Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Quantum Chemical Study of the Water Exchange Mechanism of the Neptunyl(VI) and -(V) Aqua Ions François P. Rotzinger* Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland S Supporting Information *
ABSTRACT: The water exchange reaction of the neptunyl(VI) and -(V) aqua ions was investigated with quantum chemical methods. Associative (a) and dissociative (d) exchange mechanisms were studied. The geometries and vibrational frequencies were computed with density functional theory (DFT) and multiconfiguration self-consistent field (MCSCF) methods. The Gibbs activation energies (ΔG‡) for a and d pathways were calculated with DFT and wave function theory (WFT), extended general multiconfiguration quasi-degenerate second-order perturbation theory (XGMC-QDPT2) including spin−orbit (SO) coupling at the SO configuration interaction (CI) level. The DFT−WFT agreement for ΔG‡ was poor for the investigated functionals except for LCBOP-LRD. Due to ligand-field effects, ΔG‡ for the associatively activated exchange reaction of NpO2(OH2)52+ with a fδ1 electron configuration is higher than for the actinyl(VI) aqua ions of U, Pu, and Am exhibiting f0, fδ2, and fδ2fϕ1 electron configurations.
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INTRODUCTION The water exchange reaction of the uranyl(VI) and plutonyl(VI) aqua ions, exhibiting f0 and fδ2 electron configurations, proceeds preferentially via an associative (a) substitution mechanism (A or Ia), the Gibbs activation energy (ΔG‡) for a dissociative (d) pathway (D or Id) being larger by more than 15 kJ mol−1.1−4 For the reactions of these two ions, spin−orbit (SO) effects are negligible because the ground states of all of the involved species are not degenerate or near-degenerate. In contrast, all of the species, reactant, transition states (TS), and intermediates participating in the water exchange of the americyl(VI) aqua ion with an fδ2fϕ1 electron configuration exhibit degenerate or near-degenerate ground states, which required5 four-state multiconfiguration self-consistent field (MCSCF) computations. The sizable SO effects cannot be neglected. For this reaction, ΔG‡ for the a and d activations are equal within the error limits of the calculations due to ligandfield effects, which stabilize the energy of the d TS, but do not affect ΔG‡ for the a mechanism.5 The ligand-field effects are due to the stabilization of the singly occupied fϕ orbital along the d activation. In general, ligand-field effects arise from the stabilization or destabilization of occupied d or f orbitals. In the present study, the mechanism of reaction 1, the water exchange of the neptunyl(VI) and neptunyl(V) aqua ions, was investigated with quantum chemical methods using the same techniques as in previous work.4,5
of americyl(VI). Hence, two-state quantum chemical computations are required, and SO effects are sizable. Neptunyl(V) was computed as the isoelectronic plutonyl(VI) ion,4 which does not exhibit degenerate or near-degenerate states. According to a 1H NMR study in water/acetone mixtures,6 the Gibbs activation energy at 25 °C is 40−42 kJ mol−1 for the water exchange reaction of NpO2(OH2)52+. Experimental data are not available for the corresponding NpO2(OH2)5+ ion. For neptunyl(VI), the electronic activation energy (ΔE‡) for the D and the A mechanisms was computed with quantum chemical methods7 as 70.0 and 30.0 kJ mol−1, respectively, based on Hartree−Fock (HF) geometries, being inaccurate because of the neglect of static and dynamic electron correlation, and MP2 energies, which are inaccurate due to the neglect of static electron correction. According to classical molecular dynamics (MD) simulations,8 the reaction of Np(VI) follows an a mechanism with ΔG‡ = 23.2 kJ mol−1, and for water exchange on Np(V), a d activation is preferred, exhibiting the same ΔG‡. It is an open question whether classical MD simulations are suitable for the treatment of asymmetrically occupied shells with fδ1 and fδ2fϕ1 electron configurations giving rise to degenerate or near-degenerate states. These limitations were already mentioned previously. 5 Thus, reaction 1 was reinvestigated using wave function theory (WFT) treating static and dynamic electron correlation, near-degeneracy, and SO effects.
