Combining EXAFS and Computer Simulations to Refine the Structural Description of Actinyls in Water

Abstract

EXAFS spectroscopy is one of the most used techniques to solve the structure of actinoid solutions. In this work a systematic analysis of the EXAFS spectra of four actinyl cations, [UO2]2+, [NpO2]2+, [NpO2]+ and [PuO2]2+ has been carried out by comparing experimental results with theoretical spectra. These were obtained by averaging individual contributions from snapshots taken from classical Molecular Dynamics simulations which employed a recently developed [AnO2]2+/+ –H2O force field based on the hydrated ion model using a quantum-mechanical (B3LYP) potential energy surface. Analysis of the complex EXAFS signal shows that both An-Oyl and An-OW single scattering paths as well as multiple scattering ones involving [AnO2]+/2+ molecular cation and first-shell water molecules are mixed up all together to produce a very complex signal. Simulated EXAFS from the B3LYP force field are in reasonable agreement for some of the cases studied, although the k= 6–8 Å−1 region is hard to be reproduced theoretically. Except uranyl, all studied actinyls are open-shell electron configurations, therefore it has been investigated how simulated EXAFS spectra are affected by minute changes of An-O bond distances produced by the inclusion of static and dynamic electron correlation in the quantum mechanical calculations. A [NpO2]+−H2O force field based on a NEVPT2 potential energy surface has been developed. The small structural changes incorporated by the electron correlation on the actinyl aqua ion geometry, typically smaller than 0.07 Å, leads to improve the simulated spectrum with respect to that obtained from the B3LYP force field. For the other open-shell actinyls, [NpO2]2+ and [PuO2]2+, a simplified strategy has been adopted to improve the simulated EXAFS spectrum. It is computed taking as reference structure the NEVPT2 optimized geometry and including the DW factors of their corresponding MD simulations employing the B3LYP force field. A better agreement between the experimental and the simulated EXAFS spectra is found, confirming the a priori guess that the inclusion of dynamic and static correlation refine the structural description of the open-shell actinyl aqua ions.

A Coupled EXAFS–Molecular Dynamics Study on PuO2+ and NpO2+ Hydration: The Importance of Electron Correlation in Force-Field Building

“…In the bottom of Figure 5 we have included the simulated EXAFS spectrum obtained using our previous NEVPT2-based intermolecular potential. 38 This spectrum almost matches the spectrum obtained in this work which employs a simplified version of the intramolecular actinyl potential (IMC).…”

“…58 , 25 For NpO 2 + , this work finds 2.52 Å (QM(NEVPT2) for CN = 5 row in Table 1 ), and previous B3LYP values are in the range of 2.55–2.61 Å. 25 , 38 , 58 In the case of NpO 2 + , we can compare the performance of the new formulation of the IMC potential, based on harmonic and anharmonic functions, to describe the flexibility of the actinyl entity, with respect to the previous forms, based on a set of r – n powers. The optimized geometry for [NpO 2 (H 2 O) 5 ] + using the new potential, POT5(NEVPT2), predicts distance changes smaller than 0.01 Å with respect to the previous values (see values in parentheses in the POT5(NEVPOT2) row).…”

“…All simulations were run using a modified version of DL_POLY Classic 54 which includes the functional forms of the force field employed. The convergence of MD trajectories has been checked by the analysis of structural, energetic and dynamic properties of the actinyl cations as shown in refs ( 36 ), ( 38 ), and ( 51 ).…”

“…Details of the spectrum simulation method can be found elsewhere. 23 , 31 , 38 , 51 An example of the FEFF input files can be found in Figure S5 . S 0 2 and Δ E 0 values have been chosen in the simulated NpO 2 + and PuO 2 + spectrum in order to match the first resonance of the corresponding experimental spectrum.…”

“… 39 Whereas the theoretical–experimental agreement was quite satisfactory for UO 2 2+ , NpO 2 2+ , and PuO 2 2+ using force fields derived from B3LYP potential energy surfaces, the NpO 2 + case was not satisfactory. 38 This fact compelled us to develop for NpO 2 + a new force field based on QM wave functions with explicit inclusion of the dynamic and nondynamic electron correlation, as NEVPT2 method does. 40 , 41 The simulated EXAFS spectrum gave a fair comparison with the experimental spectra.…”