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Transient Covalency in Molten Uranium(III) Chloride
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Journal of the American Chemical Society

Cite this: J. Am. Chem. Soc. 2024, 146, 31, 21220–21224
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https://doi.org/10.1021/jacs.4c05765
Published July 23, 2024

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Abstract

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Uranium is arguably the most essential element in the actinide series, serving as a crucial component of nuclear fuels. While U is recognized for engaging the 5f orbitals in chemical bonds under normal conditions, little is known about its coordination chemistry and the nature of bonding interactions at extreme conditions of high temperature. Here we report experimental and computational evidence for the shrinkage of the average U–ligand distance in UCl3 upon the solid-to-molten phase transition, leading to the formation of a significant fraction of short, transient U–Cl bonds with the enhanced involvement of U 5f valence orbitals. These findings reveal that extreme temperatures create an unusual heterogeneous bonding environment around U(III) with distinct inner- and outer-coordination subshells.

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The complex nature of chemical interactions, electronic structure, and redox properties of actinides presents deep challenges in science. (1−5) Although prior studies in the solid state and aqueous solutions emphasize the importance of f electrons in actinide bonding, (6−10) the interactions of actinides in the ionic environment at high temperatures are underresearched. (11,12) This information is exceptionally challenging to obtain due to radioactivity and the extreme conditions of such experiments. In particular, understanding the chemistry of the molten uranium trichloride (UCl3) is crucial for the development of next-generation liquid nuclear fuel and handling radioactive waste. (13) Yet, over the collection of previous experimental and computational works, (14−17) there is still no clear consensus on the bond lengths and coordination structure of molten UCl3. The first structural studies using X-ray diffraction (XRD) at 1200 K coupled with classical molecular dynamics simulations showed that a fully ionic model failed to adequately describe the local UCl3 structure, and a better agreement was achieved by incorporating a degree of covalency into the U–Cl bond. (14) Indeed, under normal conditions, uranium is known to provide its outermost 5f and 6d orbitals for mixing with ligand orbitals to afford some degree of covalency to the primarily ionic interactions. (9,18−21) Thus, from fundamental and practical points of view, it is essential to study how the structure and bonding in UCl3 (Figure 1A) change upon melting.

Figure 1

Figure 1. (A) Local structure of solid UCl3 and (B) the Fourier transform magnitude, |χ(r)|, of UCl3 L3-edge EXAFS, k3χ. The spectrum is not phase-shift adjusted. (C) Neutron pair distribution functions, G(r), measured for solid (blue line) and molten (red line) UCl3, along with (D) the RMC fit for molten UCl3 (dashed line) and decomposition of G(r) into the U–Cl (pink), Cl–Cl (green), and U–U (yellow) ion pair correlations.

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Toward this goal, we performed neutron scattering experiments and advanced ab initio molecular dynamics (AIMD) analyses to address the outstanding fundamental question of how the U–Cl bonding characteristics are affected by the extreme conditions of high temperature. Our results reveal the shrinkage of U–Cl bond lengths upon melting, leading to the formation of a highly heterogeneous coordination shell around U(III) ions. The AIMD and chemical bonding analyses in conjunction with the simulated Raman spectrum of molten UCl3 point to the presence of a large fraction of short transient U–Cl bonding interactions, which are spectroscopically active and exhibit a higher participation of U 5f orbitals than in the solid state.

According to single-crystal XRD, solid UCl3 adopts a tricapped trigonal prismatic configuration, where each U(III) center is surrounded by nine Cl atoms. Figure 1B shows our synchrotron extended X-ray absorption fine structure (EXAFS) spectroscopy results for the UCl3 sample, which was synthesized and handled under an inert atmosphere. The purity was confirmed by XRD, melting point measurements, and inductively coupled plasma optical emission spectrometry (Figures S1–S3, Table S1). Fitting the experimental EXAFS spectrum (Figure 1B, Figure S4) with a single U–Cl scattering path results in an average U–Cl distance of 2.909(9) Å (Debye–Waller factor, σ2 = 0.0079(11) Å2), consistent with the single-crystal XRD data (Table S2).

To study both local and extended structures, neutron scattering measurements were carried out on UCl3 powder that was contained in a quartz tube (Figure S5). The measured scattering intensities provide neutron structure factors, S(Q) (Figure S6), which were Fourier transformed to obtain pair distribution functions (PDFs), G(r), that are related to the probability of finding ion pairs with a given separation distance r. Figure 1C demonstrates significant changes in the PDF as UCl3 transitions from a solid (298 K) to a molten (1173 K) state. Broadening of G(r) features is observed upon heating with the complete loss of crystalline symmetries and long-range order correlations in the salt above the melting point (Figure 1C).

Based on the relative neutron weighting factors (Figure S7) and the UCl3 crystal structure, the first peak in G(r) is primarily dominated by the short-range U–Cl correlations, whereas the second peak is attributed to the Cl–Cl interactions. The features beyond 4 Å originate from the longer-range atom pair correlations. Interestingly, the first peak at 2.92 Å for solid UCl3 shifts toward a shorter distance upon melting, in direct contrast to thermal expansion expectations. This is likely associated with a decrease in the average coordination numbers on melting, often leading to shorter nearest-neighbor bond lengths. (22,23) We note, however, that the intrinsically broad features in G(r) at 1173 K, associated with the liquid state, make quantitative interpretation of scattering results difficult. Thus, reverse Monte Carlo (RMC) modeling was performed to reproduce the experimental PDF of molten UCl3 and determine partial U–Cl, Cl–Cl, and U–U PDFs (Figure 1D). Our results show that U–Cl average bond length indeed shrinks on heating to the value of 2.78(1) Å in the molten state. A similar behavior was recently reported for molten tin at 530–1323 K, where the fraction of fluctuating short Sn–Sn covalent bonds unexpectedly increased, leading to a shift of the first peak in the PDF to a shorter distance at higher temperatures. (24)

To gain more insights into the observed U–Cl bond contraction phenomenon, we performed AIMD simulations. The theoretical S(Q) and G(r) were generated directly from the 60 ps AIMD trajectory and show very good agreement with the experimental data (Figures S6 and S8), validating our model. Key structural parameters align well with those determined by the PDF measurements and RMC fit, as can be judged from the analyses of radial distribution functions, g(r) (Figure S8). Figure 2A shows that the AIMD-predicted U–Cl bond length is 2.78 Å, shorter than the U–Cl bond length in the solid state (rSolid) and in good agreement with the RMC results. The actual boundary of the first Cl– coordination shell around U(III) in the melt is where g(r) reaches the first minimum (r = r† in Figure 2A). Within this first coordination shell, we define short U–Cl contacts in the inner subshell, for which r < rSolid, and long U–Cl interactions (rSolid < r < r†) in the outer subshell. As one may see in Figure 2B, at any point of time, the former (∼55%) dominates the first coordination sphere, explaining the overall shrinkage of the average U–Cl bond seen in our PDF experiments. Interestingly, while these subshells are well preserved over time, the U(III) coordination number (CN) can rapidly deviate from its most-probable value of ∼7.5 (Figure S9), due to the high thermal energy.

Figure 2

Figure 2. (A) U–Cl radial distribution function, g(r), highlighting the shorter U–Cl distance in the melt compared to the bond length (rSolid) in the solid state. r† represents the boundary of the first coordination shell. (B) Time-dependent population of the inner (blue) and outer (teal) subshells. (C) Projection of the electron localization function for a snapshot from the UCl3 AIMD trajectory. (D) Selected short U–Cl σ-type dative bond NBO and its composition for the representative U(III) cluster in the melt.

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The nature of the U–Cl chemical bonds in the two subshells was rigorously examined using quantum chemical methods, based on electron localization function, (25) natural bond orbital (NBO), (26) and quantum theory of atoms in molecules (QTAIM) (27) analyses. Figure 2C shows that, in the inner subshell, the lone pairs of the chlorides are pointing toward U(III) with somewhat enhanced electron localization in the middle of the two atoms. In contrast, they lack directionality without being noticeably deformed in the outer subshell. This indicates the increased U participation in the short U–Cl bonds, whereas the longer U–Cl interactions are more of an ionic nature. The NBO analysis in Figure 2D for the representative cluster confirms the enhanced bonding in the inner subshell as a consequence of the high-temperature-induced shrinkage of the first coordination sphere around U(III), enabling better overlap of the Cl lone pairs with the U(III) acceptor orbitals. Although the U–Cl NBOs are strongly polarized toward Cl, our results point to the increased U(III) contribution and 5f orbital involvement in the inner subshell bonding at high temperatures as compared to the solid state (Table S3). Additionally, the enhanced bonding can be assessed employing Wiberg bond indices and QTAIM characteristics (Table S4), all showing the same trend of increased electron density sharing in the inner subshell U–Cl bonds of molten uranium trichloride. Furthermore, the projected density of states analysis shows that the overlap between U 5f and Cl 3p orbitals is stronger for the short U–Cl contacts as compared to the outer subshell interactions (Figure S10). This is consistent with our cluster model calculations and further confirms the presence of a slight orbital overlap between U and Cl, specifically at shorter distances. Thus, we anticipate that the role of the 5f valence orbitals of uranium in molten systems can be further explored using the Cl K-edge, (8) U M4,5-edge high-energy-resolution X-ray absorption near edge structure and 3d4f resonant inelastic X-ray scattering spectroscopies (28−30) in the future studies.

To gain insights into the dynamics of the U–Cl interactions dictating the stability of the two subshells, we determined the survival probability correlation function, (31) C(t), of the U-bound Cl for a range of cutoff distances, rC, corresponding to different boundaries of the subshells. Figure 3A shows that C(t) exhibits underdamped oscillations until ∼200 fs for the short U–Cl interactions (rC < rSolid). However, this feature is absent for the longer U–Cl contacts (rSolid < rC < r†). The long-time behavior of all C(t)’s is the same as can be seen from the parallel exponential curves. Thus, a Cl– ion spends ∼200 fs in the inner subshell before transitioning to the outer subshell, with the overall residence time in the first coordination sphere of ∼20 ps (obtained from the exponential fit to the long-time decay of the C(t) curve). This rather short lifetime of the U–Cl bonds at 1173 K points to their transient nature.

Figure 3

Figure 3. (A) Survival probability correlation functions, C(t), for a Cl– bound U(III) plotted for various binding cutoff distances, rC, which are Fourier-transformed to obtain (B) FT-C(ω) spectra and compared with (C) simulated Raman spectra (parallel and perpendicular polarizations).

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The important feature of covalent interactions is their directionality, usually giving rise to absorption bands in the infrared or Raman spectra. (9) Our attempts to obtain experimental Raman results for the molten UCl3 were unsuccessful, due to the strong self-absorption by the sample. (32) Nevertheless, to investigate whether the dynamic, metastable coordination environment of molten UCl3, especially the transient inner subshell, is spectroscopically resolvable, we computed Raman spectra from the AIMD trajectory using the Berry phase formalism. (33) This method has previously shown very good reproducibility of the experimental data for various molten salt systems. (33,34) Additionally, we obtained the Fourier-transformed spectra of C(t), FT-C(ω), for comparison. As depicted in Figure 3B and C, there is a striking match between FT-C(ω) for the inner subshell and the Raman spectrum with the parallel polarization. Both exhibit a well-resolved symmetric U–Cl stretch vibrational band at 200–250 cm–1 accompanied by a less noticeable high-frequency band at ∼350 cm–1. In contrast, FT-C(ω) for the outer subshell does not show any peaks. This is expected since the underdamped U–Cl oscillations are only present for the inner subshell, and thereby, only the short U–Cl contacts contribute to the distinct band. For the extended U–Cl bonds, their influence on the Raman spectra is analogous to the effect of the weakly complexing ions (e.g., alkali metal ions in molten salts), predominantly increasing the spectral intensity at the lower energy end without clearly differentiating into its own band. Thus, despite the transient nature, the inner subshell bonds are distinguishable from the outer subshell interactions and resolvable with vibrational spectroscopy.

Even the subtle yet important presence of 5f orbital covalency is frequently invoked to comprehend and rationalize the structure, reactivity, and spectroscopic properties of heavy element compounds. (35−42) Recently, it has been demonstrated that high pressure can be utilized to modify the structure and bonding in actinide complexes. (43,44) Our discovery of short transient bonds in molten UCl3, which show enhanced U 5f orbital participation and likely contributing to the highly heterogeneous coordination shell around U(III), illustrates that high temperature can also impact the fundamental characteristics of actinide compounds, including bond distances, coordination number, and local dynamics. (45) These findings are expected to improve our fundamental understanding and prediction of the structurally diverse and dynamic coordination chemistry and speciation exhibited by actinides in molten phases. (11,12)

Data Availability

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Data sets for this article are made available within 30 days of the official acceptance date of this article by the journal in the Zenodo repository under the Digital Object Identifier (DOI): 10.5281/zenodo.12668490.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c05765.