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NpO2 (OH 2)n5 + + H 2O → NpO2 (OH 2)4 OH n2+ + H 2O (n = 1, 2)
COMPUTATIONAL DETAILS
The calculations were performed with the GAMESS programs.9,10 For Np, Karlsruhe def-TZVP basis sets11 without a g function and the (1)
fδ1
The neptunyl(VI) species, with an electron configuration, exhibit degenerate or near-degenerate ground states like those © XXXX American Chemical Society
Received: September 14, 2017
A
DOI: 10.1021/acs.inorgchem.7b02373 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry corresponding small-core effective core potentials (ECP)12 with Z = 60 were used. Since GAMESS does not support ECP h functions, a modified4 version thereof (Supporting Information) was used. For O and H, modified13 Karlsruhe def2-TZVP basis sets14,15 were taken. Figure 1 was generated with MacMolPlt.16
state of the near-degenerate ground state was taken into account as described previously.5 The corresponding correction to ΔG or ΔG‡ amounted to less than 0.1 kJ mol−1. The transition states (TS) were located as described,4,5 and they exhibited a single imaginary frequency. Reactants, products, and intermediates were obtained via computation of the intrinsic reaction coordinate. All of their computed vibrational frequencies were real. The coordinates of the geometries for the WB model optimized at the DFT (LC-BOP-LRD) and the MCSCF levels are given in Tables S1− S10 (Supporting Information).
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RESULTS AND DISCUSSION Models. The A and Ia substitution mechanisms were investigated as previously4,5 based on two models: WA involves the water-adduct of the reactants, NpO2(OH2)5·OH2n+ (n = 1, 2), whereby the water molecule in the second coordiantion sphere (·OH2) is hydrogen bonded to two waters of the first coordination sphere. In the WB model, the reactions involve water from the bulk solution. The reactions were described via NpO2(OH2)5n+ + H2O → TS, whereby for the calculation of ΔG‡, corrections for the standard state of (bulk) H2O had to be included.4,28 Likewise, the NpO2(OH2) 4n+ (n = 1, 2) intermediates for the D mechanism were treated as water adducts (WA), or the eliminated water was treated as water from the bulk solution (WB). Their strengths and limitations are not reiterated here. As discussed previously,4,5 the WB model is expected to be superior. The aqua ions NpO2(OH2)5n+ (n = 1, 2) form hydrogen bonds with bulk water. Hydration was treated with the conductor polarizable continuum model (CPCM),17−19 which takes into account the electrostatic component of hydrogen bonding, but not its directionality. With explicit hydration, the stereoretentive or stereomobile Id mechanism might be found instead of D as for the water exchange reaction of Al(OH2)63+.29 Likewise, it is not possible to elucidate the steric course and the detailed, stepwise or concerted, associatively activated exchange mechanism. Quantum mechanical/molecular mechanical (QM/MM) computations are suitable to answer these questions, but they are still very demanding, if not prohibitive. Water Exchange on NpO2(OH2)5+. This fδ2 system, being isoelectronic with PuO2(OH2)52+, does not exhibit neardegenerate states, and therefore, SO effects are small. Hence, DFT is expected to yield acceptable geometries and energies, provided that static correlation and hydrogen bonding are treated sufficiently well. The NpO and Np−O(H2) bond lengths and the symmetric (νs) and asymmetric (νas) ONp O stretching frequencies (Table 1) agree with the experiment for the TPSSh-D334−36 and TPSS-D336−38 functionals, whereby D3 denotes Grimme’s dispersion corrections.36 The other functionals, LC-BOP-LRD,39,40 ωB97X,41,42 ωB97X-D3,43 and B3PW91-D3,36,44,45 as well as MCSCF produce too short NpO and too long Np−O(H2) bonds and too large O NpO vibrational frequencies. Furthermore, the νs−νas ordering is wrong for MCSCF, for which geometries and frequencies are inaccurate due to the neglect of dynamic correlation. For the most accurate NpO2(OH2)5+ geometry, the Gibbs energy (G) calculated with WFT (including static and dynamic correlation, SO coupling, hydration, and thermal corrections for 25 °C) should be lowest. Indeed, TPSS-D3 is among the best functionals (Table S11, Supporting Information). These data (Tables 1 and S11, Supporting Information) suggest that this functional is expected to yield accurate Gibbs activation
