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.