Electron-Nucleus Hyperfine Coupling Calculated from Restricted Active Space Wavefunctions and an Exact Two-Component Hamiltonian.

Exact two-component (X2C) relativistic nuclear hyperfine magnetic field operators were incorporated in X2C ab initio wavefunction calculations at the multireference restricted active space (RAS) level for calculations of nuclear hyperfine magnetic properties. Spin-orbit coupling was treated via RAS state interaction (SO-RASSI). The method was tested by calculations of electron-nucleus hyperfine coupling constants. The approach, implemented in the OpenMolcas program, overcomes a major limitation of a previous SO-RASSI implementation for hyperfine coupling that relied on nonrelativistic hyperfine operators [J. Chem. Theor. Comput. 2015, 11, 538-549] and therefore had limited applicability. Results from calculations on systems with light and heavy main group elements, transition metals, lanthanides, and one actinide complex demonstrate reasonably good agreement with experimental data, where available, as long as the active space can generate sufficient spin polarization.

Ab Initio Study of Covalency in the Ground versus Core-Excited States and X-ray Absorption Spectra of Actinide Complexes.

Relativistic multireference ab initio wave function calculations within the restricted active space (RAS) framework were performed to calculate metal and ligand X-ray absorption (XAS) near-edge spectroscopy (XANES) intensities for the metal M4,5 edges of [PuO2(H2O)5]2+, [AnVIO2]2+ (An = U, Np, Pu), and [AmCl6]3- and the Cl K edge of the Am complex. The extent of An(5f)-ligand bonding was determined via natural localized molecular orbital analyses of the relevant spin-orbit coupled multireference states. The calculated spectra are in good agreement with experiments and allow a detailed assignment of the observed spectral features. The XANES M4,5-edge spectra are representative of the actinide orbital covalency in the probed core-excited states, which may be different from the ground-state covalency. An assignment of ground-state An orbital covalency based on XAS spectra should therefore be made with caution.

A diuranium carbide cluster stabilized inside a C80 fullerene cage.

Unsupported non-bridged uranium-carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing Ih(7)-C80 fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@Ih(7)-C80. This endohedral fullerene was prepared utilizing the Krätschmer-Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (NiII-OEP) to produce UCU@Ih(7)-C80·[NiII-OEP] as single crystals. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium-carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural features of UCU@Ih(7)-C80 and the covalent nature of the U(f1)=C double bonds were further affirmed through various spectroscopic and theoretical analyses.

A Pseudotetrahedral Uranium(V) Complex.

A series of uranium amides were synthesized from N, N, N-cyclohexyl(trimethylsilyl)lithium amide [Li][N(TMS)Cy] and uranium tetrachloride to give U(NCySiMe3) x(Cl)4- x, where x = 2, 3, or 4. The diamide was isolated as a bimetallic, bridging lithium chloride adduct ((UCl2(NCyTMS)2)2-LiCl(THF)2), and the tris(amide) was isolated as the lithium chloride adduct of the monometallic species (UCl(NCyTMS)3-LiCl(THF)2). The tetraamide complex was isolated as the four-coordinate pseudotetrahedron. Cyclic voltammetry revealed an easily accessible reversible oxidation wave, and upon chemical oxidation, the UV amido cation was isolated in near-quantitative yields. The synthesis of this family of compounds allows a direct comparison of the electronic structure and properties of isostructural UIV and UV tetraamide complexes. Spectroscopic investigations consisting of UV-vis, NIR, MCD, EPR, and U L3-edge XANES, along with density functional and wave function calculations, of the four-coordinate UIV and UV complexes have been used to understand the electronic structure of these pseudotetrahedral complexes.

Influence of Substituents on the Electronic Structure of Mono- and Bis(phosphido) Thorium(IV) Complexes.