Figure 1. Structure and imaginary mode (19.5i cm−1) of the TS NpO2(OH2)5···OH222+‡ (for the A mechanism) and the singly occupied antibonding fδ MO (LC-BOP-LRD calculation). The geometries and vibrational frequencies were computed with spin-unrestricted density functional theory (DFT) and CPCM17−19 hydration. A grid finer than the default was used (in most cases, nrad = 160, nleb = 770 or 974; the respective defaults are 96 and 302). The Hessians were calculated numerically (based on analytical gradients) using the double-difference method and projected to eliminate rotational and translational contaminants.20 The Np(V) species, being isoelectronic with Pu(VI), were calculated as in a previous work.4 The computations on the Am(VI) species showed5 that the geometries obtained with single-state MCSCF are virtually equal to those with four-state MCSCF. Hence, for the Np(VI) species, the geometries and frequencies were calculated solely with single-state MCSCF (including CPCM17−19 hydration) based on the occupationally restricted multiple active space (ORMAS)21 technique with an active space composed of three subspaces as for the americyl(VI) species.5 For single-state MCSCF, the two σ(ONpO) and the four π(ONpO) molecular orbitals (MO) formed the first active space; the lowest 5f MO (one fδ), the second; and the two σ*(O NpO) and the four π*(ONpO) MOs, the third active space. Between all three subspaces, singles−doubles excitations were allowed. These calculations are abbreviated as MCSCF(10−12/6,0−2/1,0−2/ 6,13), whereby 13 is the number of electrons in the active space.13 The hydration energy was computed with two-state MCSCF exhibiting two 5f MOs (2 fδ) in the second active space, abbreviated as 2stMCSCF(10−12/6,0−3/2,0−2/6,13)-CPCM. The thermal corrections (for 25 °C) were based on DFT or single-state MCSCF vibrational frequencies. For all of the geometries, DFT and MCSCF, the electronic energies were calculated using the extended general multiconfiguration quasidegenerate second-order perturbation theory (XGMC-QDPT2),22−26 at the two-state level, 2st-XGMC-QDPT2(10−12/6,0−3/2,0−2/ 6,13), whereby the 6s and 6p MOs of Np were included in the PT2 treatment. The SO energy was evaluated as described4 for the Np(V) species being isoelectronic with Pu(VI), and for the Np(VI) species, the SO energy was calculated using SO configuration interaction (SO−CI) with a (1/2) active space (the 5f electron in the two fδ MOs) involving two doublet states. The SO−CI(1/2) calculations were based on the 2st-MCSCF wave function. The hydration energy was calculated using the conductor polarizable continuum model (CPCM).17−19 The CPCM cavity was constructed based on Batsanov’s27 van der Waals radii, whereby for Np, the same value as for U and Pu was taken. A finer tessalation than the default was used (ntsall = 960, the default is 60). At the WFT level, the hydration energy was computed with 2st-MCSCF(10−12/6,0−3/ 2,0−2/6,13)-CPCM. In the calculation of the thermal corrections to the Gibbs energy (G or G‡), the energy difference (Δε) between the first and the second B
DOI: 10.1021/acs.inorgchem.7b02373 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Table 2. Gibbs Activation (ΔG‡) and Reaction (ΔG) Energies for the Water Exchange Reactions of NpO2(OH2)5+ a
Table 1. NpO and Np−O(H2) Bond Lengths and O NpO Vibrational Frequencies of the Reactant NpO2(OH2)5+ (Experimental Data Obtained in Aqueous Solutions)
EXAFS Raman IR LC-BOP-LRD ωB97X ωB97X-D3 B3PW91-D3 TPSSh-D3 TPSS-D3 MCSCF
d(Np−O(H2)),a Å
1.822 ± 0.003 1.83c
2.488 ± 0.009 2.51c
b
DFT
νas, cm−1
mechanism
geometry
Ia
LC-BOP-LRD
b
ΔG
824 ± 4 884 862 864 850 834 815 841
e
1.771, 1.782, 1.782, 1.793, 1.806, 1.820, 1.796,
1.771 1.783 1.782 1.793 1.806 1.820 1.796
2.529 2.550 2.546 2.543 2.534 2.538 2.595
866 841 838 814 795 771 844 b
‡
WFT ΔG
TPSS-D3 MCSCF A
ωB97X
38.6 32.0 ∼36.0c ∼34.5c 37.9 30.8 27.3 27.3 32.3 32.3 30.4 30.4 19.0 19.0 23.2 23.2
ωB97X-D3
Reference 30.
