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

Journal Article · Mon Apr 21 00:00:00 EDT 2014 · Inorganic Chemistry

DOI:https://doi.org/10.1021/ic500314v· OSTI ID:1136162

Panasci, Adele F. ^[1]; Harley, Stephen J. ^[2]; Zavarin, Mavrik ^[2]; Casey, William H. ^[1]

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1. Univ. of California, Davis, CA (United States)
2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

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 k²⁹⁸_ex = 40.6(±4.3) s⁻¹, E_a =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, k²⁹⁸_ex = 143(±1.0) s⁻¹, is ~3 times faster than what we experimentally determined at pH 9.3: k²⁹⁸_ex = 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 k²⁹⁸_ex = 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.

• Research Organization: Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC52-07NA27344
• OSTI ID: 1136162
• Journal Information: Inorganic Chemistry, Vol. 53, Issue 8; ISSN 0020-1669
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY