Kinetic studies of the [NpO(2)(CO(3))(3)](4-) ion at alkaline conditions using (13)C 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 kex298 = 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, kex298 = 143(±1.0) s–1, is 3 times faster than what we experimentally determined at pH 9.3: kex298 = 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 kex298 = 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

Kinetic studies of the [NpO(2)(CO(3))(3)](4-) ion at alkaline conditions using (13)C 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 kex298 = 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, kex298 = 143(±1.0) s–1, is 3 times faster than what we experimentally determined at pH 9.3: kex298 = 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 kex298 = 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

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., we find an average rate, activation energy, enthalpy, and entropy of <i>k</i>_ex²⁹⁸ = 40.6(±4.3) s^–1, <i>E</i>_a =45.1(±3.8) kJ mol^–1, Δ<i>H</i>^‡ = 42.6(±3.8) kJ mol^–1, and Δ<i>S</i>^‡ = −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, <i>k</i>_ex²⁹⁸ = 143(±1.0) s^–1, is ∼3 times faster than what we experimentally determined at pH 9.3: <i>k</i>_ex²⁹⁸ = 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. of <i>k</i>_ex²⁹⁸ = 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

Rates of Water Exchange on the [Fe₄(OH)₂(hpdta)₂(H₂O)₄]⁰ Molecule and Its Implications for Geochemistry

The ammonium salt of [Fe₄O­(OH)­(hpdta)₂(H₂O)₄]⁻ is soluble and makes a monospecific solution of [Fe₄(OH)₂(hpdta)₂(H₂O)₄]⁰(aq) in acidic solutions (hpdta = 2-hydroxy­propane-1,3-diamino-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetraacetate). This tetramer is a diprotic acid with p<i>K</i>_a₁ estimated at 5.7 ± 0.2 and p<i>K</i>_a₂ = 8.8(5) ± 0.2. In the pH region below p<i>K</i>_a₁, the molecule is stable in solution and ¹⁷O NMR line widths can be interpreted using the Swift–Connick equations to acquire rates of ligand substitution at the four isolated bound water sites. Averaging five measurements at pH < 5, where contribution from the less-reactive conjugate base are minimal, we estimate: <i>k</i>_ex²⁹⁸ = 8.1 (±2.6) × 10⁵ s^–1, Δ<i>H</i>^⧧ = 46 (±4.6) kJ mol^–1, Δ<i>S</i>^⧧ = 22 (±18) J mol^–1 K^–1, and Δ<i>V</i>^⧧ = +1.85 (±0.2) cm³ mol^–1 for waters bound to the fully protonated, neutral molecule. Regressing the experimental rate coefficients versus 1/[H⁺] to account for the small pH variation in rate yields a similar value of <i>k</i>_ex²⁹⁸ = 8.3 (±0.8) × 10⁵ s^–1. These rates are ∼10⁴ times faster than those of the [Fe­(OH₂)₆]³⁺ ion (<i>k</i>_ex²⁹⁸ = 1.6 × 10² s^–1) but are about an order of magnitude slower than other studied aminocarboxylate complexes, although these complexes have seven-coordinated Fe­(III), not six as in the [Fe₄(OH)₂(hpdta)₂(H₂O)₄]⁰(aq) molecule. As pH approaches p<i>K</i>_a1, the rates decrease and a compensatory relation is evident between the experimental Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧ values. Such variation cannot be caused by enthalpy from the deprotonation reaction and is not well understood. A correlation between ⟨Fe^III–OH₂⟩ bond lengths and the logarithm of <i>k</i>_ex²⁹⁸ is geochemically important because it could be used to estimate rate coefficients for geochemical materials for which only DFT calculations are possible. This molecule is the only neutral, oxo-bridged Fe­(III) multimer for which rate data are available

New thermal decomposition pathway for TATB

Abstract Understanding the thermal decomposition behavior of TATB (1,3,5-triamino-2,4,6-trinitrobenzene) is a major focus in energetic materials research because of safety issues. Previous research and modelling efforts have suggested benzo-monofurazan condensation producing H2O is the initiating decomposition step. However, early evolving CO2 (m/z 44) along with H2O (m/z 18) evolution have been observed by mass spectrometric monitoring of head-space gases in both constant heating rate and isothermal decomposition studies. The source of the CO2 has not been explained, until now. With the recent successful synthesis of 13C6-TATB (13C incorporated into the benzene ring), the same experiments have been used to show the source of the CO2 is the early breakdown of the TATB ring, not adventitious C from impurities and/or adsorbed CO2. A shift in mass m/z 44 (CO2) to m/z 45 is observed throughout the decomposition process indicating the isotopically labeled 13C ring breakdown occurs at the onset of thermal decomposition along with furazan formation. Partially labeled (N18O2)3-TATB confirms at least some of the oxygen comes from the nitro-groups. This finding has a significant bearing on decomposition computational models for prediction of energy release and deflagration to detonation transitions, with respect to conditions which currently do not recognize this oxidation step