RESEARCH ARTICLE. Michael Trumm and Bernd Schimmelpfennig

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1 To appear in Molecular Physics Vol. 114, No. 6, , RESEARCH ARTICLE Towards the origin of effective An(III)/Ln(III) separation by tridentate N-donor ligands: A theoretical study on atomic charges and polarizabilities for Cm(III)/Gd(III) separation. Michael Trumm and Bernd Schimmelpfennig Institut für Nukleare Entsorgung (INE), Karlsruhe Institute of Technology (KIT), Postfach 3640, D Karlsruhe, Germany (Received 00 Month 200x; final version received 00 Month 200x) Soft N-donor ligand have shown to separate An(III) from Ln(III). The origin of the selectivity has not been entirely identified, and similar ligands show very different separation qualities. In this study we present a theoretical investigation of several relevant N-donor ligands in terms of atomic charges and polarizabilities obtained from an atoms in molecules approach. These allow new insights into the bonds between the ligands nitrogen atoms and the metal cation and explain a major part of the selectivity towards actinide ions. We deduct the superiority of 2,6- bis(1,2,4-triazine-3-yl)pyridines (BTP) in separation quality compared to similar ligands for the Cm(III)/Gd(III) separation. Furthermore improvements of existing ligands are developed that not only allow a direct experimental confirmation but also a systematic experimental study of the interactions and their influence on the selectivity. Keywords: Actinide chemistry; Separation; N-donor ligands; Polarizabilities; Quantum chemistry 1. Introduction In spent nuclear fuel, the long-term radiotoxicity is dominated by plutonium and the minor actinides. Selective separation of these elements is the first step of the Partitioning and Transmutation (P & T) scheme, a strategy to reduce the longterm radiotoxicity. Highly selective extracting ligands are required for the separation of the trivalent actinides (An(III)) from the fission lanthanides (Ln(III)) due to their high similarity in both chemical properties and ionic radii. It has been shown that high selectivity is achievable using soft S- or N-donor molecules [1 4]. Numerous SANEX (selective actinide extraction) process options based on N-donor ligands were tested in order to achieve an efficient An(III)/Ln(III) separation. A variety of such extracting agents has been developed that favour the formation of an An(III) over the Gd(III) complex in solution compared to the respective aquo ions. Among these heteroaromatic nitrogen donor ligands 2,6-bis(1,2,4-triazine- 3-yl)pyridines (BTPs) were the first extractants to achieve separation factors of more than 100 for Am(III) and Eu(III) from acidic solutions without the necessity of adding a synergistic ligand [5, 6]. BTP type ligands form highly symmetric 1:3 complexes [M(BTP) 3 ] 3+ with An(III) and Ln(III) ions (see ref. [4] and refs therein). In addition some of them (i.e. CA-BTP [7]) meet additional important requirements for an efficient extractant (e.g. reasonably fast extraction kinetics, stability towards high radiation, acidity and high solubility). The reason for the Corresponding author. Michael.Trumm@kit.edu 876

2 selectivity of the BTP ligands from both theoretical and experimental side still has not been understood entirely. For a further development and optimization of new ligands, however, it is of utmost importance to gain a deeper understanding on the molecular level. Different spectroscopic methods like extended X-ray absorption fine structure (EXAFS), time-resolved laser fluorescence spectroscopy (TRLFS), X-ray crystallography, and nuclear magnetic resonance (NMR) have been used to study the An(III)/Ln(III) complexes of several ligands [4, 8 18]. Theoretical approaches on this topic have tried to highlight differences in covalency, structure and ph-dependence [19 31]. The size of the ligands and their metal complexes restrict the possibility to apply wavefunction-based quantum-chemical (QC) methods beyond the