A Quantum Chemical and Molecular Dynamics Study of the Coordination of Cm(III) in Water

Abstract

The study of the behavior of actinide ions in aqueous solution plays a fundamental role in the quest for a better understanding of actinide waste storage problems and separation processes. Considerable progress has been made both experimentally and computationally in recent years, with special attention given to the uranyl ion, 1,2 as well as trivalent lanthanide and actinide ions. Systematic studies of the aqueous trivalent lanthanide ion first coordination sphere show the number of water molecules changes from nine to eight between Pm(III) and Dy(III) at 25°C. Similar behavior is found for the trivalent actinide ions with the transitional region occurring between Am(III) and Es(III). 3,4 Cm(III) 5 is in the middle of the actinide series and provides a bridge between the early actinides and the less common later actinides.Several studies have been performed on this species. Lindqvist-Reis et al. 6 have used time-resolved laser fluorescence spectroscopy to study the hydration of Cm(III) from 20 to 200°C. They found a strong temperature dependence for several of the spectroscopic quantities associated with the 6 D′ 7/2 -8 S′ 7/2 luminescent spectra. The emission band shifts to lower energy with increasing temperature, which is attributed to an equilibrium among hydrated Cm(III) ions with different numbers of water molecules in the first coordination sphere, namely [Cm(H 2 O) 9 ] 3+ and [Cm(H 2 O) 8 ] 3+ . More recently they have also studied 7 the ground-state and excited-state crystal field splitting of 8-fold-coordinate Cm(III) in [Y(H 2 O) 8 Cl 3 .15crown-5] crystals using laser spectroscopy, at temperatures between 20 and 293 K. Skanthakumar et al. 8 and Lindqvist-Reis et al. 9 have analyzed single-crystal X-ray diffraction data from [Cm(H 2 O) 9 ](CF 3 -SO 3 ) 3 to understand the hydration of Cm(III) in solution. According to their studies, the Cm species in the crystal is surrounded by nine coordinating waters with a tricapped-trigonal-prismatic geometry, with six short Cm-O distances at 2.453(1) Å and three longer Cm-O distances at 2.545(1) Å. Skanthakumar et al. 8 also reported high-energy X-ray scattering data for Cm(III) in perchloric acid solution. Two peaks were assigned to the oxygen atoms of H 2 O, the first peak at 2.48(1) Å was from the first coordination shell (8.8(3) oxygen atoms), and the second peak from the second water coordination shell at 4.65(10) Å (13(4) oxygen atoms). Their EXAFS measurements for Cm(III) in perchloric acid suggested that the nine coordinate structure, found in the triflate crystal, persists in solution but with an expanded splitting of the first coordination shell by ∼ 0.07 Å. Yang and Bursten 10 have studied the first and second hydration shells of Cm(III) using quantum chemical and molecular dynamics methods and they have shown that the inclusion of a complete second hydration shell has a significant effect on the primary coordination sphere. To understand the structural and chemical behavior of Cm(III) (and actinyls in general) in solution from a computational perspec...

Toward accurate solvation dynamics of lanthanides and actinides in water using polarizable force fields: from gas-phase energetics to hydration free energies

“…In addition, many recent studies highlighted the failure of traditional fixed charge force fields to capture the main physical effects that govern interaction for highly charged ions in polar solvents [8][9][10][11][12][13][14][15][16][17][18][19].…”

Collecting high-order interactions in an effective pairwise intermolecular potential using the hydrated ion concept: The hydration of Cf3+

“…54 The standard deviations for both magnitudes have been collected in Table II. The comparison of quantum and classical geometries for the smaller hydrated ions (n = 1, 2, 4) is not as good as for the aqua ions with larger hydration numbers. This may be understood on the fact that the small hydrates have not 12 , where the second hydration shell appears to be too much distant from the hexahydrate. If these data are excluded, the standard deviation goes down to 0.09 Å.…”

“…For Cm(III), Yang and Bursten 8 developed an intermolecular potential based on our hydrated ion model, [9][10][11] assuming a rigid ennea-coordination (nine-fold coordination) of the cation. Gagliardi et al 12 developed a Cm(III)-H 2 O intermolecular potential from high-level ab initio computation of this system where many-body interactions are included by means of the polarizable character of the water molecules and the ion, that are described by a perturbation theory-derived intermolecular potential (the NEMO approach). 13 Atta-Fynn et al 14 have developed another interaction potential including three-body corrections by fitting a ROHF potential energy surface.…”

Physical Chemistry at the University of Geneva

“…In such a way she can study also systems for which not enough experimental data are available, unlike when one uses semi-empirical force field. [54] L. Gagliardi has proven that multiconfigurational quantum chemistry is a powerful and useful tool in chemistry that can tackle difficult problems and not just a nice formalism, relegated to model systems. She has shown that the CASSCF/CASPT2 approach is a competitive alternative to DFT and thus a modern tool in quantum chemistry.…”