7 research outputs found
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
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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<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>]<sup>4–</sup> Ion at Alkaline Conditions Using <sup>13</sup>C NMR
Carbonate
ligand-exchange rates on the [NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>]<sup>4–</sup> ion were determined using a saturation-transfer <sup>13</sup>C 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><sub>ex</sub><sup>298</sup> = 40.6(±4.3)
s<sup>–1</sup>, <i>E</i><sub>a</sub> =45.1(±3.8)
kJ mol<sup>–1</sup>, Δ<i>H</i><sup>‡</sup> = 42.6(±3.8) kJ mol<sup>–1</sup>, and Δ<i>S</i><sup>‡</sup> = −72(±13) J mol<sup>–1</sup> K<sup>–1</sup>, 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><sub>ex</sub><sup>298</sup> = 143(±1.0) s<sup>–1</sup>, is ∼3 times faster than what we experimentally determined
at pH 9.3: <i>k</i><sub>ex</sub><sup>298</sup> = 45.4(±5.3) s<sup>–1</sup>.
Our rates for [NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>]<sup>4–</sup> are also faster by a factor of ∼3 relative to the isoelectronic
[UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>]<sup>4–</sup> as
reported by Brucher et al. of <i>k</i><sub>ex</sub><sup>298</sup> = 13(±3) s<sup>–1</sup>. Consistent with results for
the [UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>]<sup>4–</sup> ion, we find evidence for a proton-enhanced pathway for carbonate
exchange for the [NpO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>]<sup>4–</sup> ion at pH < 9.0
Rates of Water Exchange on the [Fe<sub>4</sub>(OH)<sub>2</sub>(hpdta)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>0</sup> Molecule and Its Implications for Geochemistry
The ammonium salt of [Fe<sub>4</sub>O(OH)(hpdta)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>−</sup> is soluble and makes a
monospecific solution of [Fe<sub>4</sub>(OH)<sub>2</sub>(hpdta)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>0</sup>(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><sub>a</sub><sub>1</sub> estimated at 5.7 ± 0.2 and p<i>K</i><sub>a</sub><sub>2</sub> = 8.8(5) ± 0.2. In the pH region below p<i>K</i><sub>a</sub><sub>1</sub>, the molecule is stable in solution
and <sup>17</sup>O 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><sub>ex</sub><sup>298</sup> = 8.1 (±2.6)
× 10<sup>5</sup> s<sup>–1</sup>, Δ<i>H</i><sup>⧧</sup> = 46 (±4.6) kJ mol<sup>–1</sup>,
Δ<i>S</i><sup>⧧</sup> = 22 (±18) J mol<sup>–1</sup> K<sup>–1</sup>, and Δ<i>V</i><sup>⧧</sup> = +1.85 (±0.2) cm<sup>3</sup> mol<sup>–1</sup> for waters bound to the fully protonated, neutral molecule. Regressing
the experimental rate coefficients versus 1/[H<sup>+</sup>] to account
for the small pH variation in rate yields a similar value of <i>k</i><sub>ex</sub><sup>298</sup> = 8.3 (±0.8) × 10<sup>5</sup> s<sup>–1</sup>. These rates are ∼10<sup>4</sup> times faster than those of the [Fe(OH<sub>2</sub>)<sub>6</sub>]<sup>3+</sup> ion (<i>k</i><sub>ex</sub><sup>298</sup> = 1.6
× 10<sup>2</sup> s<sup>–1</sup>) 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 [Fe<sub>4</sub>(OH)<sub>2</sub>(hpdta)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>0</sup>(aq) molecule. As pH approaches p<i>K</i><sub>a1</sub>, the rates decrease and a compensatory relation
is evident between the experimental Δ<i>H</i><sup>⧧</sup> and Δ<i>S</i><sup>⧧</sup> values.
Such variation cannot be caused by enthalpy from the deprotonation
reaction and is not well understood. A correlation between ⟨Fe<sup>III</sup>–OH<sub>2</sub>⟩ bond lengths and the logarithm
of <i>k</i><sub>ex</sub><sup>298</sup> 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