MP2 level. On the other hand molecular dynamics (MD) simulations allowing the investigation of large system sizes are based on classical energy terms. Those often lack the complexity to describe the demanding interactions apparent between the heavy actinides and their surrounding and hence have been restricted to Ln(III) ions [19, 20]. In this paper we will study the effect of different side-rings (diazene, triazine and tetrazine) as well as the influence of different side-chains and their positions on the atomic charges and polarizabilities in the molecule. We use the Hirshfeld method [32] an atoms in molecules (AIM) method to compute accurate atomic data which will explain differences in structure and complex stability. The main goal of this article is to establish a theory for gas-phase separation that will be the last step towards accurate force-field adjustment for An(III)/Gd(III) ions interacting with N-donor ligands. In future MD simulations based on these force fields this strategy will be transferred to liquid and organic phases. 2. Quantum chemical methods 2.1. Molecular structures For all investigated ligands the structure was optimized using density functional theory (DFT) with the B3-LYP functional [33, 34] as implemented in the TUR- BOMOLE software package [35]. Basis sets of triple zeta quality have been used for all atoms during the geometry optimization. To study torsional and side-chain effects separately, all ligands (if possible) have been optimized in C 2 as well as C 2v symmetry. Basis set convergence as well as torsional dependence of charges and polarizabilities were studied on a training set consisting of four ligands: 2,6-bis(1,2-diazin-3-yl)pyridine (BDP) 2,6-bis(1,2,4-triazin-3-yl)pyridine (BTP) 2,6-bis(1,2,4,5-tetrazin-3-yl)pyridine (BQP) 2,2 :6,2 -terpyridine (Terpy) For these four ligands we considered increasing basis-set sizes aug-cc-pvtz, augcc-pvqz and aug-cc-pv5z on all atoms, respectively [36]. Using the aug-cc-pvtz basis set and the optimized C 2v structure of the ligands both N 1 CCN 2 -torsion angles were varied from 0 to 10 and 20 (fig. 1). In a second step we considered several side chains substituting a hydrogen atom at the R 4, R 5 and R 6 positions of both side rings (fig. 1) and the P 4 position in the central pyridine ring. Below all other investigated ligands are listed. All molecular structures can be found in the supplementary information. 2,6-bis(4-ethyl-1,2-diazin-3-yl)pyridine (EtBDP4) R 4 = ethyl 2,6-bis(5-ethyl-1,2-diazin-3-yl)pyridine (EtBDP5) R 5 = ethyl 2,6-bis(6-ethyl-1,2-diazin-3-yl)pyridine (EtBDP6) R 6 = ethyl 2,6-bis(5,6-diethyl-1,2-diazin-3-yl)pyridine (EtBDP56) R 5 = R 6 = ethyl 877

3 P 4 R 4 R 4 R 5 Y X N 1 X Y R 5 N 2 N 2 R 6 N 3 N 3 R 6 Figure 1. Molecular structure of the ligands. X=N/C, Y=N/C. 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (nprbtp) R 5 = R 6 = n propyl 2,6-bis(5,6-diisopropyl-1,2,4-triazin-3-yl)pyridine (iprbtp) R 5 = R 6 = i propyl 2,6-bis(5-methyl-1,2,4-triazin-3-yl)pyridine (mbtp5) R 5 = methyl 2,6-bis(6-methyl-1,2,4-triazin-3-yl)pyridine (mbtp6) R 6 = methyl 2,6-bis(5,6-dimethyl-1,2,4-triazin-3-yl)pyridine (mbtp56) R 5 = R 6 = methyl 2,6-bis(1,2,4-triazin-3-yl)4-methyl-pyridine (mbtpp4) P 4 = methyl 2,6-bis(3-propyl-1,2,4,5-tetrazin-3-yl)pyridine (nprbqp) R 6 = ethyl For the methyl-substituted BTP-ligands mbtp5, mbtp6, mbtp56 and mbtpp4 as well as the ethyl-substituted BDP-ligands EtBDP5, EtBDP6 and EtBDP56 we also optimized the corresponding 1:3 complex with Cm(III) and Gd(III). For both ions small-core relativistic pseudopotentials with corresponding basis-sets (Cm/ECP60MWB and Gd/ECP28MWB[37]) were employed. Due to the poor convergence behaviour of the B3-LYP functional for complexes including heavy ions, we chose the BH-LYP functional [38] for the geometry optimizations. On the resulting structures, a single-point MP2 calculation followed determining the gas-phase interaction energies E g corrected for basis-set superposition errors by the counterpoise method [39]: E g (ML 3 ) = E(ML 3 ) E(M) +L3basis functions E(L 3 ) +Mbasis functions (1) Based on the difference in interaction energies E g = E(CmL 3 ) E(GdL 3 ) a first approximative separation factor (SF) for the exchange reaction (3) can be calculated according to ( ) Eg kcal/mol SF = exp RT Note, that in [40] we computed the difference of the corresponding aquo ions as 17.7 kcal/mol leading to a positive E in the exchange reaction (3), if E g > 17.7kcal/mol. (2) [CmL 3 ] 3+ + [Gd(H 2 O) 9 ] 3+ [GdL 3 ] 3+ + [Cm(H 2 O) 9 ] 3+ (3) This approximation has to be understood in terms of an energy decomposition. Petit et al. [29] suggest a division of the total bonding energy E bonding into parts 878

4 according to E bonding = E steric + E polarization + E covalency. (4) As we will focus on the latter two parts in the gas-phase we will not consider any solvent molecules nor study temperature dependence or entropy effects. These effects should be treated by MD techniques in a fully solvated system. Furthermore, as already pointed out by Bryantsev and Hay [24], zero-point and thermal corrections only lead to small changes in G. We have determined the corrections for some model metal/ligand exchange reaction to be smaller than 0.2 kcal/mol at ambient conditions, which is smaller than the accuracy those values can be calculated with in gas-phase. Also it should be noted, that the motivation to study Cm/Gd separation is mostly based on the ions well defined 8 S 7/2 ground state that allows the usage of single-reference methods. Although Cm/Gd separation is relevant, most experimental data focusses on Am/Eu and Cm/Eu separation. Panak et al. [4] showed, however, that differences in G for Am/Eu and Cm/Eu separation compared to Cm/Gd separation are rather small for N-donor ligands (< 0.2 kcal/mol). Hence the obtained separation factors between Gd(III) and Cm(III) can easily be compared to the experimental results for Am/Eu and Cm/Eu systems. Although there have been theoretical studies concerning these systems, multi-configurational methods would be necessary to describe them properly Hirshfeld method Many approaches have been pursued in order to divide molecular electronic properties into atomic contributions, especially for accurate force-field development. In general all AIM techniques origin in a division of the space into atomic regions, i.e. the Hirshfeld [32], LoProp [41], Mulliken [42] or Bader basin approaches [43], to name a few. In this study will we employ the Hirshfeld method to derive charges and dipole polarizabilities for a set of relevant N-donor ligands. This method has already been used successfully to study polarizabilities in aromatic systems [44]. The Hirshfeld charge q i for atom i is defined as q i = w i (r) ρ(r)dr (5) using the atomic weighting functions w i (r) = ρ0 i (r) j ρ0 j (r) (6) and the difference of molecular density ρ and promolecular density ρ 0 ρ(r) = ρ(r) ρ 0 = ρ(r) i ρ 0 i (r). (7) Here ρ 0 i (r) is the spherical ground-state density of the free atom i. Lu et al.[45] point out that for the carbon atom, the s 2 p 2 ground state density is not spherical and hence the s 1 p 3 state is used throughout their investigation. Although the charges are not effected much by this approximation, the polarizabilities strongly depend on the choice made here. Hence we use fractional occupation numbers as implemented in TURBOMOLE to create an s 2 p 2/3 x py 2/3 p 2/3 z state which has a spherical density 879

5 distribution and is closer to the actual ground-state of the atom. Having defined the Hirshfeld-charges, an atomic dipole is computed. µ γi = q i R γi (r γ R γi ) ρ i (r)dr (8) where R γi is the γ component of the positional vector. From this, the diagonal elements of the atoms static dipole polarizability tensor are calculated as a limit of the electric field F γ µ γi (F γ ) µ γi (0) α γi = lim Fγ 0. (9) F γ Here µ γi (0) represents the dipole moment of atom i without any electric field e.g. F = 0. The isotropic polarizability α i is obtained by averaging over x-, y- and z-components. Later we will investigate α i as well as its z-component α zi where the z-axis is chosen to be the ligand s C 2 symmetry axis Induced dipoles The perturbation of the electron cloud experiencing an electric field can be described considering a set of induced dipoles {µ i } located on the atomic centres. Given the electric field they are calculated according to µ i = α i E q i + T ik µ k. (10) k i Here the α i are the static dipole polarizabilities corresponding to atom i, E i the electric field generated by the charges q i and T i is the dipole tensor. Introducing a Thole damping term [46] the electric field and the dipole tensor become: E 0 i = 1 4πɛ 0 N k i λ 3 q k (r i r k ) r i r k 3 (11) r i r k T ik = 3λ 5 r i r k 5 λ 1 3 r i r k 3 (12) λ 5 = 1 exp( ar 3 ik ) (13) λ 3 = 1 (1 + ar 3 ik )exp( ar ik) (14) Assuming the polarizabilities to be isotropic and independent of the electric field the total polarization interaction can be described by the sum E pol = 1 2 i µ 2 i α i i µ i E q i 1 µ i T ik µ k. (15) 2 i k i 880

6 Figure 2. Atomic Hirshfeld charges with changing torsion angle. This energy is the part of equation (4) that is investigated here. In order to estimate the part of the separation originating from the differences in induced dipoles from both ions and ligands we carried out two short gas-phase MD runs of 10 ps using the POLARIS(MD) software package [47]. Here we use the same force field for both ions so the only differences are their polarizabilities and masses. For the ligand/ligand interaction the standard CHARMM parameters were employed [48]. Remaining ion/ligand parameters were adjusted to reproduce the gas-phase metal-nitrogen distances and will only be used for this qualitative analysis. The MD runs were performed on 1:3 complexes of [CmL 3 ] 3+ and [GdL 3 ] 3+. Along the trajectories we computed the average E pol induced by the different polarizabilities of the central cations. The resulting E pol = E pol (Gd) E pol (Cm) can be used as a first approximation for G Cm Gd M 3+ L 3 in the exchange reaction (3). 3. Results and Discussion The nprbtp and iprbtp ligands are well known extracting ligands [4], whereas apart from nprbqp, EtBDP6 [12] and mbtp56 [49] the other ligands are up to our knowledge purely theoretical model systems. Based on the results of this section, provided similar solubilities of the systems, a series of experiments can be designed to verify changes in complexation strengths directly Charges It has been shown, that one of the advantages of the Hirshfeld method is the fast convergence with the size of the basis set [45, 50]. Accordingly we found the charges for the ligands to be converged using the aug-cc-pvtz basis set. As Hirshfeld charges are known to be too small, we will also investigate the dipole moment corrected Hirshfeld charges (ADCH) [45]. The nitrogen charges q(n 1 ) and q(n 2 ) depicted in fig. 2 show a rather weak torsion-angular dependence. With increasing torsional angle from 0 to 20 q(n 1 ) increases by 5% for all ligands whereas the q(n 2 ) charges decrease by about 10%. The bond between heavy ions and N-donor ligands is known to be mainly ionic [51], hence the charges will determine the coordinating structure and dominate the binding energy. In accordance we have computed one of the highest binding energies for the Terpy 1:3 complex with Cm(III) and Gd(III) among the investigated ligands 881