TPSS-D3 D
energies for a and d activations. As it will be shown, this is not the case; ΔG‡ computed with TPSS-D3 deviates considerably from the corresponding WFT value. For TSs, it is not possible to determine the most accurate geometry based on the smallest G‡ value calculated with WFT as for the reactant NpO2(OH2)5+ because, for an inaccurate geometry, G‡ may lie above or below the real value. Hence, ΔG‡ based on the lowest G‡ computed with WFT for the various DFT and MCSCF geometries cannot be considered as the most accurate one. Thus, an alternative criterion for an appropriate functional was chosen, where the computed ΔG‡ for a given mechanism, Ia, A, or D (Table 2), is closest for the DFT and WFT methods. Such results would at least be consistent. It should be noted that no functional was parametrized for actinides. ΔG‡ for all of the investigated mechanisms, Ia, A, and D, the WB model, and the TPSS-D3 functional differs considerably from WFT (by >10 kJ mol−1) in spite of its good performance for geometries and vibrational frequencies (Tables 1 and 2). This discrepancy must arise from inaccurate TS geometries or DFT energies or both, which are rooted in the approximate static correlation of TPSS-D3. Furthermore, for the Ia and the A mechanisms, ΔG‡ calculated with WFT differs considerably (by >10 kJ mol−1) for the WA and WB models. This suggests that O−H and hydrogen bond geometries and energies might not be sufficiently accurate. Hence, it is an open question, whether TPSS-D3 yields accurate results, and this is the reason why the calculations were also performed with other functionals and MCSCF. A good DFTWFT agreement for ΔG‡ and all mechanisms was found for LC-BOP-LRD and B3PW91-D3. ωB97X and ωB97X-D3 are less accurate for A and the WB model. For WB and the best functionals (LC-BOP-LRD and B3PW91-D3) and MCSCF, respectively, ΔG‡ is equal to 44, 46, and 48 kJ mol−1 for Ia and 34, 27, and 32 kJ mol−1 for D (Table 2). The corresponding ΔG‡ for WB and TPSS-D3 are equal (A, 45; D, 33 kJ mol−1). Hence, the water exchange reaction of NpO2(OH2)5+ is likely to proceed via a dissociative mechanism. Water Exchange on NpO2(OH2)52+. In this fδ1 system, near-degeneracy and SO effects are relevant. Thus, DFT might be inaccurate. The geometries and vibrational frequencies (Table 3) were computed with the functionals yielding the most accurate ΔG‡ values for NpO2(OH2)5+ (Table 2).
LC-BOP-LRD ωB97X ωB97X-D3 B3PW91-D3 TPSS-D3
ΔG
b
47.2 40.4 42.8 39.5 43.7 36.6
B3PW91-D3
767d
Average of the five Np−O(H2) bond lengths. Reference 31. dReference 32. eReference 33.