  • Synthesis and characterization of UCl3 and additional experimental and computational details, including neutron scattering measurements, chemical bonding, density of states, and coordination number analyses (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
  • Authors
    • Dmitry S. Maltsev - Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States;  Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States
    • Darren M. Driscoll - Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States;  Orcidhttps://orcid.org/0000-0001-8859-8016
    • Yuanpeng Zhang - Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States;  Orcidhttps://orcid.org/0000-0003-4224-3361
    • Joerg C. Neuefeind - Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
    • Benjamin Reinhart - Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, United States
    • Can Agca - Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States;  Orcidhttps://orcid.org/0000-0003-1427-7812
    • Debmalya Ray - Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
    • Phillip W. Halstenberg - Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States;  Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States;  Orcidhttps://orcid.org/0000-0002-6030-4503
    • Mina Aziziha - Mechanical Engineering Department, University of South Carolina, Columbia, South Carolina 29208, United States;  Orcidhttps://orcid.org/0000-0003-4001-6413
    • Juliano Schorne-Pinto - Mechanical Engineering Department, University of South Carolina, Columbia, South Carolina 29208, United States;  Orcidhttps://orcid.org/0000-0003-4208-4815
    • Theodore M. Besmann - Mechanical Engineering Department, University of South Carolina, Columbia, South Carolina 29208, United States
    • Sheng Dai - Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States;  Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States;  Orcidhttps://orcid.org/0000-0002-8046-3931
  • Notes
    The authors declare no competing financial interest.

    This manuscript has been authored by UT-Battelle, LLC under Contract DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Acknowledgments

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This work was supported as part of the Molten Salts in Extreme Environments (MSEE) Energy Frontier Research Center, funded by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences. Work at Oak Ridge National Laboratory was supported by DOE contract DE-AC05-00OR22725. M.A., J.S.-P., and T.M.B. are supported by the U.S. DOE Office of Nuclear Energy, Nuclear Energy University Programs under award DE-NE0008985 and the Molten Salt Reactor Program. This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. A portion of this research used resources of the Advanced Photon Source at beamline 12-BM, operated by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. This research used resources of the Compute and Data Environment for Science (CADES) at the Oak Ridge National Laboratory and the National Energy Research Scientific Computing Center (NERSC), which are supported by the Office of Science of the U.S. DOE under Contract Nos. DE-AC05-00OR22725 and DE-AC02-05CH11231, respectively. A.S.I., S.R., and V.S.B. thank Dr. Margulis and Dr. Emerson for initial discussions.