7 Table 1. Atomic Hirshfeld charges [e] (ADCH charges) and isotropic polarizabilities [Å3 ] for both N 1 and N 2 atoms in the C 2v optimized structures. System q(n 1 ) q(n 2 ) α(n 1 ) α(n 2 ) BDP (-0.256) (-0.160) EtBDP (-0.220) (-0.149) EtBDP (-0.226) (-0.170) EtBDP (-0.231) (-0.172) EtBDP (-0.225) (-0.188) BTP (-0.232) (-0.169) nprbtp (-0.238) (-0.210) iprbtp (-0.238) (-0.201) mbtp (-0.231) (-0.180) mbtp (-0.235) (-0.182) mbtp (-0.237) (-0.195) BQP (-0.235) (-0.133) nprbqp (-0.241) (-0.146) in our recent study [40]. As in the case of the Terpy ligand, however, this is not sufficient to yield effective Ln(III)/An(III) separation. The introduction of electron-pushing side-chains increases the electron density over the whole aromatic system. Accordingly, the partial charges of the coordinating nitrogen atoms change depending of the substituent s position (table 1). Differences of the effect on charges are small for the three possible side-chain sites R 4, R 5 and R 6. We notice a more pronounced change in the ADCH charges. Especially for the C 2 structures the dipole correction sometimes seems to overshoot (table 2). This probably origins from the different dipoles of the molecules due to their different torsional angles. Because of this inconvenience we will continue the analysis based on the unscaled Hirshfeld charges. Table 2. Atomic Hirshfeld charges [e] (ADCH charges) and isotropic polarizabilities [Å3 ] for both N 1 and N 2 atoms, ring-ring torsion angle θ [rad] and energy difference to the C 2v structures [kcal/mol] in the C 2 optimized structures. *ligand has no C 2 symmetry. System q(n 1 ) q(n 2 ) α(n 1 ) α(n 2 ) θ E θ BDP (-0.242) (-0.323) EtBDP (-0.182) (-0.076) EtBDP (-0.250) (-0.480) EtBDP (-0.249) (-0.425) EtBDP (-0.246) (-0.180) BTP (-0.221) (-0.264) nprbtp (-0.223) (-0.373) iprbtp (-0.231) (-0.310) mbtp (-0.225) (-0.357) mbtp (-0.227) (-0.341) mbtp (-0.231) (-0.342) mbtpp4* (-0.231) (-0.328) BQP (-0.223) (-0.229) nprbqp (-0.223) (-0.157) Polarizabilites Along the Ln(III) and An(III) series, despite having the same charge, the polarizabilities of the ions change significantly. Induced by the diffuse 5f-orbitals of the An(III) ions, their polarizability is in general larger than the corresponding Ln(III) ion. For example, we found α Cm = 1.165Å3 [52] and α Gd = 0.86Å3. Polarizability studies generally focus on isotropic values only, but with regard of covalent bonds more focus should be put on the part of the polarizability tensor pointing towards the virtual metal-ion center, here the z-axis (fig. 3). Like Hirshfeld charges, the isotropic polarizabilities converge very fast with increasing basis sets. Computed changes from aug-cc-pvtz to aug-cc-pv5z basis sets are less than 2% for the BDP, BTP and BQP ligands. The z-components α z, 882

8 Figure 3. Nitrogen lone-pair of the BTP-ligand. Figure 4. Isotropic polarizabilities for different torsional angles. however, show a slightly less stable behaviour and changes up to 10% were found. Within these uncertainties we will continue the discussion on polarizabilities using the aug-cc-pvtz basis sets. Figures 4 and 5 highlight the change of the polarizabilities and their z-component with changing torsional angle. First we notice, that α(n 1 ) is significantly smaller compared to α(n 2 ). Whereas the first appear to be angular-independent, the latter change non-monotonically up to 30%. The z-components show a similar behaviour. In particular for the N 1 nitrogen atoms, the z-axis points exactly in the ideal bond-direction hence the potential of the BTP pyridine-nitrogen to form a covalent bond is supposed to be slightly higher compared to the other ligands as the higher N 1 z-polarizability allows an easier deformation of the electron-density. For the N 2 nitrogen atoms, the bond-direction is not as well defined due to different bond-lengths and angles in different complexes, hence an evaluation of the isotropic values is preferred. Again, the BTP nitrogen atoms exhibit the highest polarizabilities, especially for small torsional angles. It is important to notice, that the isotropic molecular polarizabilities decrease with amount of nitrogen atoms, i.e. for Terpy, BDP, BTP and BQP we obtain values of 31.6, 29.9, 28.1 and 26.7 Å3, respectively, from a B3-LYP/aug-cc-pVTZ calculation. Hence the computation of atomic polarizabilities is mandatory for a detailed investigation of the metal-ligand interaction. As already shown for the charges, we find a slight effect on the polarizabilities depending on the position of the substituent. In this case however, the effect of the 883