a c
d(NpO), Å
νs, cm−1
39.5 32.9 36.0 34.5 36.1 29.0 12.5 15.6 17.1 20.3 14.7 18.8 10.3 9.8 7.5 11.9
MCSCF
‡
ΔGb
model
43.2 44.4 34.5 44.3 30.3 43.1 22.5 20.0 24.3 20.8 21.7 18.9 22.3 14.9 15.2 16.0 21.2 12.2
WA WB WA WB WA WB WA WB WA WB WA WB WA WB WA WB WA WB WA WB WA WB WA WB WA WB
44.4 44.1 39.2 46.1 36.4 49.2 42.1 48.1 42.7 43.9 ∼34.5c ∼44.3c 32.5 45.2 33.6 33.6 39.7 39.7 36.4 36.4 26.8 26.8 33.3 33.3 32.0 32.0
Units: kJ/mol. bValues, for which ΔG > ΔG‡ are italicized. In such cases, it might be preferable to attribute the Ia mechanism. cThe TS exhibited virtually the same energy as the intermediate NpO2(OH2)6+. Also in this case, it might be preferable to attribute the Ia mechanism. a
Table 3. NpO and Np−O(H2) Bond Lengths and O NpO Vibrational Frequencies of the Reactant NpO2(OH2)52+ (Experimental Data Obtained in Aqueous Solutions)
EXAFS
d(NpO), Å
d(Np−O(H2)),a Å
1.754 ± 0.003 1.76c
2.414 ± 0.006 2.43c
b
νas, cm−1
b
863d 854e
Raman IR LC-BOP-LRD ωB97X-D3 B3PW91-D3 MCSCF
νs, cm−1
1.698, 1.709, 1.724, 1.705,
1.698 1.709 1.724 1.705
2.421 2.433 2.423 2.480
1020 979 932 1008
969 ± 1f 1065 1035 1001 1046
Average of the five Np−O(H2) bond lengths. bReference 30. Reference 31. dReference 32. eReference 46. fReference 33.
a c
Experimental and computed geometries and vibrational frequencies agree best for B3PW91-D3 (Table 3). For the WB model, the LC-BOP-LRD, ωB97X-D3, and B3PW91-D3 functionals, and MCSCF, ΔG‡ for an associative activation is only slightly and insignificantly lower than for the dissociative activation (by 4, 10, 3, and 3 kJ mol−1, respectively, Table 4). Hence, the present computations are not sufficiently accurate to attribute the preferred exchange mechanism. Furthermore, it C
DOI: 10.1021/acs.inorgchem.7b02373 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 4. Gibbs Activation (ΔG‡) and Reaction (ΔG) Energies for the Water Exchange Reactions of NpO2(OH2)52+ a DFT mechanism
geometry
Ia
MCSCF
A
LC-BOP-LRD ωB97X-D3 B3PW91-D3
D
LC-BOP-LRD ωB97X-D3 B3PW91-D3 MCSCF
ΔG
‡
49.2 38.8 40.0 24.0 46.9 31.9 41.7 41.7 35.8 35.8 32.3 32.3
PuO2(OH2)52+ study published earlier, which was, however, not mentioned by the authors of the DMRG study. In all of the metal−oxo complexes, including the actinyl ones, static electron correlation is present. HF and MP2 produce inaccurate geometries and energies;52 CASSCF or MCSCF are superior but not exact either, due to the neglect of dynamic electron correlation. Small inaccuracies in AnO bonds lead to sizable energetic changes, and this is due to the high AnO vibrational frequencies. For the present reactions, the changes in AnO bond lengths along the activation process need to be accurate, since their contribution to ΔG‡ is large. It is evident that TPSS-D3 producing good geometries and vibrational frequencies for NpO2(OH2)5+ does not yield accurate NpO bond lengths changes for the activation process, whereas LCBOP-LRD, producing inaccurate geometries and frequencies, seems to yield accurate NpO bond lengths changes as manifested by the WFT-DFT agreement in ΔG‡. The presently investigated functionals as well as those in previous works4,5 fail in the computation of AnO bond lengths or their changes along the activation process or both. These inaccuracies are clearly due to an inadequate treatment of static electron correlation by DFT. It was shown above that the TPSS-D3 ΔG‡ data (Table 2) based on the WB model (where inaccuracies in hydrogen bonding are smaller than for WA) differ considerably for WFT and DFT, and that this difference is likely to arise from shortcomings of TPSS-D3 in the description of static electron correlation. The other functionals, LC-BOP-LRD and B3PW91-D3, producing ΔG‡ values with smaller WFT-DFT deviations (Table 2) yielded poorer NpO and Np−O(H2) bond lengths and ONpO vibrational frequencies for NpO2(OH2)5+, where (near-)degeneracy and SO effects are absent