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    Smith, A. L. Structure-property Relationships in Actinide Containing Molten Salts – A Review: Understanding and Modelling the Chemistry of Nuclear Fuel Salts. J. Mol. Liq. 2022, 360, 119426  DOI: 10.1016/j.molliq.2022.119426
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    Hombourger, B.; Křepel, J.; Pautz, A. Breed-and-burn Fuel Cycle in Molten Salt Reactors. EPJ. Nuclear Sciences & Technologies 2019, 5, 15,  DOI: 10.1051/epjn/2019026
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    Okamoto, Y.; Kobayashi, F.; Ogawa, T. Structure and Dynamic Properties of Molten Uranium Trichloride. J. Alloys Compd. 1998, 271–273, 355– 358,  DOI: 10.1016/S0925-8388(98)00087-5
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    Structure and dynamic properties of molten uranium trichloride
    Okamoto, Yoshihiro; Kobayashi, Fumiaki; Ogawa, Toru
    Journal of Alloys and Compounds (1998), 271-273 (), 355-358CODEN: JALCEU; ISSN:0925-8388. (Elsevier Science S.A.)
    High temp. X-ray diffraction anal. was performed to study structure of molten UCl3. The nearest neighbor distance and coordination no. of U-Cl pair were 0.284 nm and 6, resp. Short range structure of molten UCl3 is described as octahedral coordination where six Cl- ions surrounds U3+ ion. The obtained structural data were analyzed by using a Debye scattering equation and a mol. dynamics (MD) simulation. Dynamic properties such as shear viscosity and elec. cond. calcd. from structurally optimized MD simulation were compared with the exptl. data in the literature.
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    Okamoto, Y.; Madden, P. A.; Minato, K. X-ray Diffraction and Molecular Dynamics Simulation Studies of Molten Uranium Chloride. J. Nucl. Mater. 2005, 344 (1–3), 109– 114,  DOI: 10.1016/j.jnucmat.2005.04.026
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    X-ray diffraction and molecular dynamics simulation studies of molten uranium chloride
    Okamoto, Y.; Madden, P. A.; Minato, K.
    Journal of Nuclear Materials (2005), 344 (1-3), 109-114CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
    The structure of molten UCl3 at 1200 K was studied by high-temp. X-ray diffraction and mol. dynamics simulation. The XRD data was reproduced by the simulation with a polarizable ionic model. The nearest U3+-Cl- distance was 2.85 Å with coordination no. 8.0, implying that the 8-fold structure (UCl8)5- is predominant in the melt - in contradiction to earlier suggestions of octahedral coordination. The potential model, which had been optimized by comparison with the structural data, was also found to reproduce the exptl. information on transport properties like the diffusion coeff., elec. cond. and shear viscosity.
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    Adya, A. K.; Matsuura, H.; Takagi, R.; Rycerz, L.; Gaune-Escard, M. Structural and Thermodynamic Properties of Molten UCl3 and UCl3-MCl (M = Li, Na, K, and Cs) Systems. ECS Proceedings 1999, 1999–41 (1), 341,  DOI: 10.1149/199941.0341PV
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    Neilson, G. W.; Adya, A. K.; Ansell, S. 8 Neutron and X-ray Diffraction Studies on Complex Liquids. Annual Reports Section “C” (Physical Chemistry) 2002, 98 (0), 271– 322,  DOI: 10.1039/b111168j
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    Yu, X.; Sergentu, D.-C.; Feng, R.; Autschbach, J. Covalency of Trivalent Actinide Ions with Different Donor Ligands: Do Density Functional and Multiconfigurational Wavefunction Calculations Corroborate the Observed “Breaks”?. Inorg. Chem. 2021, 60 (23), 17744– 17757,  DOI: 10.1021/acs.inorgchem.1c02374
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    18
    Covalency of Trivalent Actinide Ions with Different Donor Ligands: Do Density Functional and Multiconfigurational Wavefunction Calculations Corroborate the Observed "Breaks"?
    Yu, Xiaojuan; Sergentu, Dumitru-Claudiu; Feng, Rulin; Autschbach, Jochen
    Inorganic Chemistry (2021), 60 (23), 17744-17757CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    A comprehensive ab initio study of periodic actinide-ligand bonding trends for trivalent actinides is performed. Relativistic d. functional theory (DFT) and complete active-space (CAS) SCF wavefunction calcns. are used to dissect the chem. bonding in the [AnCl6]3-, [An(CN)6]3-, [An(NCS)6]3-, [An(S2PMe2)3], [An(DPA)3]3-, and [An(HOPO)]- series of actinide (An = U-Es) complexes. Except for some differences for the early actinide complexes with DPA, bond orders and excess 5f-shell populations from donation bonding show qual. similar trends in 5fn active-space CAS vs DFT calcns. The influence of spin-orbit coupling on donation bonding is small for the tested systems. Along the actinide series, chem. soft vs chem. harder ligands exhibit clear differences in bonding trends. There are pronounced changes in the 5f populations when moving from Pu to Am or Cm, which correlate with previously noted "breaks" in chem. trends. Bonding involving 5f becomes very weak beyond Cm/Bk. We propose that Cm(III) is a borderline case among the trivalent actinides that can be meaningfully considered to be involved in ground-state 5f covalent bonding.
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    Butorin, S. M.; Modin, A.; Vegelius, J. R.; Kvashnina, K. O.; Shuh, D. K. Probing Chemical Bonding in Uranium Dioxide by Means of High-Resolution X-ray Absorption Spectroscopy. J. Phys. Chem. C 2016, 120 (51), 29397– 29404,  DOI: 10.1021/acs.jpcc.6b09335
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    19
    Probing Chemical Bonding in Uranium Dioxide by Means of High-Resolution X-ray Absorption Spectroscopy
    Butorin, Sergei M.; Modin, Anders; Vegelius, Johan R.; Kvashnina, Kristina O.; Shuh, David K.
    Journal of Physical Chemistry C (2016), 120 (51), 29397-29404CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    A systematic x-ray absorption study at the U 3d, 4d, and 4f edges of UO2 was performed, and the data were analyzed within framework of the Anderson impurity model. By applying the high-energy-resoln. fluorescence-detection (HERFD) mode of x-ray absorption spectroscopy (XAS) at the U 3d3/2 edge and conducting the XAS measurements at the shallower U 4f levels, fine details of the XAS spectra were resolved resulting from reduced core-hole lifetime broadening. This multiedge study enabled a far more effective anal. of the electronic structure at the U sites and characterization of the chem. bonding and degree of the 5f localization in UO2. The results support the covalent character of UO2 and do not agree with the suggestions of rather ionic bonding in this compd. as expressed in some publications.
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  20. 20
    Amidani, L.; Retegan, M.; Volkova, A.; Popa, K.; Martin, P. M.; Kvashnina, K. O. Probing the Local Coordination of Hexavalent Uranium and the Splitting of 5f Orbitals Induced by Chemical Bonding. Inorg. Chem. 2021, 60 (21), 16286– 16293,  DOI: 10.1021/acs.inorgchem.1c02107
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    20
    Probing the Local Coordination of Hexavalent Uranium and the Splitting of 5f Orbitals Induced by Chemical Bonding
    Amidani, Lucia; Retegan, Marius; Volkova, Anna; Popa, Karin; Martin, Philippe M.; Kvashnina, Kristina O.
    Inorganic Chemistry (2021), 60 (21), 16286-16293CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The authors report here a detailed exptl. and theor. investigation of hexavalent uranium in various local configurations with a high-energy-resoln. fluorescence-detected X-ray absorption near-edge structure at the U M4 edge. The authors show the pronounced sensitivity of the technique to the arrangement of atoms around the absorber and provide a detailed theor. interpretation revealing the nature of spectral features. Calcns. based on d. functional theory and on crystal field multiplet theory indicate that for all local configurations analyzed, the main peak corresponds to nonbonding 5f orbitals, and the highest energy peak corresponds to antibonding 5f orbitals. The authors' findings agree with the accepted interpretation of uranyl spectral features and embed the latter in a broader field of view, which interprets the spectra of a large variety of U6+-contg. samples on a common theor. ground.
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  21. 21
    Dai, S.; Toth, L. M.; Del Cul, G. D.; Metcalf, D. H. Near-IR absorption transition of 3H4 to 3F2 for UCl62– complex in M2ZrCl6 host crystals (M = K+, Rb+, and Cs+): an experimental and theoretical study. Chem. Phys. 1995, 200 (3), 271– 279,  DOI: 10.1016/0301-0104(95)00228-6
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    Angell, C. A.; Gruen, D. M. Short-Range Order in Fused Salts. I. Coordination States of Nickel(II) in Molten Zinc Chloride-Potassium Chloride Mixtures1. J. Phys. Chem. 1966, 70 (5), 1601– 1609,  DOI: 10.1021/j100877a045
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    Roy, S.; Sharma, S.; Karunaratne, W. V.; Wu, F.; Gakhar, R.; Maltsev, D. S.; Halstenberg, P.; Abeykoon, M.; Gill, S. K.; Zhang, Y. X-Ray Scattering Reveals Ion Clustering of Dilute Chromium Species in Molten Chloride Medium. Chemical Science 2021, 12 (23), 8026– 8035,  DOI: 10.1039/D1SC01224J
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    Xu, L.; Wang, Z.; Chen, J.; Chen, S.; Yang, W.; Ren, Y.; Zuo, X.; Zeng, J.; Wu, Q.; Sheng, H. Folded Network and Structural Transition in Molten Tin. Nat. Commun. 2022, 13 (1), 126,  DOI: 10.1038/s41467-021-27742-2
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    Silvi, B.; Savin, A. Classification of Chemical Bonds Based on Topological Analysis of Electron Localization Functions. Nature 1994, 371 (6499), 683– 686,  DOI: 10.1038/371683a0
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    Classification of chemical bonds based on topological analysis of electron localization functions
    Silvi, B.; Savin, A.
    Nature (London, United Kingdom) (1994), 371 (6499), 683-6CODEN: NATUAS; ISSN:0028-0836.
    The definitions currently used to classify chem. bonds (in terms of bond order, covalency vs. ionicity and so forth) are derived from approx. theories and are often imprecise. Here we outline a first step towards a more rigorous means of classification based on topol. anal. of local quantum-mech. functions related to the Pauli exclusion principle. The local max. of these functions define localization attractors, of which there are only three basic types: bonding, non-bonding and core. Bonding attractors lie between the core attractors (which themselves surround the at. nuclei) and characterize the shared-electron interactions. The no. of bond attractors is related to the bond multiplicity. The spatial organization of localization attractors provides a basis for a well-defined classification of bonds, allowing an abs. characterization of covalency vs. ionicity to be obtained from observable properties such as electron densities.
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    Glendening, E. D.; Landis, C. R.; Weinhold, F. Natural Bond Orbital Methods. WIREs Computational Molecular Science 2012, 2 (1), 1– 42,  DOI: 10.1002/wcms.51
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    Bader, R. F. W. Atoms in Molecules. Acc. Chem. Res. 1985, 18 (1), 9– 15,  DOI: 10.1021/ar00109a003
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    Atoms in molecules
    Bader, R. F. W.
    Accounts of Chemical Research (1985), 18 (1), 9-15CODEN: ACHRE4; ISSN:0001-4842.
    A review with 21 refs.
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    Vitova, T.; Pidchenko, I.; Fellhauer, D.; Bagus, P. S.; Joly, Y.; Pruessmann, T.; Bahl, S.; Gonzalez-Robles, E.; Rothe, J.; Altmaier, M. The role of the 5f valence orbitals of early actinides in chemical bonding. Nat. Commun. 2017, 8 (1), 16053  DOI: 10.1038/ncomms16053
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    The role of the 5f valence orbitals of early actinides in chemical bonding
    Vitova, T.; Pidchenko, I.; Fellhauer, D.; Bagus, P. S.; Joly, Y.; Pruessmann, T.; Bahl, S.; Gonzalez-Robles, E.; Rothe, J.; Altmaier, M.; Denecke, M. A.; Geckeis, H.
    Nature Communications (2017), 8 (), 16053CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    One of the long standing debates in actinide chem. is the level of localization and participation of the actinide 5f valence orbitals in covalent bonds across the actinide series. Here we illuminate the role of the 5f valence orbitals of uranium, neptunium and plutonium in chem. bonding using advanced spectroscopies: actinide M4,5 HR-XANES and 3d4f RIXS. Results reveal that the 5f orbitals are active in the chem. bonding for uranium and neptunium, shown by significant variations in the level of their localization evidenced in the spectra. In contrast, the 5f orbitals of plutonium appear localized and surprisingly insensitive to different bonding environments. We envisage that this report of using relative energy differences between the 5fδ/φ and 5fπ*/5fσ* orbitals as a qual. measure of overlap-driven actinyl bond covalency will spark activity, and extend to numerous applications of RIXS and HR-XANES to gain new insights into the electronic structures of the actinide elements.
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    Vitova, T.; Kvashnina, K. O.; Nocton, G.; Sukharina, G.; Denecke, M. A.; Butorin, S. M.; Mazzanti, M.; Caciuffo, R.; Soldatov, A.; Behrends, T. High energy resolution x-ray absorption spectroscopy study of uranium in varying valence states. Phys. Rev. B 2010, 82 (23), 235118  DOI: 10.1103/PhysRevB.82.235118
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    Kvashnina, K.; Silva, C.; Amidani, L.; Retegan, M.; Bazarkina, E.; Weiss, S.; Graubner, T.; Kraus, F. On the origin of low-valent uranium oxidation state; Research Square Platform LLC: 2024.
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    Luzar, A.; Chandler, D. Hydrogen-Bond Kinetics in Liquid Water. Nature 1996, 379 (6560), 55– 57,  DOI: 10.1038/379055a0
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    Hydrogen-bond kinetics in liquid water
    Luzar, Alenka; Chandler, David
    Nature (London) (1996), 379 (6560), 55-7CODEN: NATUAS; ISSN:0028-0836. (Macmillan Magazines)
    Hydrogen bonds play a crucial role in the behavior of water; their spatial patterns and fluctuations characterize the structure and dynamics of the liq. The processes of breaking and making hydrogen bonds in the condensed phase can be probed indirectly by a variety of exptl. techniques, and more quant. information can be obtained from computer simulations. In particular, simulations have revealed that on long timescales the relaxation behavior of hydrogen bonds in liq. water exhibit non-exponential kinetics, suggesting that bond making and breaking are not simple processes characterized by well defined rate consts. Here we show that these kinetics can be understood in terms of an interplay between diffusion and hydrogen-bond dynamics. In our model, which can be extended to other hydrogen-bonded liqs., diffusion governs whether a specific pair of water mols. are near neighbors, and hydrogen bonds between such pairs form and persist at random with av. lifetimes detd. by rate consts. for bond making and breaking.
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    Strzelecki, A. C.; Wang, G.; Hickam, S. M.; Parker, S. S.; Batrice, R.; Jackson, J. M.; Conroy, N. A.; Mitchell, J. N.; Andersson, D. A.; Monreal, M. J. In Situ High-Temperature Raman Spectroscopy of UCl3: A Combined Experimental and Theoretical Study. Inorg. Chem. 2023, 62 (45), 18724– 18731,  DOI: 10.1021/acs.inorgchem.3c03139
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    Roy, S.; Brehm, M.; Sharma, S.; Wu, F.; Maltsev, D. S.; Halstenberg, P.; Gallington, L. C.; Mahurin, S. M.; Dai, S.; Ivanov, A. S. Unraveling Local Structure of Molten Salts via X-ray Scattering, Raman Spectroscopy, and Ab Initio Molecular Dynamics. J. Phys. Chem. B 2021, 125 (22), 5971– 5982,  DOI: 10.1021/acs.jpcb.1c03786
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    Unraveling Local Structure of Molten Salts via X-ray Scattering, Raman Spectroscopy, and Ab Initio Molecular Dynamics
    Roy, Santanu; Brehm, Martin; Sharma, Shobha; Wu, Fei; Maltsev, Dmitry S.; Halstenberg, Phillip; Gallington, Leighanne C.; Mahurin, Shannon M.; Dai, Sheng; Ivanov, Alexander S.; Margulis, Claudio J.; Bryantsev, Vyacheslav S.
    Journal of Physical Chemistry B (2021), 125 (22), 5971-5982CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
    In this work, we resolve a long-standing issue concerning the local structure of molten MgCl2 by employing a multimodal approach, including X-ray scattering and Raman spectroscopy, along with the theor. modeling of the exptl. spectra based on ab initio mol. dynamics (AIMD) simulations utilizing several d. functional theory (DFT) methods. We demonstrate the reliability of AIMD simulations in achieving excellent agreement between the exptl. and simulated spectra for MgCl2 and 50 mol % MgCl2 + 50 mol % KCl, and ZnCl2, thus allowing structural insights not directly available from expt. alone. A thorough computational anal. using five DFT methods provides a convergent view that octahedrally coordinated magnesium in pure MgCl2 upon melting preferentially coordinates with five chloride anions to form distorted square pyramidal polyhedra that are connected via corners and to a lesser degree via edges. This is contrasted with the results for ZnCl2, which does not change its tetrahedral coordination on melting. Although the five-coordinate MgCl53- complex was not considered in the early literature, together with an increasing tendency to form a tetrahedrally coordinated complex with decreasing the MgCl2 content in the mixt. with alkali metal chloride systems, current work reconciles the results of most previous seemingly contradictory exptl. studies.
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    Emerson, M. S.; Sharma, S.; Roy, S.; Bryantsev, V. S.; Ivanov, A. S.; Gakhar, R.; Woods, M. E.; Gallington, L. C.; Dai, S.; Maltsev, D. S. Complete Description of the LaCl3–NaCl Melt Structure and the Concept of a Spacer Salt That Causes Structural Heterogeneity. J. Am. Chem. Soc. 2022, 144 (47), 21751– 21762,  DOI: 10.1021/jacs.2c09987
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    Cantat, T.; Graves, C. R.; Jantunen, K. C.; Burns, C. J.; Scott, B. L.; Schelter, E. J.; Morris, D. E.; Hay, P. J.; Kiplinger, J. L. Evidence for the Involvement of 5f Orbitals in the Bonding and Reactivity of Organometallic Actinide Compounds: Thorium(IV) and Uranium(IV) Bis(hydrazonato) Complexes. J. Am. Chem. Soc. 2008, 130 (51), 17537– 17551,  DOI: 10.1021/ja8067287
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    35
    Evidence for the Involvement of 5f Orbitals in the Bonding and Reactivity of Organometallic Actinide Compounds: Thorium(IV) and Uranium(IV) Bis(hydrazonato) Complexes
    Cantat, Thibault; Graves, Christopher R.; Jantunen, Kimberly C.; Burns, Carol J.; Scott, Brian L.; Schelter, Eric J.; Morris, David E.; Hay, P. Jeffrey; Kiplinger, Jaqueline L.
