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...