Slow proton exchange kinetics in aqueous solutions of hexaaquarhodium(III): Influence of the second hydration sphere

I. Bányai, Julius Glaser, Michael C. Read, Magnus Sandström

Research output: Contribution to journalArticle

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Abstract

The exchange processes between water protons in the first hydration sphere of the rhodium(III) ion and water protons in the bulk solvent have been studied using 1H NMR spectroscopy. The pseudo-first-order rate constants for proton exchange between bulk water and the first hydration sphere of rhodium(III) have been determined as a function of pH at 293 K from 1H NMR line-broadening experiments on aqueous solutions of rhodium(III), at magnetic field strengths of 5.87 T (250 MHz) and 9.40 T (400 MHz). A minimum in the rate of proton exchange is observed at pH ≈ 3 where the average lifetime of a specific proton in the first hydration sphere is τHRh = 7 ms in an 0.1 M solution of Rh(III). The rate of proton exchange increases with increasing pH when pH > 3 indicating a reaction path involving exchange between [Rh(H2O)5OH]2+ and bulk water protons. When pH <3, the rate of proton exchange increases asymptotically with decreasing pH. The pH dependence in this acidic region is explained by a mechanism for which the rate-determining step is the exchange of a proton from a hydronium ion in the second hydration sphere of rhodium(III) with one in the bulk. At very low pH (2O)6]3+ to the second sphere of hydrogen-bonded water molecules is proposed. The rate constant for this process is k1 = 6.0 (±0.2) × 104 s-1. The direct exchange between first sphere water protons in [Rh(H2O)6]3+ and bulk water protons is too slow to be detected. The acid dissociation constants for [Rh(H2O)6]3+, pKa1 = 3.6 ± 0.1(2σ), and [Rh(H2O)5OH]2+, pKa2 = 4.7 ± 0.2(2ó), have been determined by potentiometry in the ionic medium used in the kinetic experiments {[ClO4-] = 3 M; 3[Rh3+] + [Li+] + [H+] = 3 M}.

Original languageEnglish
Pages (from-to)2423-2429
Number of pages7
JournalInorganic Chemistry
Volume34
Issue number9
Publication statusPublished - 1995

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Hydration
hydration
Protons
Ion exchange
aqueous solutions
Kinetics
protons
kinetics
Rhodium
Water
rhodium
water
Rate constants
hydronium ions
potentiometric analysis
nuclear magnetic resonance
Nuclear magnetic resonance spectroscopy
Hydrogen
field strength
Experiments

ASJC Scopus subject areas

  • Inorganic Chemistry

Cite this

Slow proton exchange kinetics in aqueous solutions of hexaaquarhodium(III) : Influence of the second hydration sphere. / Bányai, I.; Glaser, Julius; Read, Michael C.; Sandström, Magnus.

In: Inorganic Chemistry, Vol. 34, No. 9, 1995, p. 2423-2429.

