Tiferet Eitan, Gil Adrià, Bo Carles, Shvareva Tatiana Y, Nyman May, Navrotsky Alexandra
Peter A. Rock Thermochemistry Laboratory, University of California, Davis, One Shields Avenue, Davis, CA 95616 (USA); Nuclear Research Center-Negev, Be'er-Sheva 84190, Israel Institution (Israel).
Chemistry. 2014 Mar 24;20(13):3646-51. doi: 10.1002/chem.201304076. Epub 2014 Mar 6.
Nanoscale uranyl peroxide clusters containing UO2(2+) groups bonded through peroxide bridges to form polynuclear molecular species (polyoxometalates) exist both in solution and in the solid state. There is an extensive family of clusters containing 28 uranium atoms (U28 clusters), with an encapsulated anion in the center, for example, [UO2(O2)(3-x)(OH)(x)(4-)], [Nb(O2)4(3-)], or [Ta(O2)4(3-)]. The negative charge of these clusters is balanced by alkali ions, both encapsulated, and located exterior to the cluster. The present study reports measurement of enthalpy of formation for two such U28 compounds, one of which is uranyl centered and the other is peroxotantalate centered. The [(Ta(O2)4]-centered U28 capsule is energetically more stable than the [(UO2)(O2)3]-centered capsule. These data, along with our prior studies on other uranyl-peroxide solids, are used to explore the energy landscape and define thermochemical trends in alkali-uranyl-peroxide systems. It was suggested that the energetic role of charge-balancing alkali ions and their electrostatic interactions with the negatively charged uranyl-peroxide species is the dominant factor in defining energetic stability. These experimental data were supported by DFT calculations, which agree that the [(Ta(O2)4]-centered U28 capsule is more stable than the uranyl-centered capsule. Moreover, the relative stability is controlled by the interactions of the encapsulated alkalis with the encapsulated anion. Thus, the role of alkali-anion interactions was shown to be important at all length scales of uranyl-peroxide species: in both comparing clusters to clusters; and clusters to monomers or extended solids.
含有通过过氧化物桥键合的UO₂²⁺基团以形成多核分子物种（多金属氧酸盐）的纳米级过氧化铀酰簇既存在于溶液中，也存在于固态中。存在一个包含28个铀原子的广泛的簇家族（U28簇），其中心有一个包封的阴离子，例如，[UO₂(O₂)₃₋ₓ(OH)ₓ⁴⁻]、[Nb(O₂)₄³⁻]或[Ta(O₂)₄³⁻]。这些簇的负电荷由碱金属离子平衡，碱金属离子既包封在簇内，也位于簇的外部。本研究报告了两种此类U28化合物的生成焓的测量结果，其中一种以铀酰为中心，另一种以过氧钽酸盐为中心。以[(Ta(O₂)₄]为中心的U28胶囊在能量上比以[(UO₂)(O₂)₃]为中心的胶囊更稳定。这些数据，连同我们之前对其他过氧化铀酰固体的研究，被用于探索能量格局并确定碱 - 过氧化铀酰体系中的热化学趋势。有人提出，电荷平衡碱金属离子的能量作用及其与带负电荷的过氧化铀酰物种的静电相互作用是定义能量稳定性的主导因素。这些实验数据得到了密度泛函理论（DFT）计算的支持，该计算结果一致认为以[(Ta(O₂)₄]为中心的U28胶囊比以铀酰为中心的胶囊更稳定。此外，相对稳定性由包封的碱与包封的阴离子之间的相互作用控制。因此，碱 - 阴离子相互作用的作用在过氧化铀酰物种的所有长度尺度上都很重要：在比较簇与簇之间；以及簇与单体或扩展固体之间。
