for heavier actinides, reflecting increased energy degeneracy driven covalency and concomitant orbital mixing between the 5f 24 orbitals of the An ions and the π orbitals of the ligand. Notably, the ability of this ligand to either accept or donate electron 25 density as needed from its pyridine rings is found to be key to its extraordinary stability across the actinide series. ■ INTRODUCTION27 Radiological contamination incidents can result in widespread 28 radiation exposure to both local and remote regions. 29 Representing an extreme recent example, the 2011 Fukushima 30 Daiichi Nuclear Power Plant accident resulted in the dispersal 31 of several radionuclides across a wide area, including portions of 32 the continental U.S.1 Actinide (An) and lanthanide (Ln) fission 33 product species are likely to be major components of such 34 contamination events, and it is therefore necessary to 35 thoroughly understand and study the behavior of these ions 36 in environmental and biological systems. Internal contamination of human populations in the event of 38 a radiological incident, whether accidental or intentional, is of 39 critical concern. Once internalized, An ions transit rapidly 40 throughout the bloodstream and are primarily deposited in the 41 liver and bones (uranium is an exception and preferentially 42 deposits in the kidneys rather than in the liver), from which 43 elimination occurs very slowly. 3,4 Uptake and deposition of 44 these ions present severe health risks due to both their 45 The present work uses density functional theory (DFT) 120 combined with extended X-ray absorption fine structure 121 (EXAFS) measurements to advance our current understanding 122 of An-3,4,3-LI(1,2-HOPO) complexes. Their structure, ther-123 modynamics, electronic structures, and redox properties are 124 investigated across the An series up to Es, with both formally 125 An(III) and An(IV) ions. The similarity of 3,4,3-LI(1,2-126 HOPO) to other biological complexants and the wealth of 127 excellent experimental information on this system ensures that 128 a fundamental understanding of An-3,4,3-LI(1,2-HOPO) 129 complexation will have applications beyond this single ligand, 130 and presents an opportunity to study trends in An-ligand 131 Other than the exceptions described above, the fit model 243 obtained from calculated structures describes the data well 244 (Figure 2). Generally speaking, the EXAFS data are consistent 245 with the HOPO complex structure calculations for the nearest 246 neighbor M(III)−O pair distances with the largest deviation 247 occurring with [Am(HOPO)] − . In addition, the Cf−C/N 248 average bond length differs from calculation, but the calculation 249 also shows a broad distribution width for the 8 bonds in this t1 250 shell. Table 1 compares the EXAFS bond length results to 251 those derived from the DFT calculations for the first two shells. 252 Further details of the methods and results are available in 253 Supporting Information (pp S11−S15). Figure S6 for thermodynamic calculations...

Recent reports have suggested the late actinides participate in more covalent interactions than the earlier actinides, yet the origin of this shift in chemistry is not understood. This report considers the chemistry of actinide dipicolinate complexes to identify why covalent interactions become more prominent for heavy actinides. A modest increase in measured actinide:dipicolinate stability constants is coincident with a significant increase in An 5f energy degeneracy with the dipicolinate molecular orbitals for Bk and Cf relative to Am and Cm. While the interactions in the actinide-dipicolinate complex are largely ionic, the decrease in 5f orbital energy across the series manifests in orbital-mixing and, hence, covalency driven by energy degeneracy. This observation suggests the origin of covalency in heavy actinide interactions stems from the degeneracy of 5f orbitals with ligand molecular orbitals rather than spatial orbital overlap. These findings suggest that the limiting radial extension of the 5f orbitals later in the actinide series could make the heavy actinides ideal elements to probe and tune effects of energy degeneracy driven covalency.

Radiometric and mass spectrometric analyses of Cs contamination in the environment can reveal the location of Cs emission sources, release mechanisms, modes of transport, prediction of future contamination migration, and attribution of contamination to specific generator(s) and/or process(es). The Subsurface Disposal Area (SDA) at Idaho National Laboratory (INL) represents a complicated case study for demonstrating the current capabilities and limitations to environmental Cs analyses. (137)Cs distribution patterns, (135)Cs/(137)Cs isotope ratios, known Cs chemistry at this site, and historical records enable narrowing the list of possible emission sources and release events to a single source and event, with the SDA identified as the emission source and flood transport of material from within Pit 9 and Trench 48 as the primary release event. These data combined allow refining the possible number of waste generators from dozens to a single generator, with INL on-site research and reactor programs identified as the most likely waste generator. A discussion on the ultimate limitations to the information that (135)Cs/(137)Cs ratios alone can provide is presented and includes (1) uncertainties in the exact date of the fission event and (2) possibility of mixing between different Cs source terms (including nuclear weapons fallout and a source of interest).

Ion pairing can have profound effects upon the ionic strength of electrolyte solutions but is poorly understood in solutions containing more than one solvent. Herein a combined density functional theory and molecular dynamics approach is used to examine the effect of both methanol concentration and interionic distance upon the structure and dynamics within successive solvation shells of Na(+) and Cl(-) in water/methanol binary solutions. The structure and dynamics of the first and second solvation shells were studied along a reaction coordinate associated with ion pair formation using potential of mean force simulations. The lifetimes of the solvent-solvent hydrogen bonds become perturbed when the second solvation shells of the ions begin to interact. In contrast, the structural properties within the first and second solvation shells of the ions were found to be largely independent of both methanol concentration and interionic distance until a contact ion pair is formed. Thus, as the ions are brought together, the effect of the opposing ion manifests itself in the solvation dynamics before any structural changes are observed. As anticipated based upon the decreased dielectric constant of the binary solution, ion pair formation becomes energetically more favorable as the concentration of methanol increases.

In all known examples of metal-ligand (M-L) δ and φ bonds, the metal orbitals are aligned to the ligand orbitals in a "head-to-head" or "side-to-head" fashion. Here, we report two fundamentally new types of M-L δ and φ interactions; "head-to-side" δ and "side-to-side" φ back-bonding, found in complexes of metallacyclopropenes and metallacyclocumulenes of actinides (Pa-Pu) that makes them distinct from their corresponding Group 4 analogues. In addition to the known Th and U complexes, our calculations include complexes of Pa, Np, and Pu. In contrast with conventional An-C bond decreasing, due to the actinide contraction, the An-C distance increases from Pa to Pu. We demonstrate that the direct L-An σ and π donations combined with the An-L δ or φ back-donations are crucial in explaining this nonclassical trend of the An-L bond lengths in both series, underscoring the significance of these δ/φ back-donation interactions, and their importance for complexes of Pa and U in particular.