9 Figure 5. Z-component of the polarizability tensor for different torsional angles. R 5 position seems to be overall stronger. Both N 1 and N 2 polarizabilities increase, which is partially compensated when a second side-chain is introduced. An alkyl substituent on the R 6 position, although increasing the nitrogen electron density has a negative effect on both α(n 1 ) and α(n 2 ). It would hence be very interesting to investigate a mono-substituted species at the R 5 position experimentally. Such systems have already been synthesized for bis-triazine bis-pyridine (BTBP) based ligands but no comparison to the alternative substitution at R 6 has been drawn [53]. The R 4 position, up to our knowledge, has not been target to substitutions, yet. Although a to the R 5 position similar effect on α(n 1 ) is observed in C 2v it vanishes when the ligand is relaxed in C 2 symmetry. This proves that the effect, different to R 5 originated by a deformation of the pyridine ring by the close proximity of the side-chain due to the symmetry restrictions. For all substituents, we have determined the main effect on the polarizabilities to be on the meta-positions in the aromatic ring Separation Within the 10 ps time-frame of the MD runs, the 1:3 complex was stable although the geometry, in particular the bis-angle, varies with changing polarizabilities. It will be interesting to see how solvent molecules influence the dynamics in future studies, after a proper adjustment of all relevant force fields. Considering classic charge-dipole and dipole-dipole interactions via the formula (15) a separation between the two ions is achieved due to the larger interaction of the Cm(III) ion with the ligand. It should hence in principle be possible to assess and improve the separation quality by tuning the polarizabilities of the coordinating nitrogen atoms. The optimal separation area based on the ADCH charges computed for BTP is located around α(n 1 ) = 0.35 and α(n 2 ) = 0.95 (fig. 6). Using the generally smaller unscaled Hirshfeld charges instead the profile flattens significantly and the minimum is shifted towards α(n 1 ) = 0.31 and α(n 2 ) = 0.65 (see SI fig. 1). Within these uncertainties we continue the discussion based on the ADCH charges. In fig. 6 we also plot the position of some ligands on the contour map. Although solvation effects are not included it can already be seen, that nprbtp is clearly superior to most other ligands considered. The mbtp5 species is, behind nprbtp, the next closest to the determined minimum in the contour plot. Its qualities also show in the corresponding 1:3 complexes. 884

10 Figure 6. Contour map of E pol from the MD trajectory [kcal/mol] based on the ADCH charges of BTP. Ligands are marked based on their C 2v polarizability values. Table 3. Cm-N bond distances d 1 and d 2 [Å] and interaction energies E g [kcal/mol] for the Cm/Gd 1:3 complexes. SF denotes the gas-phase separation factor. d 1 d 2 E g(cm) E g(gd) E g SF mbtp mbtp mbtp mbtpp BTP[40] Table 4. Cm-N bond distances d 1 and d 2 [Å] and interaction energies E g [kcal/mol] for the Cm/Gd 1:3 complexes. SF denotes the gas-phase separation factor. d 1 d 2 E g(cm) E g(gd) E g SF EtBDP EtBDP EtBDP BDP[40] The ligand-series mbtp5, mbtp6 and mbtp56 allows a very constructive analysis of the roles that charges and polarizabilities play for the structure and binding energy. Table 3 summarizes the optimized distances d 1 and d 2 of the metal-ion to the coordinating N 1 and N 2 nitrogen atoms as well as the interaction energies E g for both Cm- and Gd-complexes. As predicted by the polarizability analysis, the mbtp5 ligand has better separation qualities in gas-phase with a E g of kcal/mol compared to mbtp6 (16.58 kcal/mol). Table 3 also highlights the differences in binding energy gained by adding side-chains. Again, the R 5 -position induces the positive effect in both energy and separation quality. The larger energy gain might be an indicator for covalency