or small (Table 1). The above-described considerations show that the presence of static electron correlation in actinyl complexes is established, and this is the reason why DFT does not perform well for such compounds. The present data (Table 1−4) show that LCBOP-LRD is the best functional, and that B3PW91-D3 yields acceptable results in spite of their shortcomings mentioned above. None of the present functionals yields accurate geometries, vibrational frequencies, and ΔG‡ values simultaneously: TPSS-D3, for example, performs well for geometries and frequencies, but not for ΔG‡, whereas for LC-BOP-LRD, it is the opposite (poor geometries and frequencies, but good ΔG‡). Other functionals performing in general well in benchmark tests (Tables S12 and S13, Supporting Information) were tested for NpO2(OH2)5+. They produced inaccurate NpO and Np−O(H2) bond lengths, ONpO vibrational frequencies, and ΔG‡ values (differing strongly from the WFT values). To conclude, none of the presently and previously4,5 investigated functionals yields accurate results for actinyl complexes (Tables 1−4). The reliability and applicability of single-configuration methods like DFT and MP2, for example, has to be established explicitly by comparison with high-level WFT. Ligand-Field Effects in Actinyl Ions. ΔG‡ values for the water exchange reaction of AnO2(OH2)52+ (An = U, Np, Pu, Am) and NpO2(OH2)5+ via an associative (a) or a dissociative (d) mechanism are represented in Figure 2. For an a activation, ΔG‡ for NpO2(OH2)52+ is significantly larger than for the other actinyl(VI) ions for the reasons discussed above, whereas for a d activation, ΔG‡ is significantly smaller for AmO2(OH2)52+ 5 compared with the other actinyl(VI) ions (Figure 2). For these
WFT
ΔG
b
42.1 31.7 42.3 26.4 43.3 28.3 33.1 42.0 30.7 38.0 11.1 34.1
ΔG
‡
41.3 42.7 46.0 40.6 39.7 29.8 44.6 36.5 44.6 44.6 39.7 39.7 39.5 39.5 46.4 46.4
ΔGb
model
38.9 33.5 42.7 32.8 40.5 32.3 38.0 40.4 35.5 35.4 22.0 37.4 40.3 34.0
WA WB WA WB WA WB WA WB WA WB WA WB WA WB WA WB
Units: kJ/mol. bValues, for which ΔG > ΔG‡ are italicized. In such cases, it might be preferable to attribute the Ia or Id mechanism. a
cannot be excluded that both an associative and a dissociative mechanism operate. ΔG‡ for the associatively activated water exchange reaction of NpO2(OH2)52+ is significantly higher than for UO2(OH2)52+, PuO2(OH2)52+, and AmO2(OH2)52+.4,5 This is due to ligandfield effects. In the TS for A, NpO2(OH2)5···OH22+‡, the water ligands lying above and below the equatorial plane give rise to an antibonding interaction with the singly occupied fδ MO (Figure 1). Its MO energy is higher by 33.9 and 23.9 kJ mol−1 than in the reactant NpO2(OH2)52+ as calculated using the LCBOP-LRD and B3PW91-D3 functionals, respectively. In contrast, for the dissociatively activated water exchange reaction of AmO2(OH2)52+,5 exhibiting a fδ2fϕ1 electron configuration, ligand-field effects give rise to a lower ΔG‡. Electron Correlation. In actinyl ions, AnO2+ and AnO22+, static correlation is present, which requires multiconfiguration wave functions. Usually, CASSCF or MCSCF is used, followed by the treatment of dynamic electron correlation using perturbation theory. For the present systems, the active space is composed by the σ(OAnO), σ*(OAnO), π(O AnO), π*(OAnO), and f MOs of An, depending on their occupation. In 2005, the water exchange reaction of the UO2(OH2)52+ ion was investigated by Hagberg et al.47 with classical MD simulations, whereby the force field was computed using CASPT2.48,49 Later, this water exchange reaction was reinvestigated50 with CASSCF/MCQDPT2 and continuum solvation models. In a comment, Vallet et al.51 contested the presence of static electron correlation in uranyl(VI) compounds. In a reply,52 it was shown that static correlation is indeed present in uranyl(VI) complexes. In further studies4,5 on UO2(OH2)52+, PuO2(OH2)52+, and AmO2(OH2)52+, static correlation involving the above-mentioned σ, σ*, π, π*, and f MOs (for PuVI and AmVI) in the active space was treated with XGMC-QDPT2. Recently, the multireference nature of plutonium oxides was established by Boguslawsky et al.53 based on Density Matrix Renormalization Group (DMRG) calculations. For (bare) PuO22+, these authors found a similar active space to that used for the above-mentioned 4 D