    Journal of the American Chemical Society (2008), 130 (51), 17537-17551CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Migratory insertion of diphenyldiazomethane into both metal-C bonds of the bis(alkyl) and bis(aryl) complexes (C5Me5)2AnR2 yields the 1st f-element bis(hydrazonato) complexes (C5Me5)2An[η2-(N,N')-R-N-N:CPh2]2 [An = Th, R = CH3 (18), PhCH2 (15), Ph (16); An = U, R = CH3 (17), PhCH2 (14)], which were characterized by a combination of spectroscopy, electrochem., and x-ray crystallog. The two hydrazonato ligands adopt an η2-coordination mode leading to 20-electron (for Th) and 22-electron (for U) complexes that have no transition-metal analogs. In fact, reaction of (C5H5)2ZrMe2 or (C5Me5)2HfMe2 with diphenyldiazomethane is limited to the formation of the corresponding mono(hydrazonato) complex (C5R5)2M[η2-(N,N')-CH3-N-N:CPh2]Me (M = Zr, R = H or M = Hf, R = CH3). The difference in the reactivities of the Group 4 metal complexes and the actinides was used as a unique platform for studying in depth the role of 5f orbitals on the reactivity and bonding in actinide organometallic complexes. The electronic structure of the (C5H5)2M[η2-(N,N')-CH3-N-N:CH2]2 (M = Zr, Th, U) model complexes was studied using d. functional theory (DFT) calcns. and compared to exptl. structural, electrochem., and spectroscopic results. Whereas transition-metal bis(cyclopentadienyl) complexes are known to stabilize three ligands in the metallocene girdle to form satd. (C5H5)2ML3 species, in a bis(hydrazonato) system, a 4th ligand is coordinated to the metal center to give (C5H5)2ML4. DFT calcns. showed that 5f orbitals in the actinide complexes play a crucial role in stabilizing this 4th ligand by stabilizing both the σ and π electrons of the two η2-coordinated hydrazonato ligands. In contrast, the stabilization of the hydrazonato ligands is significantly less effective for the putative bis(hydrazonato) Zr(IV) complex, yielding a higher energy structure. However, the difference in the reactivities of the Group 4 metal and actinide complexes does not arise on thermodn. grounds but is primarily of kinetic origin. Unfavorable steric factors were ruled out as the sole influence to explain these different behaviors, and electronic factors govern the reactivity. For the actinides, both the C5H5 and more realistic C5Me5 ligands were taken into account in computing the energy surface. The reaction profile for the C5Me5 system differs from that with the C5H5 ligand by a uniform shift of ∼5 kcal/mol in the relative energies of the transition state and products. The insertion of a 2nd diazoalkane mol. into the sole metal-C bond in the mono(hydrazonato) complexes involves a high energy barrier (∼20 kcal/mol) for the Zr(IV) system, whereas the actinides can facilitate the approach of the diazoalkane by coordination (formation of an adduct) and its insertion into the An-C bond with a very low barrier on the potential energy surface.
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    Yang, P.; Warnke, I.; Martin, R. L.; Hay, P. J. Theoretical Studies of the sp2 versus sp3 C–H Bond Activation Chemistry of 2-Picoline by (C5Me5)2An(CH3)2 Complexes (An = Th, U). Organometallics 2008, 27 (7), 1384– 1392,  DOI: 10.1021/om700927n
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    36
    Theoretical Studies of the sp2 versus sp3 C-H Bond Activation Chemistry of 2-Picoline by (C5Me5)2An(CH3)2 Complexes (An = Th, U)
    Yang, Ping; Warnke, Ingolf; Martin, Richard L.; Hay, P. Jeffrey
    Organometallics (2008), 27 (7), 1384-1392CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    The C-H bond activation chem. of (C5Me5)2ThMe2 and (C5Me5)2UMe2 with 2-picoline (2-methylpyridine) was examd. using d. functional techniques. In particular, the differences between insertion into the ortho ring sp2 C-H bond and the Me sp3 C-H bond are explored. The energies to form the η2-(N,C)-pyridyl products resulting from the activation of the arom. ring sp2 C-H bond are calcd. as thermodn. products for both Th and U systems with similar reaction energies of -15.8 kcal/mol. The products corresponding to insertion into the Me sp3 C-H bond are higher in energy by 3.5 and 5.4 kcal/mol for Th and U, resp. In the transition states the actinide atom mediates the H migration from 2-picoline to the leaving Me group by forming an agostic five-centered C-H complex. The relative activation energies between sp2 and sp3 C-H bond activation differ slightly between Th, ΔE⧺(sp2) > ΔE⧺(sp3) and U, ΔE⧺(sp2) < ΔE⧺(sp3). These results are in agreement with the exptl. observations that the sp2 insertion product is the thermodn. product in both cases, but that the sp3 insertion product is the kinetic product in the case of Th.
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  37. 37
    Su, J.; Batista, E. R.; Boland, K. S.; Bone, S. E.; Bradley, J. A.; Cary, S. K.; Clark, D. L.; Conradson, S. D.; Ditter, A. S.; Kaltsoyannis, N. Energy-Degeneracy-Driven Covalency in Actinide Bonding. J. Am. Chem. Soc. 2018, 140 (51), 17977– 17984,  DOI: 10.1021/jacs.8b09436
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    37
    Energy-Degeneracy-Driven Covalency in Actinide Bonding
    Su, Jing; Batista, Enrique R.; Boland, Kevin S.; Bone, Sharon E.; Bradley, Joseph A.; Cary, Samantha K.; Clark, David L.; Conradson, Steven D.; Ditter, Alex S.; Kaltsoyannis, Nikolas; Keith, Jason M.; Kerridge, Andrew; Kozimor, Stosh A.; Loble, Matthias W.; Martin, Richard L.; Minasian, Stefan G.; Mocko, Veronika; La Pierre, Henry S.; Seidler, Gerald T.; Shuh, David K.; Wilkerson, Marianne P.; Wolfsberg, Laura E.; Yang, Ping
    Journal of the American Chemical Society (2018), 140 (51), 17977-17984CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Evaluating the nature of chem. bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic d. functional theory study that quant. evaluates An-Cl covalency in AnCl62- (AnIV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and AnIV 5f- and 6d-orbitals (t1u*/t2u* and t2g*/eg*), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amt. of M-Cl orbital mixing, such that AnIV 6d- and Cl 3p-mixing decreased and metal 5f- and Cl 3p-orbital mixing increased across this series.
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    Formanuik, A.; Ariciu, A.-M.; Ortu, F.; Beekmeyer, R.; Kerridge, A.; Tuna, F.; McInnes, E. J. L.; Mills, D. P. Actinide Covalency Measured by Pulsed Electron Paramagnetic Resonance Spectroscopy. Nat. Chem. 2017, 9 (6), 578– 583,  DOI: 10.1038/nchem.2692
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    Actinide covalency measured by pulsed electron paramagnetic resonance spectroscopy
    Formanuik, Alasdair; Ariciu, Ana-Maria; Ortu, Fabrizio; Beekmeyer, Reece; Kerridge, Andrew; Tuna, Floriana; McInnes, Eric J. L.; Mills, David P.
    Nature Chemistry (2017), 9 (6), 578-583CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    Our knowledge of actinide chem. bonds lags far behind our understanding of the bonding regimes of any other series of elements. This is a major issue given the technol. as well as fundamental importance of f-block elements. Some key chem. differences between actinides and lanthanides-and between different actinides-can be ascribed to minor differences in covalency, i.e., the degree to which electrons are shared between the f-block element and coordinated ligands. Yet there are almost no direct measures of such covalency for actinides. Here we report the first pulsed ESR spectra of actinide compds. We apply the hyperfine sublevel correlation technique to quantify the electron-spin d. at ligand nuclei (via the weak hyperfine interactions) in mol. thorium and uranium species and therefore the extent of covalency. Such information will be important in developing our understanding of the chem. bonding, and therefore the reactivity, of actinides.
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    Smiles, D. E.; Wu, G.; Hrobárik, P.; Hayton, T. W. Use of 77Se and 125Te NMR Spectroscopy to Probe Covalency of the Actinide-Chalcogen Bonding in [Th(En){N(SiMe3)2}3]– (E = Se, Te; n = 1, 2) and Their Oxo-Uranium(VI) Congeners. J. Am. Chem. Soc. 2016, 138 (3), 814– 825,  DOI: 10.1021/jacs.5b07767
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    Use of 77Se and 125Te NMR Spectroscopy to Probe Covalency of the Actinide-Chalcogen Bonding in [Th(En){N(SiMe3)2}3]- (E = Se, Te; n = 1, 2) and Their Oxo-Uranium(VI) Congeners
    Smiles, Danil E.; Wu, Guang; Hrobarik, Peter; Hayton, Trevor W.
    Journal of the American Chemical Society (2016), 138 (3), 814-825CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Reaction of [Th(I)(NR2)3] (R = SiMe3) (1) with 1 equiv of either [K(18-crown-6)]2[Se4] or [K(18-crown-6)]2[Te2] affords the thorium dichalcogenides, [K(18-crown-6)][Th(η2-E2)(NR2)3] (E = Se, 2; E = Te, 3), resp. Removal of one chalcogen atom via reaction with Et3P, or Et3P and Hg, affords the monoselenide and monotelluride complexes of thorium, [K(18-crown-6)][Th(E)(NR2)3] (E = Se, 4; E = Te, 5), resp. Both 4 and 5 were characterized by X-ray crystallog. and were found to feature the shortest known Th-Se and Th-Te bond distances. The electronic structure and nature of the actinide-chalcogen bonds were investigated with 77Se and 125Te NMR spectroscopy accompanied by detailed quantum-chem. anal. We also recorded the 77Se NMR shift for a U(VI) oxo-selenido complex, [U(O)(Se)(NR2)3]- (δ(77Se) = 4905 ppm), which features the highest frequency 77Se NMR shift yet reported, and expands the known 77Se chem. shift range for diamagnetic substances from ∼3300 ppm to almost 6000 ppm. Both 77Se and 125Te NMR chem. shifts of given chalcogenide ligands were identified as quant. measures of the An-E bond covalency within an isoelectronic series and supported significant 5f-orbital participation in actinide-ligand bonding for uranium(VI) complexes in contrast to those involving thorium(IV). Moreover, X-ray diffraction studies together with NMR spectroscopic data and d. functional theory (DFT) calcns. provide convincing evidence for the actinide-chalcogen multiple bonding in the title complexes. Larger An-E covalency is obsd. in the [U(O)(E)(NR2)3]- series, which decreases as the chalcogen atom becomes heavier.
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    Kaltsoyannis, N. Does Covalency Increase or Decrease across the Actinide Series? Implications for Minor Actinide Partitioning. Inorg. Chem. 2013, 52 (7), 3407– 3413,  DOI: 10.1021/ic3006025
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    Does Covalency Increase or Decrease across the Actinide Series? Implications for Minor Actinide Partitioning
    Kaltsoyannis, Nikolas
    Inorganic Chemistry (2013), 52 (7), 3407-3413CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    A review. A covalent chem. bond carries the connotation of overlap of AOs between bonded atoms, leading to a buildup of the electron d. in the internuclear region. Stabilization of the valence 5f orbitals as the actinide series is crossed leads, in compds. of the minor actinides americium and curium, to their becoming approx. degenerate with the highest occupied ligand levels and hence to the unusual situation in which the resultant valence MOs have significant contributions from both actinide and the ligand yet in which there is little AO overlap. In such cases, the traditional quantum-chem. tools for assessing the covalency, e.g., population anal. and spin densities, predict significant metal-ligand covalency, although whether this orbital mixing is really covalency in the generally accepted chem. view is an interesting question. This review discusses our recent analyses of the bonding in AnCp3 and AnCp4 (An = Th-Cm; Cp = η5-C5H5) using both the traditional tools and also topol. anal. of the electron d. via the quantum theory of atoms-in-mols. I will show that the two approaches yield rather different conclusions and suggest that care must be taken when using quantum chem. to assess metal-ligand covalency in this part of the periodic table. The implications of this work for minor actinide partitioning from nuclear wastes are discussed; minor actinide extractant ligands based on nitrogen donors have received much attention in recent years, as have comparisons of the extent of covalency in actinide-nitrogen bonding with that in analogous lanthanide systems via quantum-chem. studies employing the traditional tools for assessing the covalency.
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    Kerridge, A. Quantification of f-Element Covalency Through Analysis of the Electron Density: Insights from Simulation. Chem. Commun. 2017, 53 (50), 6685– 6695,  DOI: 10.1039/C7CC00962C
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    Quantification of f-element covalency through analysis of the electron density: insights from simulation
    Kerridge, A.
    Chemical Communications (Cambridge, United Kingdom) (2017), 53 (50), 6685-6695CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    The electronic structure of f-element compds. is complex due to a combination of relativistic effects, strong electron correlation and weak crystal field environments. However, a quant. understanding of bonding in these compds. is becoming increasingly technol. relevant. Recently, bonding interpretations based on analyses of the phys. observable electronic d. have gained popularity and, in this Feature Article, the utility of such d.-based approaches is demonstrated. Application of Bader's Quantum Theory of Atoms in Mols. (QTAIM) is shown to elucidate many properties including bonding trends, orbital overlap and energy degeneracy-driven covalency, oxidn. state identification and bond stability, demonstrating the increasingly important role that simulation and anal. play in the area of f-element bond characterization.
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    Berryman, V. E. J.; Whalley, Z. J.; Shephard, J. J.; Ochiai, T.; Price, A. N.; Arnold, P. L.; Parsons, S.; Kaltsoyannis, N. Computational Analysis of M–O Covalency in M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, U). Dalton Transactions 2019, 48 (9), 2939– 2947,  DOI: 10.1039/C8DT05094E
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    Sperling, J. M.; Warzecha, E. J.; Celis-Barros, C.; Sergentu, D.-C.; Wang, X.; Klamm, B. E.; Windorff, C. J.; Gaiser, A. N.; White, F. D.; Beery, D. A. Compression of Curium Pyrrolidine-Dithiocarbamate Enhances Covalency. Nature 2020, 583 (7816), 396– 399,  DOI: 10.1038/s41586-020-2479-2
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    Compression of curium pyrrolidine-dithiocarbamate enhances covalency
    Sperling, Joseph M.; Warzecha, Evan J.; Celis-Barros, Cristian; Sergentu, Dumitru-Claudiu; Wang, Xiaoyu; Klamm, Bonnie E.; Windorff, Cory J.; Gaiser, Alyssa N.; White, Frankie D.; Beery, Drake A.; Chemey, Alexander T.; Whitefoot, Megan A.; Long, Brian N.; Hanson, Kenneth; Kogerler, Paul; Speldrich, Manfred; Zurek, Eva; Autschbach, Jochen; Albrecht-Schonzart, Thomas E.
    Nature (London, United Kingdom) (2020), 583 (7816), 396-399CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Abstr.: curium is unique in the actinide series because its half-filled 5f 7 shell has lower energy than other 5f n configurations, rendering it both redox-inactive and resistant to forming chem. bonds that engage the 5f shell1-3. This is even more pronounced in gadolinium, curium's lanthanide analog, owing to the contraction of the 4f orbitals with respect to the 5f orbitals4. However, at high pressures metallic curium undergoes a transition from localized to itinerant 5f electrons5. This transition is accompanied by a crystal structure dictated by the magnetic interactions between curium atoms5,6. Therefore, the question arises of whether the frontier metal orbitals in curium(III)-ligand interactions can also be modified by applying pressure, and thus be induced to form metal-ligand bonds with a degree of covalency. Here the authors report exptl. and computational evidence for changes in the relative roles of the 5f/6d orbitals in curium-sulfur bonds in [Cm(pydtc)4]- (pydtc, pyrrolidinedithiocarbamate) at high pressures (up to 11 gigapascals). The authors compare these results to the spectra of [Nd(pydtc)4]- and of a Cm(III) mellitate that possesses only curium-oxygen bonds. Compared with the changes obsd. in the [Cm(pydtc)4]- spectra, smaller changes in the f-f transitions in the [Nd(pydtc)4]- absorption spectrum and in the f-f emission spectrum of the Cm(III) mellitate upon pressurization, which are related to the smaller perturbation of the nature of their bonds. were obsd. These results reveal that the metal orbital contributions to the curium-sulfur bonds are considerably enhanced at high pressures and that the 5f orbital involvement doubles between 0 and 11 gigapascal. The authors' work implies that covalency in actinides is complex even when dealing with the same ion, but it could guide the selection of ligands to study the effect of pressure on actinide compds.
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    Shephard, J. J.; Berryman, V. E. J.; Ochiai, T.; Walter, O.; Price, A. N.; Warren, M. R.; Arnold, P. L.; Kaltsoyannis, N.; Parsons, S. Covalent Bond Shortening and Distortion Induced by Pressurization of Thorium, Uranium, and Neptunium Tetrakis Aryloxides. Nat. Commun. 2022, 13 (1), 5923,  DOI: 10.1038/s41467-022-33459-7
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    Shinohara, Y.; Ivanov, A. S.; Maltsev, D.; Granroth, G. E.; Abernathy, D. L.; Dai, S.; Egami, T. Real-Space Local Dynamics of Molten Inorganic Salts Using Van Hove Correlation Function. J. Phys. Chem. Lett. 2022, 13 (25), 5956– 5962,  DOI: 10.1021/acs.jpclett.2c01230
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    Real-Space Local Dynamics of Molten Inorganic Salts Using Van Hove Correlation Function
    Shinohara, Yuya; Ivanov, Alexander S.; Maltsev, Dmitry; Granroth, Garrett E.; Abernathy, Douglas L.; Dai, Sheng; Egami, Takeshi
    Journal of Physical Chemistry Letters (2022), 13 (25), 5956-5962CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Molten inorg. salts are attracting resurgent attention because of their unique physicochem. properties, making them promising media for next-generation concg. solar power systems and molten salt reactors. The dynamics of these highly disordered ionic media is largely studied by theor. simulations, while the robust exptl. techniques capable of observing local dynamics are not well-developed. To provide fundamental insights into the at.-scale transport properties of molten salts, we report the real-space dynamics of molten magnesium chloride at high temps. employing the Van Hove correlation function obtained by inelastic neutron scattering. Our results directly depict the distance-dependent dynamics of a molten salt on the picosecond time scale. This study demonstrates the capability of the developed approach to describe the locally correlated- and self-dynamics in molten salts, significantly improving our understanding of the interplay between microscopic structural parameters and their dynamics that ultimately control phys. properties of condensed matter in extreme environments.
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  1. Jun-Bo Lu, Yang-Yang Zhang, Jian-Biao Liu, Jun Li. Norm-Conserving 5f-in-Core Pseudopotentials and Gaussian Basis Sets Optimized for Tri- and Tetra-Valent Actinides (An = Pa–Lr). Journal of Chemical Theory and Computation 2025, 21 (1) , 170-182. https://doi.org/10.1021/acs.jctc.4c01189
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  • Abstract

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    Figure 1

    Figure 1. (A) Local structure of solid UCl3 and (B) the Fourier transform magnitude, |χ(r)|, of UCl3 L3-edge EXAFS, k3χ. The spectrum is not phase-shift adjusted. (C) Neutron pair distribution functions, G(r), measured for solid (blue line) and molten (red line) UCl3, along with (D) the RMC fit for molten UCl3 (dashed line) and decomposition of G(r) into the U–Cl (pink), Cl–Cl (green), and U–U (yellow) ion pair correlations.

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    Figure 2

    Figure 2. (A) U–Cl radial distribution function, g(r), highlighting the shorter U–Cl distance in the melt compared to the bond length (rSolid) in the solid state. r† represents the boundary of the first coordination shell. (B) Time-dependent population of the inner (blue) and outer (teal) subshells. (C) Projection of the electron localization function for a snapshot from the UCl3 AIMD trajectory. (D) Selected short U–Cl σ-type dative bond NBO and its composition for the representative U(III) cluster in the melt.

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    Figure 3

    Figure 3. (A) Survival probability correlation functions, C(t), for a Cl– bound U(III) plotted for various binding cutoff distances, rC, which are Fourier-transformed to obtain (B) FT-C(ω) spectra and compared with (C) simulated Raman spectra (parallel and perpendicular polarizations).

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  • References

    This article references 45 other publications.