induced by the higher polarizability. The contour map (fig. 6) also suggests a possible improvement on the the BDP ligand by substituting at the R 5 rather than the at R 6 position as investigated by Beele et al. [12] Optimized gas-phase complexes confirm an increase of the SF by a factor of 2 from 44 (EtBDP6) to 80 (EtBDP5). It is further interesting, that the substitution at both R 5 and R 6 positions does not lead to a further improvement as in the BTP case as polarizabilities drop out of the optimal separation zone (table 4). Regarding the polarizabilities of nprbtp, nprbqp and EtBDP6 we see increasing distances to the optimal separation zone in fig. 6. These correlated well to the experimental results obtained for these ligands [4, 12] with separation factors of 885

11 SF(nPr-BTP) = , SF(nPrBQP) = 9 and SF(EtBDP6) = 5. A direct comparison of the gas-phase data for ligands from different families (BDP, BTP, etc.), however, is unadvisable as their interaction with the solvent differs and hence demand a study in solution. Additionally the SFs obtained by Beele et al. on nprbqp and EtBDP6 were obtained at very low ligand concentrations (< 10 mm), whereas gas-phase calculations correspond to highly saturated ligand solutions. 4. Conclusions We have shown, that charges and polarizabilities determined by the Hirshfeld method can be connected to separation quality in gas-phase. As already proven for Hirshfeld charges, also the polarizabilities converge very fast with increasing basis set and we obtained stable values using the aug-cc-pvtz basis sets. A rather weak torsion-angular dependency of both charges and polarizabilities was determined allowing the usage of those values in MD simulations, where the ligand s structure will change rather quickly due to thermodynamic effects. This study is an essential step towards the creation of accurate force fields. Corresponding MD simulations will include dynamic and solvent effects in the investigation and help to understand the extraction processes. The optimal area of polarizabilities in gas-phase was determined by short MD runs and will be recomputed in solution in future studies to include temperature and entropy changes. We have also shown, that the inclusion of side-chains, especially their position and length is of high importance and should not be omitted when performing studies on separation. For example we have deducted, that the performance of the EtBDP ligand can be improved significantly by changing the position of the ethyl residue. Based on the presented results, experiments have been designed to confirm the findings. 5. Acknowledgement This study was supported by the European FP7 TALISMAN project, under contract with the European Commission and by the German Federal Ministry of Education and Research (BMBF) under contract number 02NUK020A. We acknowledge access to the computing resources provided by the Steinbuch Centre for Computing (SCC) at KIT. We would also like to thank Dr. Feiwu Chen from the University of Beijing, Dr. Michael Patzschke from the Helmholtz Zentrum Dresden-Rossendorf, Dr. Andreas Geist and Dr. Petra Panak from KIT for helpful discussions. References [1] C. Musikas, P. Vitorge and D. Pattee, Proc. Internat. Solvent Extr. Conf. (1983). [2] C. Hill, D. Guillaneux, L. Berthon and C. Madic, J. Nucl. Sci. Technol. Supplement 3, 309 (2002). [3] D. Magnusson, B. Christiansen, M. Foreman, A. Geist, J. Glatz, R. Malmbeck, G. Modolo, D. Serrano-Purroy and C. Sorel, Solvent Extr. Ion Ech. 27, 97 (2009). [4] P. Panak and A. Geist, Chem. Rev. 113, 1199 (2013). [5] Z. Kolarik, U. Müllich and F. Gassner, Solvent Extr. Ion Exch. 17, 1155 (1999). [6] S. Trumm, G. Wipff, A. Geist, P. Panak and T. Fanghänel, Radiochim. Acta 99, 13 (2011). [7] S. Trumm, A. Geist, P.J. Panak and T. Fanghänel, Solvent Extr. Ion Exch. 29, 213 (2011). [8] M. Denecke, A. Rossberg, P. Panak, M. Weigl, B. Schimmelpfennig and A. Geist., Inorg. Chem. 44, 8418 (2005). 886

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