measurements indicate that the kinetics are dominated by F- exchange rather than complex formation. The detected reactions and their rate constants are AlF22+ + *F- ⇆ AlF*F2+ + F- (k02 = (1.8 ± 0.3) x 106 M-1 s-1), AlF30 + *F- ⇆ AlF2*F0 + F- (k03 = (3.9 ± 0.9) x 106 M-1 s-1), and AlF30 + H*F ⇆ AlF2*F0 + HF (k(H)03 = (6.6 ± 0.5) x 104 M-1 s-1). The rates of these exchange reactions increase markedly with increasing F- substitution. Thus, the reactions of AlF2+(aq) were too inert to be detected even on the T1 NMR time scale, while some of the reactions of AlF30(aq) were fast, causing large line broadening. The ligand exchange appears to follow an associative interchange mechanism. The cis-trans isomerization of AlF2+(aq), consistent with octahedral geometry for that complex, is slowed sufficiently to be observed at temperatures around 270 K. Difference between the Al3+/F- system and the much studied Al3+/OH- system are briefly commented on.",
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AU - Tóth, I.

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AU - Hefter, G. T.

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N2 - A careful reinvestigation by high-field 19F NMR (470 MHz) spectroscopy has been made of the Al3+/F- system in aqueous solution under carefully controlled conditions of pH, concentration, ionic strength (I), and temperature. The 19F NMR spectra show five distinct signals at 278 K and I = 0.6 M (TMACl) which have been attributed to the complexes AlF(i)((3-i)+)(aq) with i ≤ 5. There was no need to invoke AlF(i)(OH)(j)((3-i-j)+) mixed complexes in the model under our experimental conditions (pH ≤ 6.5), nor was any evidence obtained for the formation of AlF63-(aq) at very high ratios of F-/Al3+. The stepwise equilibrium constants obtained for the complexes by integration of the 19F signals are in good agreement with literature data given the differences in medium and temperature. In I = 0.6 M TMACl at 278 K and in I = 3 M KCl at 298 K the log K(i) values are 6.42, 5.41, 3.99, 2.50, and 0.84 (for species i = 1 - 5) and 6.35, 5.25, and 4.11 (for species i = 1 - 3), respectively. Disappearance of the 19F NMR signals under certain conditions was shown to be due to precipitation. Certain 19F NMR signals exhibit temperature- and concentration-dependent exchange broadening. Detailed line shape analysis of the spectra and magnetization transfer measurements indicate that the kinetics are dominated by F- exchange rather than complex formation. The detected reactions and their rate constants are AlF22+ + *F- ⇆ AlF*F2+ + F- (k02 = (1.8 ± 0.3) x 106 M-1 s-1), AlF30 + *F- ⇆ AlF2*F0 + F- (k03 = (3.9 ± 0.9) x 106 M-1 s-1), and AlF30 + H*F ⇆ AlF2*F0 + HF (k(H)03 = (6.6 ± 0.5) x 104 M-1 s-1). The rates of these exchange reactions increase markedly with increasing F- substitution. Thus, the reactions of AlF2+(aq) were too inert to be detected even on the T1 NMR time scale, while some of the reactions of AlF30(aq) were fast, causing large line broadening. The ligand exchange appears to follow an associative interchange mechanism. The cis-trans isomerization of AlF2+(aq), consistent with octahedral geometry for that complex, is slowed sufficiently to be observed at temperatures around 270 K. Difference between the Al3+/F- system and the much studied Al3+/OH- system are briefly commented on.

AB - A careful reinvestigation by high-field 19F NMR (470 MHz) spectroscopy has been made of the Al3+/F- system in aqueous solution under carefully controlled conditions of pH, concentration, ionic strength (I), and temperature. The 19F NMR spectra show five distinct signals at 278 K and I = 0.6 M (TMACl) which have been attributed to the complexes AlF(i)((3-i)+)(aq) with i ≤ 5. There was no need to invoke AlF(i)(OH)(j)((3-i-j)+) mixed complexes in the model under our experimental conditions (pH ≤ 6.5), nor was any evidence obtained for the formation of AlF63-(aq) at very high ratios of F-/Al3+. The stepwise equilibrium constants obtained for the complexes by integration of the 19F signals are in good agreement with literature data given the differences in medium and temperature. In I = 0.6 M TMACl at 278 K and in I = 3 M KCl at 298 K the log K(i) values are 6.42, 5.41, 3.99, 2.50, and 0.84 (for species i = 1 - 5) and 6.35, 5.25, and 4.11 (for species i = 1 - 3), respectively. Disappearance of the 19F NMR signals under certain conditions was shown to be due to precipitation. Certain 19F NMR signals exhibit temperature- and concentration-dependent exchange broadening. Detailed line shape analysis of the spectra and magnetization transfer measurements indicate that the kinetics are dominated by F- exchange rather than complex formation. The detected reactions and their rate constants are AlF22+ + *F- ⇆ AlF*F2+ + F- (k02 = (1.8 ± 0.3) x 106 M-1 s-1), AlF30 + *F- ⇆ AlF2*F0 + F- (k03 = (3.9 ± 0.9) x 106 M-1 s-1), and AlF30 + H*F ⇆ AlF2*F0 + HF (k(H)03 = (6.6 ± 0.5) x 104 M-1 s-1). The rates of these exchange reactions increase markedly with increasing F- substitution. Thus, the reactions of AlF2+(aq) were too inert to be detected even on the T1 NMR time scale, while some of the reactions of AlF30(aq) were fast, causing large line broadening. The ligand exchange appears to follow an associative interchange mechanism. The cis-trans isomerization of AlF2+(aq), consistent with octahedral geometry for that complex, is slowed sufficiently to be observed at temperatures around 270 K. Difference between the Al3+/F- system and the much studied Al3+/OH- system are briefly commented on.

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