DOI: 10.1021/acs.inorgchem.7b02373 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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Article
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02373. Modified ECPs for Np; atomic coordinates (LC-BOPLRD and MCSCF geometries) of the reactants, TSs, and intermediates for the Ia, A, and D mechanisms and the WB model; NpO and Np−O(H2) bond lengths and ONpO vibrational frequencies of NpO2(OH2)5+; G of NpO2(OH2)5+, calculated with WFT for various DFT and MCSCF geometries; ΔG‡ values for the water exchange reactions of NpO2(OH2)5+ investigated with functionals not listed in Tables 1 and 2 (PDF)
Figure 2. Calculated ΔG‡ for the a and d activated water exchange pathways of actinyl aqua complexes AnO2(OH2)52+ (An = U, Np, Pu, Am) and NpO2(OH2)5+ with various fn electron configurations.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: francois.rotzinger@epfl.ch. ORCID
François P. Rotzinger: 0000-0001-8759-4427
two systems, exhibiting an odd f electron number, a partially filled fδ or fϕ shell, ΔG‡’s for the a and d activated water exchange are close. As shown in this and previous5 work, ΔG‡ for the water exchange reaction of AmO2(OH2)52+ via a d activation is lowered by ligand-field effects, whereas for the exchange of NpO2(OH2)52+ via an a activation, ΔG‡ is increased. For these exchange reactions of actinyl ions, ligand-field effects are small, but present. Recently, ligandfield effects have also been reported for actinide(III) (Th−No) and actinide(IV) (Pa−Bk) ions.54
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Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS One reviewer contributed valuable comments to the manuscript. REFERENCES
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CONCLUSIONS
(i) Water exchange on NpO2(OH2)5+ follows a dissociative mechanism. (ii) For the water exchange on NpO2(OH2)52+, ΔG‡ for an associative activation is higher than for the corresponding U(IV), Pu(VI), and Am(VI) ions,4,5 and higher than predicted in earlier studies7,8 due to ligand-field effects. (iii) ΔG‡ for a dissociative activation of NpO2(OH2)52+ is considerably lower than the value (70.0 kJ mol−1) calculated by Vallet et al.7 (iv) According to the present computations on NpO2(OH2)52+, ΔG‡ for an a activation is only slightly and insignificantly lower than ΔG‡ for a d activation. Hence, it is not possible to attribute the exchange mechanism of this reaction. (v) A d activation cannot be ruled out. (vi) The AnO bond lengths, OAnO vibrational frequencies, the AnO bond lengths changes along the activation process, and the ΔG‡ values are sensitive to the treatment of static (and dynamic) electron correlation. The presently investigated functionals (GGA, hybrid-GGA, metaGGA, hybrid-meta-GGA, and range-separated hybrids) fail in the simultaneous description of all of the above-mentioned properties. (vii) Due to the inaccuracies of the properties calculated with DFT, ΔG‡ has to be computed with WFT (taking into account static and dynamic electron correlation, near-degeneracy effects, if present, and SO coupling). (viii) Acceptable results were obtained with TPSS-D3 (good geometries and frequencies) and LC-BOP-LRD (producing good ΔG‡ values), for example. E
DOI: 10.1021/acs.inorgchem.7b02373 Inorg. Chem. XXXX, XXX, XXX−XXX
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G
DOI: 10.1021/acs.inorgchem.7b02373 Inorg. Chem. XXXX, XXX, XXX−XXX