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      A review. The overarching goal here is to review the body of theor. studies on actinide oxide mols. As to date only the binary oxides have been investigated by systematic studies on the whole set of light actinides, An = Th-Cm, or in a few cases on the entire actinide series, these species alone can provide consistent information on changes in mol. properties across the series. In this review, we present trends for several mol. properties utilizing data obtained at consistent theor. levels. The accuracy of the computed data is assessed by comparison with available exptl. results.
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      Inorganic Chemistry (2019), 58 (3), 1721-1723CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      There is no expanded citation for this reference.
      >> More from SciFinder ®
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      Neidig, M. L.; Clark, D. L.; Martin, R. L. Covalency in f-Element Complexes. Coord. Chem. Rev. 2013, 257, 394– 406,  DOI: 10.1016/j.ccr.2012.04.029
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      Covalency in f-element complexes
      Neidig, Michael L.; Clark, David L.; Martin, Richard L.
      Coordination Chemistry Reviews (2013), 257 (2), 394-406CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. The presence of covalency in complexes of the 4f and 5f elements has been a source of intense research and controversy. In addn. to academic interest in this debate, there is an industrial motivation for better understanding of bonding in f-element complexes due to the need to sep. trivalent trans-plutonium elements from trivalent lanthanide fission products in advanced nuclear fuel cycles. This review discusses the key evidence for covalency in f-element bonds derived from structural, spectroscopic and theor. studies of some selected classes of mols., including octahedral hexahalides, linear actinyl and organometallic sandwich complexes. This evidence is supplemented by a discussion of covalency, including the possibility of both overlap and near-degeneracy driven covalency and the need to quantify their relative contributions in actinide metal-ligand bonds.
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      Kelley, M. P.; Su, J.; Urban, M.; Luckey, M.; Batista, E. R.; Yang, P.; Shafer, J. C. On the Origin of Covalent Bonding in Heavy Actinides. J. Am. Chem. Soc. 2017, 139 (29), 9901– 9908,  DOI: 10.1021/jacs.7b03251
      7
      On the Origin of Covalent Bonding in Heavy Actinides
      Kelley, Morgan P.; Su, Jing; Urban, Matthew; Luckey, Morgan; Batista, Enrique R.; Yang, Ping; Shafer, Jenifer C.
      Journal of the American Chemical Society (2017), 139 (29), 9901-9908CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Recent reports suggested the late actinides participate in more covalent interactions than the earlier actinides, yet the origin of this shift in chem. is not understood. This report considers the chem. of actinide dipicolinate complexes to identify why covalent interactions become more prominent for heavy actinides. A modest increase in measured actinide:dipicolinate stability consts. is coincident with a significant increase in An 5f energy degeneracy with the dipicolinate MOs for Bk and Cf relative to Am and Cm. While the interactions in the actinide-dipicolinate complex are largely ionic, the decrease in 5f orbital energy across the series manifests in orbital-mixing and, hence, covalency driven by energy degeneracy. This observation suggests the origin of covalency in heavy actinide interactions stems from the degeneracy of 5f orbitals with ligand MOs rather than spatial orbital overlap. Probably the limiting radial extension of the 5f orbitals later in the actinide series could make the heavy actinides ideal elements to probe and tune effects of energy degeneracy driven covalency.
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      Cross, J. N.; Su, J.; Batista, E. R.; Cary, S. K.; Evans, W. J.; Kozimor, S. A.; Mocko, V.; Scott, B. L.; Stein, B. W.; Windorff, C. J. Covalency in Americium(III) Hexachloride. J. Am. Chem. Soc. 2017, 139 (25), 8667– 8677,  DOI: 10.1021/jacs.7b03755
      8
      Covalency in Americium(III) Hexachloride
      Cross, Justin N.; Su, Jing; Batista, Enrique R.; Cary, Samantha K.; Evans, William J.; Kozimor, Stosh A.; Mocko, Veronika; Scott, Brian L.; Stein, Benjamin W.; Windorff, Cory J.; Yang, Ping
      Journal of the American Chemical Society (2017), 139 (25), 8667-8677CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Developing a better understanding of covalency (or orbital mixing) is of fundamental importance. Covalency occupies a central role in directing chem. and phys. properties for almost any given compd. or material. Hence, the concept of covalency has potential to generate broad and substantial scientific advances, ranging from biol. applications to condensed matter physics. Given the importance orbital mixing combined with the difficultly in measuring covalency, estg. or inferring covalency often leads to fiery debate. Consider the 60-yr controversy sparked by Seaborg and co-workers (1954) when it was proposed that covalency from 5f-orbitals contributed to the unique behavior of americium in chloride matrixes. Herein, we describe the use of ligand K-edge X-ray absorption spectroscopy (XAS) and electronic structure calcns. to quantify the extent of covalent bonding in - arguably - one of the most difficult systems to study, the Am-Cl interaction within AmCl63-. We obsd. both 5f- and 6d-orbital mixing with the Cl-3p orbitals; however, contributions from the 6d-orbitals were more substantial. Comparisons with the isoelectronic EuCl63- indicated similar bonding for the AmIII 6d- and 5d-orbitals. Meanwhile, the results confirmed Seaborg's 1954 hypothesis that AmIII 5f-orbital covalency was more substantial than 4f-orbital mixing for EuIII.
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    9. 9
      Pace, K. A.; Klepov, V. V.; Berseneva, A. A.; zur Loye, H.-C. Covalency in Actinide Compounds. Chemistry – A European Journal 2021, 27 (19), 5835– 5841,  DOI: 10.1002/chem.202004632
      There is no corresponding record for this reference.
    10. 10
      Galley, S. S.; Pattenaude, S. A.; Gaggioli, C. A.; Qiao, Y.; Sperling, J. M.; Zeller, M.; Pakhira, S.; Mendoza-Cortes, J. L.; Schelter, E. J.; Albrecht-Schmitt, T. E. Synthesis and Characterization of Tris-chelate Complexes for Understanding f-Orbital Bonding in Later Actinides. J. Am. Chem. Soc. 2019, 141 (6), 2356– 2366,  DOI: 10.1021/jacs.8b10251
      10
      Synthesis and Characterization of Tris-chelate Complexes for Understanding f-Orbital Bonding in Later Actinides
      Galley, Shane S.; Pattenaude, Scott A.; Gaggioli, Carlo Alberto; Qiao, Yusen; Sperling, Joseph M.; Zeller, Matthias; Pakhira, Srimanta; Mendoza-Cortes, Jose L.; Schelter, Eric J.; Albrecht-Schmitt, Thomas E.; Gagliardi, Laura; Bart, Suzanne C.
      Journal of the American Chemical Society (2019), 141 (6), 2356-2366CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      An isostructural family of f-element compds. (Ce, Nd, Sm, Gd; Am, Bk, Cf) of the redox-active dioxophenoxazine ligand (DOPOq; DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate) was prepd. This family, of the form M(DOPOq)3, represents the first nonaq. isostructural series, including the later actinides berkelium and californium. The lanthanide derivs. were fully characterized using 1H NMR spectroscopy and SQUID magnetometry, while all species were structurally characterized by x-ray crystallog. and electronic absorption spectroscopy. In order to probe the electronic structure of this new family, CASSCF calcns. were performed and revealed these systems to be largely ionic in contrast to previous studies, where berkelium and californium typically have a small degree of covalent character. To validate the zeroth order regular approxn. (ZORA) method, the same CASSCF anal. using exptl. structures vs. UDFT-ZORA optimized structures does not exhibit sizable changes in bonding patterns. UDFT-ZORA combined with CASSCF could be a useful first approxn. to predict and investigate the structure and electronic properties of actinides and lanthanides that are difficult to synthesize or characterize.
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    11. 11
      Jiang, S.; Liu, Y.; Wang, L.; Chai, Z.; Shi, W.-Q. The Coordination Chemistry of f-Block Elements in Molten Salts. Chemistry – A European Journal 2022, 28 (60), e202201145  DOI: 10.1002/chem.202201145
      There is no corresponding record for this reference.
    12. 12
      Smith, A. L. Structure-property Relationships in Actinide Containing Molten Salts – A Review: Understanding and Modelling the Chemistry of Nuclear Fuel Salts. J. Mol. Liq. 2022, 360, 119426  DOI: 10.1016/j.molliq.2022.119426
      There is no corresponding record for this reference.
    13. 13
      Hombourger, B.; Křepel, J.; Pautz, A. Breed-and-burn Fuel Cycle in Molten Salt Reactors. EPJ. Nuclear Sciences & Technologies 2019, 5, 15,  DOI: 10.1051/epjn/2019026
      There is no corresponding record for this reference.
    14. 14
      Okamoto, Y.; Kobayashi, F.; Ogawa, T. Structure and Dynamic Properties of Molten Uranium Trichloride. J. Alloys Compd. 1998, 271–273, 355– 358,  DOI: 10.1016/S0925-8388(98)00087-5
      14
      Structure and dynamic properties of molten uranium trichloride
      Okamoto, Yoshihiro; Kobayashi, Fumiaki; Ogawa, Toru
      Journal of Alloys and Compounds (1998), 271-273 (), 355-358CODEN: JALCEU; ISSN:0925-8388. (Elsevier Science S.A.)
      High temp. X-ray diffraction anal. was performed to study structure of molten UCl3. The nearest neighbor distance and coordination no. of U-Cl pair were 0.284 nm and 6, resp. Short range structure of molten UCl3 is described as octahedral coordination where six Cl- ions surrounds U3+ ion. The obtained structural data were analyzed by using a Debye scattering equation and a mol. dynamics (MD) simulation. Dynamic properties such as shear viscosity and elec. cond. calcd. from structurally optimized MD simulation were compared with the exptl. data in the literature.
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    15. 15
      Okamoto, Y.; Madden, P. A.; Minato, K. X-ray Diffraction and Molecular Dynamics Simulation Studies of Molten Uranium Chloride. J. Nucl. Mater. 2005, 344 (1–3), 109– 114,  DOI: 10.1016/j.jnucmat.2005.04.026
      15
      X-ray diffraction and molecular dynamics simulation studies of molten uranium chloride
      Okamoto, Y.; Madden, P. A.; Minato, K.
      Journal of Nuclear Materials (2005), 344 (1-3), 109-114CODEN: JNUMAM; ISSN:0022-3115. (Elsevier B.V.)
      The structure of molten UCl3 at 1200 K was studied by high-temp. X-ray diffraction and mol. dynamics simulation. The XRD data was reproduced by the simulation with a polarizable ionic model. The nearest U3+-Cl- distance was 2.85 Å with coordination no. 8.0, implying that the 8-fold structure (UCl8)5- is predominant in the melt - in contradiction to earlier suggestions of octahedral coordination. The potential model, which had been optimized by comparison with the structural data, was also found to reproduce the exptl. information on transport properties like the diffusion coeff., elec. cond. and shear viscosity.
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    16. 16
      Adya, A. K.; Matsuura, H.; Takagi, R.; Rycerz, L.; Gaune-Escard, M. Structural and Thermodynamic Properties of Molten UCl3 and UCl3-MCl (M = Li, Na, K, and Cs) Systems. ECS Proceedings 1999, 1999–41 (1), 341,  DOI: 10.1149/199941.0341PV
      There is no corresponding record for this reference.
    17. 17
      Neilson, G. W.; Adya, A. K.; Ansell, S. 8 Neutron and X-ray Diffraction Studies on Complex Liquids. Annual Reports Section “C” (Physical Chemistry) 2002, 98 (0), 271– 322,  DOI: 10.1039/b111168j
      There is no corresponding record for this reference.
    18. 18
      Yu, X.; Sergentu, D.-C.; Feng, R.; Autschbach, J. Covalency of Trivalent Actinide Ions with Different Donor Ligands: Do Density Functional and Multiconfigurational Wavefunction Calculations Corroborate the Observed “Breaks”?. Inorg. Chem. 2021, 60 (23), 17744– 17757,  DOI: 10.1021/acs.inorgchem.1c02374
      18
      Covalency of Trivalent Actinide Ions with Different Donor Ligands: Do Density Functional and Multiconfigurational Wavefunction Calculations Corroborate the Observed "Breaks"?
      Yu, Xiaojuan; Sergentu, Dumitru-Claudiu; Feng, Rulin; Autschbach, Jochen
      Inorganic Chemistry (2021), 60 (23), 17744-17757CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A comprehensive ab initio study of periodic actinide-ligand bonding trends for trivalent actinides is performed. Relativistic d. functional theory (DFT) and complete active-space (CAS) SCF wavefunction calcns. are used to dissect the chem. bonding in the [AnCl6]3-, [An(CN)6]3-, [An(NCS)6]3-, [An(S2PMe2)3], [An(DPA)3]3-, and [An(HOPO)]- series of actinide (An = U-Es) complexes. Except for some differences for the early actinide complexes with DPA, bond orders and excess 5f-shell populations from donation bonding show qual. similar trends in 5fn active-space CAS vs DFT calcns. The influence of spin-orbit coupling on donation bonding is small for the tested systems. Along the actinide series, chem. soft vs chem. harder ligands exhibit clear differences in bonding trends. There are pronounced changes in the 5f populations when moving from Pu to Am or Cm, which correlate with previously noted "breaks" in chem. trends. Bonding involving 5f becomes very weak beyond Cm/Bk. We propose that Cm(III) is a borderline case among the trivalent actinides that can be meaningfully considered to be involved in ground-state 5f covalent bonding.
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    19. 19
      Butorin, S. M.; Modin, A.; Vegelius, J. R.; Kvashnina, K. O.; Shuh, D. K. Probing Chemical Bonding in Uranium Dioxide by Means of High-Resolution X-ray Absorption Spectroscopy. J. Phys. Chem. C 2016, 120 (51), 29397– 29404,  DOI: 10.1021/acs.jpcc.6b09335
      19
      Probing Chemical Bonding in Uranium Dioxide by Means of High-Resolution X-ray Absorption Spectroscopy
      Butorin, Sergei M.; Modin, Anders; Vegelius, Johan R.; Kvashnina, Kristina O.; Shuh, David K.
      Journal of Physical Chemistry C (2016), 120 (51), 29397-29404CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      A systematic x-ray absorption study at the U 3d, 4d, and 4f edges of UO2 was performed, and the data were analyzed within framework of the Anderson impurity model. By applying the high-energy-resoln. fluorescence-detection (HERFD) mode of x-ray absorption spectroscopy (XAS) at the U 3d3/2 edge and conducting the XAS measurements at the shallower U 4f levels, fine details of the XAS spectra were resolved resulting from reduced core-hole lifetime broadening. This multiedge study enabled a far more effective anal. of the electronic structure at the U sites and characterization of the chem. bonding and degree of the 5f localization in UO2. The results support the covalent character of UO2 and do not agree with the suggestions of rather ionic bonding in this compd. as expressed in some publications.
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    20. 20
      Amidani, L.; Retegan, M.; Volkova, A.; Popa, K.; Martin, P. M.; Kvashnina, K. O. Probing the Local Coordination of Hexavalent Uranium and the Splitting of 5f Orbitals Induced by Chemical Bonding. Inorg. Chem. 2021, 60 (21), 16286– 16293,  DOI: 10.1021/acs.inorgchem.1c02107
      20
      Probing the Local Coordination of Hexavalent Uranium and the Splitting of 5f Orbitals Induced by Chemical Bonding
