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Title: Kinetic studies of the [NpO₂ (CO₃)₃]⁴⁻ ion at alkaline conditions using ¹³C NMR

Carbonate ligand-exchange rates on the [NpO₂ (CO₃)₃]⁴⁻ ion were determined using a saturation-transfer ¹³C nuclear magnetic resonance (NMR) pulse sequence in the pH range of 8.1 ≤ pH ≤ 10.5. Over the pH range 9.3 ≤ pH ≤ 10.5, which compares most directly with previous work of Stout et al.,1 we find an average rate, activation energy, enthalpy, and entropy of k298ex = 40.6(±4.3) s⁻¹, Ea =45.1(±3.8) kJ mol⁻¹, ΔH = 42.6(±3.8) kJ mol⁻¹, and ΔS = -72(±13) J mol⁻¹ K⁻¹, respectively. These activation parameters are similar to the Stout et al. results at pH 9.4. However, their room-temperature rate at pH 9.4, k298ex = 143(±1.0) s⁻¹, is ~3 times faster than what we experimentally determined at pH 9.3: k298ex = 45.4(±5.3) s⁻¹. Our rates for [NpO₂ (CO₃)₃]⁴⁻ are also faster by a factor of ~3 relative to the isoelectronic [UO₂(CO₃)₃]⁴⁻ as reported by Brucher et al.2 of k298ex = 13(±3) s⁻¹. Consistent with results for the [UO₂(CO₃)₃]⁴⁻ ion, we find evidence for a proton-enhanced pathway for carbonate exchange for the [NpO₂(CO₃)₃]⁴⁻ ion at pH < 9.0.
Authors:
 [1] ;  [2] ;  [2] ;  [1]
  1. Univ. of California, Davis, CA (United States)
  2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Publication Date:
OSTI Identifier:
1136162
DOE Contract Number:
AC52-07NA27344
Resource Type:
Journal Article
Resource Relation:
Journal Name: Inorganic Chemistry; Journal Volume: 53; Journal Issue: 8
Publisher:
American Chemical Society (ACS)
Research Org:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY