Quantifying Actinide-Ligand Covalency with Resonant Inelastic X-ray Scattering

英国投资了世界领先的同步加速器X射线设施。这包括最先进的共振非弹性X射线散射（RIXS）技术，以前所未有的能量分辨率测量电子结构。然而，由于缺乏限制定量见解的分析专业知识，RIXS很少充分发挥其潜力。最不发达的领域是锕系元素RIXS分析。锕系元素是元素周期表中最大的元素之一，其化学性质非常难以预测。在适合获取锕系元素的能量下，RIXS的发展为实验测量锕系元素的物理和化学性质开辟了一条未知的途径。RIXS是一种针对外锕系元素轨道的元素特异性技术。因此，该方法对锕系轨道如何在分子水平上参与成键具有选择性敏感性。该项目旨在将RIXS发展成为一种定量工具，以促进对锕系元素如何参与化学键合的理解。键共价的程度和性质是特别重要的，因为这深刻地影响物理性质，反应性和锕系元素键形成的选择性。关于锕系元素成键的共价性有很多争论。一方面，共价可以被认为是原子之间电子密度的混合，另一方面，共价可以被理解为当锕系元素和成键原子轨道的能量匹配时发生。很少有实验技术对锕系元素的共价性敏感，并且那些存在的技术仅限于特定的情况，即仅某些氧化态或键合原子的类型。RIXS有可能弥合锕系化合物的合成分离和第一性原理理论之间的差距。PI的初步研究证实了RIXS对铀键共价性的敏感性，确定了进一步了解锕系元素键合的机会。锕系元素复杂的化学键合特性是核能部门面临的一个重大挑战。该项目将开发各种方法和新知识，从而改进在核废料处理中将锕系元素与其他元素分离的过程，并更好地了解锕系元素如何在环境中相互作用。拟议的研究需要RIXS测量和理论模拟的平等结合。多层次的理论将被应用于确定最准确的手段来模拟光谱。合成化学的进展提供了系统的化合物家族，将用于识别光谱趋势，帮助RIXS分析的发展。重点将放在铀和钍上，但开发的方法同样适用于超铀化合物的研究。RIXS的测量方法、分析和模拟方法将以一个易于使用的软件包的形式发布，从而将RIXS的全部潜力交给X射线界。初步研究将探索单键和双键中的供体共价性，以阐明RIXS光谱形状与电子结构之间的关系。将研究较少探索的情况，包括预测为高度共价的化合物。然后，将开发的RIXS分析方法应用于合作者和项目合作伙伴制备的新型分子。这将包括一系列分子，这些分子可以在周期表的整个列中采用各种金属离子，这样我们对U和Th键的新发现就可以放在周期表的更大背景下。最后，L和M-边RIXS的互补使用将被应用于确定锕系元素电子结构中最有争议和最难以捉摸的问题之一：5 f与6d轨道对共价的贡献的不同程度。

The UK has invested in world-leading synchrotron X-ray facilities. This includes state-of-the-art resonant inelastic X-ray scattering (RIXS) techniques that measure electronic structure with unprecedented energy resolution. RIXS is, however, infrequently applied to its full potential due to a lack of analytical expertise that limits quantitative insights. The least developed area of all is the analysis of actinide RIXS. Actinides are amongst the largest elements within the periodic table, and their chemistry is notoriously challenging to predict. The development of RIXS at energies suitable for accessing actinides has opened up an unexplored avenue to experimentally measure actinide physical and chemical properties. RIXS is an element-specific technique that targets the outer actinide orbitals. The method is therefore selectively sensitive to how actinide orbitals engage in bonding at the molecular level. This project aims to develop RIXS into a quantitative tool to advance understanding of how actinides engage in chemical bonding. The extent and nature of bond-covalency is of particular importance since this profoundly influences physical properties, reactivity and the selectivity of actinide bond formation. There is much debate concerning the covalency of actinide bonding. On one hand, covalency can be considered as the mixing of electron density between atoms, and on the other covalency can be understood as occurring when the energy of the actinide and bonding atom orbitals match up. There are few experimental techniques with sensitivity to actinide covalency, and those that exist are limited to specific cases, i.e. only certain oxidation states, or types of bonded atom. RIXS has the potential to bridge the gap between the synthetic isolation of actinide compounds and first principle theory. Preliminary research by the PI has confirmed the sensitivity of RIXS to uranium bond covalency, identifying opportunities to advance understanding of actinide bonding.The complex chemical bonding properties of actinides represent a major challenge to the nuclear energy sector. This project will develop methodologies and new knowledge that could lead to improved processes for the separation of actinides from other elements in nuclear waste processing and to better understand how actinides interact within the environment. The proposed research requires an equal combination of RIXS measurement and theoretical simulations. Multiple levels of theory will be applied to identify the most accurate means to simulate spectra. Advances in synthetic chemistry have provided systematic families of compounds, that will be used to identify spectral trends, aiding the development of RIXS analysis. The focus will be on uranium and thorium, but the methods developed will be equally relevant to the study of transuranic compounds. The RIXS measurement methodologies, analysis and simulation methods developed will be distributed in an easy to use software package, to put the full potential of RIXS into the hands of the X-ray community.Initial studies will explore donor covalency in single and double bonds to elucidate how RIXS spectral shape correlates with electronic structure. Less explored situations will be investigated, including compounds predicted as being highly covalent.The developed RIXS analysis methods will then be applied to novel molecules prepared by collaborators and project partners. This will include a series of molecules that can adopt a variety of metal ions down a full column of the periodic table, such that our newfound understanding of U and Th bonding can be placed within the larger context of the periodic table. Finally, the complementary use of L and M-edge RIXS will be applied to pin down one of the most controversial and elusive problems in actinide electronic structure: the varying extent of 5f versus 6d orbital contributions to covalency.