      Amidani, Lucia; Retegan, Marius; Volkova, Anna; Popa, Karin; Martin, Philippe M.; Kvashnina, Kristina O.
      Inorganic Chemistry (2021), 60 (21), 16286-16293CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The authors report here a detailed exptl. and theor. investigation of hexavalent uranium in various local configurations with a high-energy-resoln. fluorescence-detected X-ray absorption near-edge structure at the U M4 edge. The authors show the pronounced sensitivity of the technique to the arrangement of atoms around the absorber and provide a detailed theor. interpretation revealing the nature of spectral features. Calcns. based on d. functional theory and on crystal field multiplet theory indicate that for all local configurations analyzed, the main peak corresponds to nonbonding 5f orbitals, and the highest energy peak corresponds to antibonding 5f orbitals. The authors' findings agree with the accepted interpretation of uranyl spectral features and embed the latter in a broader field of view, which interprets the spectra of a large variety of U6+-contg. samples on a common theor. ground.
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    21. 21
      Dai, S.; Toth, L. M.; Del Cul, G. D.; Metcalf, D. H. Near-IR absorption transition of 3H4 to 3F2 for UCl62– complex in M2ZrCl6 host crystals (M = K+, Rb+, and Cs+): an experimental and theoretical study. Chem. Phys. 1995, 200 (3), 271– 279,  DOI: 10.1016/0301-0104(95)00228-6
      There is no corresponding record for this reference.
    22. 22
      Angell, C. A.; Gruen, D. M. Short-Range Order in Fused Salts. I. Coordination States of Nickel(II) in Molten Zinc Chloride-Potassium Chloride Mixtures1. J. Phys. Chem. 1966, 70 (5), 1601– 1609,  DOI: 10.1021/j100877a045
      There is no corresponding record for this reference.
    23. 23
      Roy, S.; Sharma, S.; Karunaratne, W. V.; Wu, F.; Gakhar, R.; Maltsev, D. S.; Halstenberg, P.; Abeykoon, M.; Gill, S. K.; Zhang, Y. X-Ray Scattering Reveals Ion Clustering of Dilute Chromium Species in Molten Chloride Medium. Chemical Science 2021, 12 (23), 8026– 8035,  DOI: 10.1039/D1SC01224J
      There is no corresponding record for this reference.
    24. 24
      Xu, L.; Wang, Z.; Chen, J.; Chen, S.; Yang, W.; Ren, Y.; Zuo, X.; Zeng, J.; Wu, Q.; Sheng, H. Folded Network and Structural Transition in Molten Tin. Nat. Commun. 2022, 13 (1), 126,  DOI: 10.1038/s41467-021-27742-2
      There is no corresponding record for this reference.
    25. 25
      Silvi, B.; Savin, A. Classification of Chemical Bonds Based on Topological Analysis of Electron Localization Functions. Nature 1994, 371 (6499), 683– 686,  DOI: 10.1038/371683a0
      25
      Classification of chemical bonds based on topological analysis of electron localization functions
      Silvi, B.; Savin, A.
      Nature (London, United Kingdom) (1994), 371 (6499), 683-6CODEN: NATUAS; ISSN:0028-0836.
      The definitions currently used to classify chem. bonds (in terms of bond order, covalency vs. ionicity and so forth) are derived from approx. theories and are often imprecise. Here we outline a first step towards a more rigorous means of classification based on topol. anal. of local quantum-mech. functions related to the Pauli exclusion principle. The local max. of these functions define localization attractors, of which there are only three basic types: bonding, non-bonding and core. Bonding attractors lie between the core attractors (which themselves surround the at. nuclei) and characterize the shared-electron interactions. The no. of bond attractors is related to the bond multiplicity. The spatial organization of localization attractors provides a basis for a well-defined classification of bonds, allowing an abs. characterization of covalency vs. ionicity to be obtained from observable properties such as electron densities.
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    26. 26
      Glendening, E. D.; Landis, C. R.; Weinhold, F. Natural Bond Orbital Methods. WIREs Computational Molecular Science 2012, 2 (1), 1– 42,  DOI: 10.1002/wcms.51
      There is no corresponding record for this reference.
    27. 27
      Bader, R. F. W. Atoms in Molecules. Acc. Chem. Res. 1985, 18 (1), 9– 15,  DOI: 10.1021/ar00109a003
      27
      Atoms in molecules
      Bader, R. F. W.
      Accounts of Chemical Research (1985), 18 (1), 9-15CODEN: ACHRE4; ISSN:0001-4842.
      A review with 21 refs.
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    28. 28
      Vitova, T.; Pidchenko, I.; Fellhauer, D.; Bagus, P. S.; Joly, Y.; Pruessmann, T.; Bahl, S.; Gonzalez-Robles, E.; Rothe, J.; Altmaier, M. The role of the 5f valence orbitals of early actinides in chemical bonding. Nat. Commun. 2017, 8 (1), 16053  DOI: 10.1038/ncomms16053
      28
      The role of the 5f valence orbitals of early actinides in chemical bonding
      Vitova, T.; Pidchenko, I.; Fellhauer, D.; Bagus, P. S.; Joly, Y.; Pruessmann, T.; Bahl, S.; Gonzalez-Robles, E.; Rothe, J.; Altmaier, M.; Denecke, M. A.; Geckeis, H.
      Nature Communications (2017), 8 (), 16053CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      One of the long standing debates in actinide chem. is the level of localization and participation of the actinide 5f valence orbitals in covalent bonds across the actinide series. Here we illuminate the role of the 5f valence orbitals of uranium, neptunium and plutonium in chem. bonding using advanced spectroscopies: actinide M4,5 HR-XANES and 3d4f RIXS. Results reveal that the 5f orbitals are active in the chem. bonding for uranium and neptunium, shown by significant variations in the level of their localization evidenced in the spectra. In contrast, the 5f orbitals of plutonium appear localized and surprisingly insensitive to different bonding environments. We envisage that this report of using relative energy differences between the 5fδ/φ and 5fπ*/5fσ* orbitals as a qual. measure of overlap-driven actinyl bond covalency will spark activity, and extend to numerous applications of RIXS and HR-XANES to gain new insights into the electronic structures of the actinide elements.
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    29. 29
      Vitova, T.; Kvashnina, K. O.; Nocton, G.; Sukharina, G.; Denecke, M. A.; Butorin, S. M.; Mazzanti, M.; Caciuffo, R.; Soldatov, A.; Behrends, T. High energy resolution x-ray absorption spectroscopy study of uranium in varying valence states. Phys. Rev. B 2010, 82 (23), 235118  DOI: 10.1103/PhysRevB.82.235118
      There is no corresponding record for this reference.
    30. 30
      Kvashnina, K.; Silva, C.; Amidani, L.; Retegan, M.; Bazarkina, E.; Weiss, S.; Graubner, T.; Kraus, F. On the origin of low-valent uranium oxidation state; Research Square Platform LLC: 2024.
      There is no corresponding record for this reference.
    31. 31
      Luzar, A.; Chandler, D. Hydrogen-Bond Kinetics in Liquid Water. Nature 1996, 379 (6560), 55– 57,  DOI: 10.1038/379055a0
      31
      Hydrogen-bond kinetics in liquid water
      Luzar, Alenka; Chandler, David
      Nature (London) (1996), 379 (6560), 55-7CODEN: NATUAS; ISSN:0028-0836. (Macmillan Magazines)
      Hydrogen bonds play a crucial role in the behavior of water; their spatial patterns and fluctuations characterize the structure and dynamics of the liq. The processes of breaking and making hydrogen bonds in the condensed phase can be probed indirectly by a variety of exptl. techniques, and more quant. information can be obtained from computer simulations. In particular, simulations have revealed that on long timescales the relaxation behavior of hydrogen bonds in liq. water exhibit non-exponential kinetics, suggesting that bond making and breaking are not simple processes characterized by well defined rate consts. Here we show that these kinetics can be understood in terms of an interplay between diffusion and hydrogen-bond dynamics. In our model, which can be extended to other hydrogen-bonded liqs., diffusion governs whether a specific pair of water mols. are near neighbors, and hydrogen bonds between such pairs form and persist at random with av. lifetimes detd. by rate consts. for bond making and breaking.
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    32. 32
      Strzelecki, A. C.; Wang, G.; Hickam, S. M.; Parker, S. S.; Batrice, R.; Jackson, J. M.; Conroy, N. A.; Mitchell, J. N.; Andersson, D. A.; Monreal, M. J. In Situ High-Temperature Raman Spectroscopy of UCl3: A Combined Experimental and Theoretical Study. Inorg. Chem. 2023, 62 (45), 18724– 18731,  DOI: 10.1021/acs.inorgchem.3c03139
      There is no corresponding record for this reference.
    33. 33
      Roy, S.; Brehm, M.; Sharma, S.; Wu, F.; Maltsev, D. S.; Halstenberg, P.; Gallington, L. C.; Mahurin, S. M.; Dai, S.; Ivanov, A. S. Unraveling Local Structure of Molten Salts via X-ray Scattering, Raman Spectroscopy, and Ab Initio Molecular Dynamics. J. Phys. Chem. B 2021, 125 (22), 5971– 5982,  DOI: 10.1021/acs.jpcb.1c03786
      33
      Unraveling Local Structure of Molten Salts via X-ray Scattering, Raman Spectroscopy, and Ab Initio Molecular Dynamics
      Roy, Santanu; Brehm, Martin; Sharma, Shobha; Wu, Fei; Maltsev, Dmitry S.; Halstenberg, Phillip; Gallington, Leighanne C.; Mahurin, Shannon M.; Dai, Sheng; Ivanov, Alexander S.; Margulis, Claudio J.; Bryantsev, Vyacheslav S.
      Journal of Physical Chemistry B (2021), 125 (22), 5971-5982CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      In this work, we resolve a long-standing issue concerning the local structure of molten MgCl2 by employing a multimodal approach, including X-ray scattering and Raman spectroscopy, along with the theor. modeling of the exptl. spectra based on ab initio mol. dynamics (AIMD) simulations utilizing several d. functional theory (DFT) methods. We demonstrate the reliability of AIMD simulations in achieving excellent agreement between the exptl. and simulated spectra for MgCl2 and 50 mol % MgCl2 + 50 mol % KCl, and ZnCl2, thus allowing structural insights not directly available from expt. alone. A thorough computational anal. using five DFT methods provides a convergent view that octahedrally coordinated magnesium in pure MgCl2 upon melting preferentially coordinates with five chloride anions to form distorted square pyramidal polyhedra that are connected via corners and to a lesser degree via edges. This is contrasted with the results for ZnCl2, which does not change its tetrahedral coordination on melting. Although the five-coordinate MgCl53- complex was not considered in the early literature, together with an increasing tendency to form a tetrahedrally coordinated complex with decreasing the MgCl2 content in the mixt. with alkali metal chloride systems, current work reconciles the results of most previous seemingly contradictory exptl. studies.
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    34. 34
      Emerson, M. S.; Sharma, S.; Roy, S.; Bryantsev, V. S.; Ivanov, A. S.; Gakhar, R.; Woods, M. E.; Gallington, L. C.; Dai, S.; Maltsev, D. S. Complete Description of the LaCl3–NaCl Melt Structure and the Concept of a Spacer Salt That Causes Structural Heterogeneity. J. Am. Chem. Soc. 2022, 144 (47), 21751– 21762,  DOI: 10.1021/jacs.2c09987
      There is no corresponding record for this reference.
    35. 35
      Cantat, T.; Graves, C. R.; Jantunen, K. C.; Burns, C. J.; Scott, B. L.; Schelter, E. J.; Morris, D. E.; Hay, P. J.; Kiplinger, J. L. Evidence for the Involvement of 5f Orbitals in the Bonding and Reactivity of Organometallic Actinide Compounds: Thorium(IV) and Uranium(IV) Bis(hydrazonato) Complexes. J. Am. Chem. Soc. 2008, 130 (51), 17537– 17551,  DOI: 10.1021/ja8067287
      35
      Evidence for the Involvement of 5f Orbitals in the Bonding and Reactivity of Organometallic Actinide Compounds: Thorium(IV) and Uranium(IV) Bis(hydrazonato) Complexes
      Cantat, Thibault; Graves, Christopher R.; Jantunen, Kimberly C.; Burns, Carol J.; Scott, Brian L.; Schelter, Eric J.; Morris, David E.; Hay, P. Jeffrey; Kiplinger, Jaqueline L.
      Journal of the American Chemical Society (2008), 130 (51), 17537-17551CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Migratory insertion of diphenyldiazomethane into both metal-C bonds of the bis(alkyl) and bis(aryl) complexes (C5Me5)2AnR2 yields the 1st f-element bis(hydrazonato) complexes (C5Me5)2An[η2-(N,N')-R-N-N:CPh2]2 [An = Th, R = CH3 (18), PhCH2 (15), Ph (16); An = U, R = CH3 (17), PhCH2 (14)], which were characterized by a combination of spectroscopy, electrochem., and x-ray crystallog. The two hydrazonato ligands adopt an η2-coordination mode leading to 20-electron (for Th) and 22-electron (for U) complexes that have no transition-metal analogs. In fact, reaction of (C5H5)2ZrMe2 or (C5Me5)2HfMe2 with diphenyldiazomethane is limited to the formation of the corresponding mono(hydrazonato) complex (C5R5)2M[η2-(N,N')-CH3-N-N:CPh2]Me (M = Zr, R = H or M = Hf, R = CH3). The difference in the reactivities of the Group 4 metal complexes and the actinides was used as a unique platform for studying in depth the role of 5f orbitals on the reactivity and bonding in actinide organometallic complexes. The electronic structure of the (C5H5)2M[η2-(N,N')-CH3-N-N:CH2]2 (M = Zr, Th, U) model complexes was studied using d. functional theory (DFT) calcns. and compared to exptl. structural, electrochem., and spectroscopic results. Whereas transition-metal bis(cyclopentadienyl) complexes are known to stabilize three ligands in the metallocene girdle to form satd. (C5H5)2ML3 species, in a bis(hydrazonato) system, a 4th ligand is coordinated to the metal center to give (C5H5)2ML4. DFT calcns. showed that 5f orbitals in the actinide complexes play a crucial role in stabilizing this 4th ligand by stabilizing both the σ and π electrons of the two η2-coordinated hydrazonato ligands. In contrast, the stabilization of the hydrazonato ligands is significantly less effective for the putative bis(hydrazonato) Zr(IV) complex, yielding a higher energy structure. However, the difference in the reactivities of the Group 4 metal and actinide complexes does not arise on thermodn. grounds but is primarily of kinetic origin. Unfavorable steric factors were ruled out as the sole influence to explain these different behaviors, and electronic factors govern the reactivity. For the actinides, both the C5H5 and more realistic C5Me5 ligands were taken into account in computing the energy surface. The reaction profile for the C5Me5 system differs from that with the C5H5 ligand by a uniform shift of ∼5 kcal/mol in the relative energies of the transition state and products. The insertion of a 2nd diazoalkane mol. into the sole metal-C bond in the mono(hydrazonato) complexes involves a high energy barrier (∼20 kcal/mol) for the Zr(IV) system, whereas the actinides can facilitate the approach of the diazoalkane by coordination (formation of an adduct) and its insertion into the An-C bond with a very low barrier on the potential energy surface.
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    36. 36
      Yang, P.; Warnke, I.; Martin, R. L.; Hay, P. J. Theoretical Studies of the sp2 versus sp3 C–H Bond Activation Chemistry of 2-Picoline by (C5Me5)2An(CH3)2 Complexes (An = Th, U). Organometallics 2008, 27 (7), 1384– 1392,  DOI: 10.1021/om700927n
      36
      Theoretical Studies of the sp2 versus sp3 C-H Bond Activation Chemistry of 2-Picoline by (C5Me5)2An(CH3)2 Complexes (An = Th, U)
      Yang, Ping; Warnke, Ingolf; Martin, Richard L.; Hay, P. Jeffrey
