Moll Henry, Rossberg André, Steudtner Robin, Drobot Björn, Müller Katharina, Tsushima Satoru
Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf (HZDR) , Bautzner Landstraße 400, Dresden 01328, Germany.
Inorg Chem. 2014 Feb 3;53(3):1585-93. doi: 10.1021/ic402664n. Epub 2014 Jan 15.
The mechanism by which oxygen bound in UO2(2+) exchanges with that from water under strong alkaline conditions remains a subject of controversy. Two recent NMR studies independently revealed that the key intermediate species is a binuclear uranyl(VI) hydroxide, presumably of the stoichiometry [(UO2(OH)4(2-))(UO2(OH)5(3-))]. The presence of UO2(OH)5(3-) in highly alkaline solution was postulated in earlier experimental studies, yet the species has been little characterized. Quantum-chemical calculations (DFT and MP2) show that hydrolysis of UO2(OH)4(2-) yields UO3(OH)3(3-) preferentially over UO2(OH)5(3-). X-ray absorption spectroscopy was used to study the uranium(VI) speciation in a highly alkaline solution supporting the existence of a species with three U-O bonds, as expected for UO3(OH)3(3-). Therefore, we explored the oxygen exchange pathway through the binuclear adduct [(UO2(OH)4(2-))(UO3(OH)3(3-))] by quantum-chemical calculations. Assuming that the rate-dominating step is proton transfer between the oxygen atoms, the activation Gibbs energy for the intramolecular proton transfer within [(UO2(OH)4(2-))(UO3(OH)3(3-))] at the B3LYP level was estimated to be 64.7 kJ mol(-1). This value is in good agreement with the activation energy for "yl"-oxygen exchange in [(UO2(OH)4(2-))(UO2(OH)5(3-))] obtained from experiment by Szabó and Grenthe (Inorg. Chem. 2010, 49, 4928-4933), which is 60.8 ± 2.4 kJ mol(-1). Both the presence of UO3(OH)3(3-) and the scenario of an "yl"-oxygen exchange through a binuclear species in strong alkaline solution are supported by the present study.
在强碱性条件下，UO2(2+)中结合的氧与水中的氧发生交换的机制仍然存在争议。最近的两项核磁共振研究独立揭示，关键的中间物种是一种双核氢氧化铀酰(VI)，其化学计量可能为[(UO2(OH)4(2-))(UO2(OH)5(3-))]。早期的实验研究推测在高碱性溶液中存在UO2(OH)5(3-)，但对该物种的表征很少。量子化学计算(DFT和MP2)表明，UO2(OH)4(2-)的水解优先产生UO3(OH)3(3-)而非UO2(OH)5(3-)。X射线吸收光谱用于研究高碱性溶液中的铀(VI)形态，支持了存在具有三个U-O键的物种的观点，这与UO3(OH)3(3-)的预期一致。因此，我们通过量子化学计算探索了通过双核加合物[(UO2(OH)4(2-))(UO3(OH)3(3-))]的氧交换途径。假设速率主导步骤是氧原子之间的质子转移，在B3LYP水平下，[(UO2(OH)4(2-))(UO3(OH)3(3-))]内分子内质子转移的活化吉布斯自由能估计为64.7 kJ mol(-1)。该值与Szabó和Grenthe通过实验得到的[(UO2(OH)4(2-))(UO2(OH)5(3-))]中“yl”-氧交换的活化能60.8±2.4 kJ mol(-1)非常吻合。本研究支持了UO3(OH)3(3-)的存在以及在强碱性溶液中通过双核物种进行“yl”-氧交换的情况。