Research output: Contribution to journalArticle

Bányai, I. ; Glaser, Julius ; Read, Michael C. ; Sandström, Magnus. / Slow proton exchange kinetics in aqueous solutions of hexaaquarhodium(III) : Influence of the second hydration sphere. In: Inorganic Chemistry. 1995 ; Vol. 34, No. 9. pp. 2423-2429.
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abstract = "The exchange processes between water protons in the first hydration sphere of the rhodium(III) ion and water protons in the bulk solvent have been studied using 1H NMR spectroscopy. The pseudo-first-order rate constants for proton exchange between bulk water and the first hydration sphere of rhodium(III) have been determined as a function of pH at 293 K from 1H NMR line-broadening experiments on aqueous solutions of rhodium(III), at magnetic field strengths of 5.87 T (250 MHz) and 9.40 T (400 MHz). A minimum in the rate of proton exchange is observed at pH ≈ 3 where the average lifetime of a specific proton in the first hydration sphere is τHRh = 7 ms in an 0.1 M solution of Rh(III). The rate of proton exchange increases with increasing pH when pH > 3 indicating a reaction path involving exchange between [Rh(H2O)5OH]2+ and bulk water protons. When pH <3, the rate of proton exchange increases asymptotically with decreasing pH. The pH dependence in this acidic region is explained by a mechanism for which the rate-determining step is the exchange of a proton from a hydronium ion in the second hydration sphere of rhodium(III) with one in the bulk. At very low pH (2O)6]3+ to the second sphere of hydrogen-bonded water molecules is proposed. The rate constant for this process is k1 = 6.0 (±0.2) × 104 s-1. The direct exchange between first sphere water protons in [Rh(H2O)6]3+ and bulk water protons is too slow to be detected. The acid dissociation constants for [Rh(H2O)6]3+, pKa1 = 3.6 ± 0.1(2σ), and [Rh(H2O)5OH]2+, pKa2 = 4.7 ± 0.2(2{\'o}), have been determined by potentiometry in the ionic medium used in the kinetic experiments {[ClO4-] = 3 M; 3[Rh3+] + [Li+] + [H+] = 3 M}.",
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N2 - The exchange processes between water protons in the first hydration sphere of the rhodium(III) ion and water protons in the bulk solvent have been studied using 1H NMR spectroscopy. The pseudo-first-order rate constants for proton exchange between bulk water and the first hydration sphere of rhodium(III) have been determined as a function of pH at 293 K from 1H NMR line-broadening experiments on aqueous solutions of rhodium(III), at magnetic field strengths of 5.87 T (250 MHz) and 9.40 T (400 MHz). A minimum in the rate of proton exchange is observed at pH ≈ 3 where the average lifetime of a specific proton in the first hydration sphere is τHRh = 7 ms in an 0.1 M solution of Rh(III). The rate of proton exchange increases with increasing pH when pH > 3 indicating a reaction path involving exchange between [Rh(H2O)5OH]2+ and bulk water protons. When pH <3, the rate of proton exchange increases asymptotically with decreasing pH. The pH dependence in this acidic region is explained by a mechanism for which the rate-determining step is the exchange of a proton from a hydronium ion in the second hydration sphere of rhodium(III) with one in the bulk. At very low pH (2O)6]3+ to the second sphere of hydrogen-bonded water molecules is proposed. The rate constant for this process is k1 = 6.0 (±0.2) × 104 s-1. The direct exchange between first sphere water protons in [Rh(H2O)6]3+ and bulk water protons is too slow to be detected. The acid dissociation constants for [Rh(H2O)6]3+, pKa1 = 3.6 ± 0.1(2σ), and [Rh(H2O)5OH]2+, pKa2 = 4.7 ± 0.2(2ó), have been determined by potentiometry in the ionic medium used in the kinetic experiments {[ClO4-] = 3 M; 3[Rh3+] + [Li+] + [H+] = 3 M}.

AB - The exchange processes between water protons in the first hydration sphere of the rhodium(III) ion and water protons in the bulk solvent have been studied using 1H NMR spectroscopy. The pseudo-first-order rate constants for proton exchange between bulk water and the first hydration sphere of rhodium(III) have been determined as a function of pH at 293 K from 1H NMR line-broadening experiments on aqueous solutions of rhodium(III), at magnetic field strengths of 5.87 T (250 MHz) and 9.40 T (400 MHz). A minimum in the rate of proton exchange is observed at pH ≈ 3 where the average lifetime of a specific proton in the first hydration sphere is τHRh = 7 ms in an 0.1 M solution of Rh(III). The rate of proton exchange increases with increasing pH when pH > 3 indicating a reaction path involving exchange between [Rh(H2O)5OH]2+ and bulk water protons. When pH <3, the rate of proton exchange increases asymptotically with decreasing pH. The pH dependence in this acidic region is explained by a mechanism for which the rate-determining step is the exchange of a proton from a hydronium ion in the second hydration sphere of rhodium(III) with one in the bulk. At very low pH (2O)6]3+ to the second sphere of hydrogen-bonded water molecules is proposed. The rate constant for this process is k1 = 6.0 (±0.2) × 104 s-1. The direct exchange between first sphere water protons in [Rh(H2O)6]3+ and bulk water protons is too slow to be detected. The acid dissociation constants for [Rh(H2O)6]3+, pKa1 = 3.6 ± 0.1(2σ), and [Rh(H2O)5OH]2+, pKa2 = 4.7 ± 0.2(2ó), have been determined by potentiometry in the ionic medium used in the kinetic experiments {[ClO4-] = 3 M; 3[Rh3+] + [Li+] + [H+] = 3 M}.

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