      Organometallics (2008), 27 (7), 1384-1392CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      The C-H bond activation chem. of (C5Me5)2ThMe2 and (C5Me5)2UMe2 with 2-picoline (2-methylpyridine) was examd. using d. functional techniques. In particular, the differences between insertion into the ortho ring sp2 C-H bond and the Me sp3 C-H bond are explored. The energies to form the η2-(N,C)-pyridyl products resulting from the activation of the arom. ring sp2 C-H bond are calcd. as thermodn. products for both Th and U systems with similar reaction energies of -15.8 kcal/mol. The products corresponding to insertion into the Me sp3 C-H bond are higher in energy by 3.5 and 5.4 kcal/mol for Th and U, resp. In the transition states the actinide atom mediates the H migration from 2-picoline to the leaving Me group by forming an agostic five-centered C-H complex. The relative activation energies between sp2 and sp3 C-H bond activation differ slightly between Th, ΔE⧺(sp2) > ΔE⧺(sp3) and U, ΔE⧺(sp2) < ΔE⧺(sp3). These results are in agreement with the exptl. observations that the sp2 insertion product is the thermodn. product in both cases, but that the sp3 insertion product is the kinetic product in the case of Th.
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    37. 37
      Su, J.; Batista, E. R.; Boland, K. S.; Bone, S. E.; Bradley, J. A.; Cary, S. K.; Clark, D. L.; Conradson, S. D.; Ditter, A. S.; Kaltsoyannis, N. Energy-Degeneracy-Driven Covalency in Actinide Bonding. J. Am. Chem. Soc. 2018, 140 (51), 17977– 17984,  DOI: 10.1021/jacs.8b09436
      37
      Energy-Degeneracy-Driven Covalency in Actinide Bonding
      Su, Jing; Batista, Enrique R.; Boland, Kevin S.; Bone, Sharon E.; Bradley, Joseph A.; Cary, Samantha K.; Clark, David L.; Conradson, Steven D.; Ditter, Alex S.; Kaltsoyannis, Nikolas; Keith, Jason M.; Kerridge, Andrew; Kozimor, Stosh A.; Loble, Matthias W.; Martin, Richard L.; Minasian, Stefan G.; Mocko, Veronika; La Pierre, Henry S.; Seidler, Gerald T.; Shuh, David K.; Wilkerson, Marianne P.; Wolfsberg, Laura E.; Yang, Ping
      Journal of the American Chemical Society (2018), 140 (51), 17977-17984CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Evaluating the nature of chem. bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic d. functional theory study that quant. evaluates An-Cl covalency in AnCl62- (AnIV = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and AnIV 5f- and 6d-orbitals (t1u*/t2u* and t2g*/eg*), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals. Moving from Th to U, Np, and Pu markedly changed the amt. of M-Cl orbital mixing, such that AnIV 6d- and Cl 3p-mixing decreased and metal 5f- and Cl 3p-orbital mixing increased across this series.
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    38. 38
      Formanuik, A.; Ariciu, A.-M.; Ortu, F.; Beekmeyer, R.; Kerridge, A.; Tuna, F.; McInnes, E. J. L.; Mills, D. P. Actinide Covalency Measured by Pulsed Electron Paramagnetic Resonance Spectroscopy. Nat. Chem. 2017, 9 (6), 578– 583,  DOI: 10.1038/nchem.2692
      38
      Actinide covalency measured by pulsed electron paramagnetic resonance spectroscopy
      Formanuik, Alasdair; Ariciu, Ana-Maria; Ortu, Fabrizio; Beekmeyer, Reece; Kerridge, Andrew; Tuna, Floriana; McInnes, Eric J. L.; Mills, David P.
      Nature Chemistry (2017), 9 (6), 578-583CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Our knowledge of actinide chem. bonds lags far behind our understanding of the bonding regimes of any other series of elements. This is a major issue given the technol. as well as fundamental importance of f-block elements. Some key chem. differences between actinides and lanthanides-and between different actinides-can be ascribed to minor differences in covalency, i.e., the degree to which electrons are shared between the f-block element and coordinated ligands. Yet there are almost no direct measures of such covalency for actinides. Here we report the first pulsed ESR spectra of actinide compds. We apply the hyperfine sublevel correlation technique to quantify the electron-spin d. at ligand nuclei (via the weak hyperfine interactions) in mol. thorium and uranium species and therefore the extent of covalency. Such information will be important in developing our understanding of the chem. bonding, and therefore the reactivity, of actinides.
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    39. 39
      Smiles, D. E.; Wu, G.; Hrobárik, P.; Hayton, T. W. Use of 77Se and 125Te NMR Spectroscopy to Probe Covalency of the Actinide-Chalcogen Bonding in [Th(En){N(SiMe3)2}3]– (E = Se, Te; n = 1, 2) and Their Oxo-Uranium(VI) Congeners. J. Am. Chem. Soc. 2016, 138 (3), 814– 825,  DOI: 10.1021/jacs.5b07767
      39
      Use of 77Se and 125Te NMR Spectroscopy to Probe Covalency of the Actinide-Chalcogen Bonding in [Th(En){N(SiMe3)2}3]- (E = Se, Te; n = 1, 2) and Their Oxo-Uranium(VI) Congeners
      Smiles, Danil E.; Wu, Guang; Hrobarik, Peter; Hayton, Trevor W.
      Journal of the American Chemical Society (2016), 138 (3), 814-825CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Reaction of [Th(I)(NR2)3] (R = SiMe3) (1) with 1 equiv of either [K(18-crown-6)]2[Se4] or [K(18-crown-6)]2[Te2] affords the thorium dichalcogenides, [K(18-crown-6)][Th(η2-E2)(NR2)3] (E = Se, 2; E = Te, 3), resp. Removal of one chalcogen atom via reaction with Et3P, or Et3P and Hg, affords the monoselenide and monotelluride complexes of thorium, [K(18-crown-6)][Th(E)(NR2)3] (E = Se, 4; E = Te, 5), resp. Both 4 and 5 were characterized by X-ray crystallog. and were found to feature the shortest known Th-Se and Th-Te bond distances. The electronic structure and nature of the actinide-chalcogen bonds were investigated with 77Se and 125Te NMR spectroscopy accompanied by detailed quantum-chem. anal. We also recorded the 77Se NMR shift for a U(VI) oxo-selenido complex, [U(O)(Se)(NR2)3]- (δ(77Se) = 4905 ppm), which features the highest frequency 77Se NMR shift yet reported, and expands the known 77Se chem. shift range for diamagnetic substances from ∼3300 ppm to almost 6000 ppm. Both 77Se and 125Te NMR chem. shifts of given chalcogenide ligands were identified as quant. measures of the An-E bond covalency within an isoelectronic series and supported significant 5f-orbital participation in actinide-ligand bonding for uranium(VI) complexes in contrast to those involving thorium(IV). Moreover, X-ray diffraction studies together with NMR spectroscopic data and d. functional theory (DFT) calcns. provide convincing evidence for the actinide-chalcogen multiple bonding in the title complexes. Larger An-E covalency is obsd. in the [U(O)(E)(NR2)3]- series, which decreases as the chalcogen atom becomes heavier.
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    40. 40
      Kaltsoyannis, N. Does Covalency Increase or Decrease across the Actinide Series? Implications for Minor Actinide Partitioning. Inorg. Chem. 2013, 52 (7), 3407– 3413,  DOI: 10.1021/ic3006025
      40
      Does Covalency Increase or Decrease across the Actinide Series? Implications for Minor Actinide Partitioning
      Kaltsoyannis, Nikolas
      Inorganic Chemistry (2013), 52 (7), 3407-3413CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A review. A covalent chem. bond carries the connotation of overlap of AOs between bonded atoms, leading to a buildup of the electron d. in the internuclear region. Stabilization of the valence 5f orbitals as the actinide series is crossed leads, in compds. of the minor actinides americium and curium, to their becoming approx. degenerate with the highest occupied ligand levels and hence to the unusual situation in which the resultant valence MOs have significant contributions from both actinide and the ligand yet in which there is little AO overlap. In such cases, the traditional quantum-chem. tools for assessing the covalency, e.g., population anal. and spin densities, predict significant metal-ligand covalency, although whether this orbital mixing is really covalency in the generally accepted chem. view is an interesting question. This review discusses our recent analyses of the bonding in AnCp3 and AnCp4 (An = Th-Cm; Cp = η5-C5H5) using both the traditional tools and also topol. anal. of the electron d. via the quantum theory of atoms-in-mols. I will show that the two approaches yield rather different conclusions and suggest that care must be taken when using quantum chem. to assess metal-ligand covalency in this part of the periodic table. The implications of this work for minor actinide partitioning from nuclear wastes are discussed; minor actinide extractant ligands based on nitrogen donors have received much attention in recent years, as have comparisons of the extent of covalency in actinide-nitrogen bonding with that in analogous lanthanide systems via quantum-chem. studies employing the traditional tools for assessing the covalency.
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    41. 41
      Kerridge, A. Quantification of f-Element Covalency Through Analysis of the Electron Density: Insights from Simulation. Chem. Commun. 2017, 53 (50), 6685– 6695,  DOI: 10.1039/C7CC00962C
      41
      Quantification of f-element covalency through analysis of the electron density: insights from simulation
      Kerridge, A.
      Chemical Communications (Cambridge, United Kingdom) (2017), 53 (50), 6685-6695CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      The electronic structure of f-element compds. is complex due to a combination of relativistic effects, strong electron correlation and weak crystal field environments. However, a quant. understanding of bonding in these compds. is becoming increasingly technol. relevant. Recently, bonding interpretations based on analyses of the phys. observable electronic d. have gained popularity and, in this Feature Article, the utility of such d.-based approaches is demonstrated. Application of Bader's Quantum Theory of Atoms in Mols. (QTAIM) is shown to elucidate many properties including bonding trends, orbital overlap and energy degeneracy-driven covalency, oxidn. state identification and bond stability, demonstrating the increasingly important role that simulation and anal. play in the area of f-element bond characterization.
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    42. 42
      Berryman, V. E. J.; Whalley, Z. J.; Shephard, J. J.; Ochiai, T.; Price, A. N.; Arnold, P. L.; Parsons, S.; Kaltsoyannis, N. Computational Analysis of M–O Covalency in M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, U). Dalton Transactions 2019, 48 (9), 2939– 2947,  DOI: 10.1039/C8DT05094E
      There is no corresponding record for this reference.
    43. 43
      Sperling, J. M.; Warzecha, E. J.; Celis-Barros, C.; Sergentu, D.-C.; Wang, X.; Klamm, B. E.; Windorff, C. J.; Gaiser, A. N.; White, F. D.; Beery, D. A. Compression of Curium Pyrrolidine-Dithiocarbamate Enhances Covalency. Nature 2020, 583 (7816), 396– 399,  DOI: 10.1038/s41586-020-2479-2
      43
      Compression of curium pyrrolidine-dithiocarbamate enhances covalency
      Sperling, Joseph M.; Warzecha, Evan J.; Celis-Barros, Cristian; Sergentu, Dumitru-Claudiu; Wang, Xiaoyu; Klamm, Bonnie E.; Windorff, Cory J.; Gaiser, Alyssa N.; White, Frankie D.; Beery, Drake A.; Chemey, Alexander T.; Whitefoot, Megan A.; Long, Brian N.; Hanson, Kenneth; Kogerler, Paul; Speldrich, Manfred; Zurek, Eva; Autschbach, Jochen; Albrecht-Schonzart, Thomas E.
      Nature (London, United Kingdom) (2020), 583 (7816), 396-399CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Abstr.: curium is unique in the actinide series because its half-filled 5f 7 shell has lower energy than other 5f n configurations, rendering it both redox-inactive and resistant to forming chem. bonds that engage the 5f shell1-3. This is even more pronounced in gadolinium, curium's lanthanide analog, owing to the contraction of the 4f orbitals with respect to the 5f orbitals4. However, at high pressures metallic curium undergoes a transition from localized to itinerant 5f electrons5. This transition is accompanied by a crystal structure dictated by the magnetic interactions between curium atoms5,6. Therefore, the question arises of whether the frontier metal orbitals in curium(III)-ligand interactions can also be modified by applying pressure, and thus be induced to form metal-ligand bonds with a degree of covalency. Here the authors report exptl. and computational evidence for changes in the relative roles of the 5f/6d orbitals in curium-sulfur bonds in [Cm(pydtc)4]- (pydtc, pyrrolidinedithiocarbamate) at high pressures (up to 11 gigapascals). The authors compare these results to the spectra of [Nd(pydtc)4]- and of a Cm(III) mellitate that possesses only curium-oxygen bonds. Compared with the changes obsd. in the [Cm(pydtc)4]- spectra, smaller changes in the f-f transitions in the [Nd(pydtc)4]- absorption spectrum and in the f-f emission spectrum of the Cm(III) mellitate upon pressurization, which are related to the smaller perturbation of the nature of their bonds. were obsd. These results reveal that the metal orbital contributions to the curium-sulfur bonds are considerably enhanced at high pressures and that the 5f orbital involvement doubles between 0 and 11 gigapascal. The authors' work implies that covalency in actinides is complex even when dealing with the same ion, but it could guide the selection of ligands to study the effect of pressure on actinide compds.
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    44. 44
      Shephard, J. J.; Berryman, V. E. J.; Ochiai, T.; Walter, O.; Price, A. N.; Warren, M. R.; Arnold, P. L.; Kaltsoyannis, N.; Parsons, S. Covalent Bond Shortening and Distortion Induced by Pressurization of Thorium, Uranium, and Neptunium Tetrakis Aryloxides. Nat. Commun. 2022, 13 (1), 5923,  DOI: 10.1038/s41467-022-33459-7
      There is no corresponding record for this reference.
    45. 45
      Shinohara, Y.; Ivanov, A. S.; Maltsev, D.; Granroth, G. E.; Abernathy, D. L.; Dai, S.; Egami, T. Real-Space Local Dynamics of Molten Inorganic Salts Using Van Hove Correlation Function. J. Phys. Chem. Lett. 2022, 13 (25), 5956– 5962,  DOI: 10.1021/acs.jpclett.2c01230
      45
      Real-Space Local Dynamics of Molten Inorganic Salts Using Van Hove Correlation Function
      Shinohara, Yuya; Ivanov, Alexander S.; Maltsev, Dmitry; Granroth, Garrett E.; Abernathy, Douglas L.; Dai, Sheng; Egami, Takeshi
      Journal of Physical Chemistry Letters (2022), 13 (25), 5956-5962CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Molten inorg. salts are attracting resurgent attention because of their unique physicochem. properties, making them promising media for next-generation concg. solar power systems and molten salt reactors. The dynamics of these highly disordered ionic media is largely studied by theor. simulations, while the robust exptl. techniques capable of observing local dynamics are not well-developed. To provide fundamental insights into the at.-scale transport properties of molten salts, we report the real-space dynamics of molten magnesium chloride at high temps. employing the Van Hove correlation function obtained by inelastic neutron scattering. Our results directly depict the distance-dependent dynamics of a molten salt on the picosecond time scale. This study demonstrates the capability of the developed approach to describe the locally correlated- and self-dynamics in molten salts, significantly improving our understanding of the interplay between microscopic structural parameters and their dynamics that ultimately control phys. properties of condensed matter in extreme environments.
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    • Synthesis and characterization of UCl3 and additional experimental and computational details, including neutron scattering measurements, chemical bonding, density of states, and coordination number analyses (PDF)


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