A series of metallocene thorium complexes with mono- and bis(phosphido) ligands have been investigated with varying hues: (C5Me5)2Th(Cl)[P(Mes)2] (Mes = mesityl = 2,4,6-(CH3)3C6H2; dark red-purple), (C5Me5)2Th[P(Mes)(CH3)]2 (dark red-purple), (C5Me5)2Th(CH3)[P(Mes)2] (dark red-purple), (C5Me5)2Th(CH3)[P(Mes)(SiMe3)] (orange), (C5Me5)2Th(Cl)[P(Mes)(SiMe3)] (orange), (C5Me5)2Th[P(Mes)(SiMe3)]2 (orange), and (C5Me5)2Th[PH(Mes)]2 (pale yellow). While all of these complexes bear a mesityl group on phosphorus, the electronic structure observed differs depending on the other substituent (mesityl, methyl, trimethylsilyl, or hydrogen). This sparked an investigation of the electronic structure of these complexes using 31P NMR and electronic absorption spectroscopy in concert with time-dependent density functional theory calculations.

Calculation of linear and nonlinear optical properties of azobenzene derivatives with Kohn-Sham and coupled-cluster methods.

Linear polarizabilities (α) and second hyperpolarizabilities (γ) of unsubstituted azobenzenes and 'push-pull' azobenzene derivatives are investigated using Kohn-Sham theory (KST) and coupled-cluster (CC) approaches. Various standard exchange-correlation functionals as well as a non-empirically 'tuned' long-range corrected (LC) functional with range-separated exchange are used in the KST calculations. When compared to correlated ab initio calculations and measurements, the tuned functional gives accurate low-energy excitation energies, especially for charge transfer (CT) transitions, and performs well for α. Basis set and solvent effects are also studied. In contrast to expectations, but in agreement with a prior study of π-conjugated systems that do not have low-energy CT excitations, the improvements of the CT excitation energies for the push-pull π-chromophores due to tuning do not go along with clear improvements of γ toward the CC reference data, likely due to the importance of the dynamic electron correlation for this property.

Assessment of Tuned Range Separated Exchange Functionals for Spectroscopies and Properties of Uranium Complexes.

The Kohn-Sham delocalization error (DE) is quantified in select uranium compounds for various functionals and shown to correlate with the magnitude of dative ligand donation into the 5f shell. Range separated exchange functionals are reparametrized to minimize the DE and analyzed for their spectroscopic predictive capabilities. Valence excitation spectra of occupied 5f systems exhibit noticeable improvement upon reparametrization, e.g. UCl6-, UCl62-, and UO2+. Less sensitivity to the reparameterization was observed for closed shell 5f systems and core excitation spectra. A general parametrization is proposed to perform well for valence excitation spectra with small DE.

Magnetic circular dichroism of UCl6- in the ligand-to-metal charge-transfer spectral region.

We present a combined ab initio theoretical and experimental study of the magnetic circular dichroism (MCD) spectrum of the octahedral UCl6- complex ion in the UV-Vis spectral region. The ground state is an orbitally non-degenerate doublet E5/2u and the MCD is a -term spectrum caused by spin-orbit coupling. Calculations of the electronic spectrum at various levels of theory indicate that differential dynamic electron correlation has a strong influence on the energies of the dipole-allowed transitions and the envelope of the MCD spectrum. The experimentally observed bands are assigned to dipole-allowed ligand-to-metal charge transfer into the 5f shell, and 5f to 6d transitions. Charge transfer excitations into the U 6d shell appear at much higher energies. The MCD-allowed transitions can be assigned via their signs of the -terms: Under Oh double group symmetry, E5/2u → E5/2g transitions have negative -terms whereas E5/2u → F3/2g transitions have positive -terms if the ground state g-factor is negative, as it is the case for UCl6-.

Impact of the Kohn-Sham Delocalization Error on the 4f Shell Localization and Population in Lanthanide Complexes.

The extent of ligand to metal donation bonding and mixing of 4f (and 5d) orbitals with ligand orbitals is studied by Kohn-Sham (KS) calculations for LaX3 (X = F, Cl, Br, I), GdX3, and LuX3 model complexes, CeCl6(2-), YbCp3, and selected lanthanide complexes with larger ligands. The KS delocalization error (DE) is quantified via the curvature of the energy for noninteger electron numbers. The extent of donation bonding and 4f-ligand mixing correlates well with the DE. For Lu complexes, the DE also correlates with the extent of mixing of ligand and 4f orbitals in the canonical molecular orbitals (MOs). However, the localized set of MOs and population analyses indicate that the closed 4f shell is localized. Attempts to create situations where mixing of 4f and ligand orbitals occurs due to a degeneracy of fragment orbitals were unsuccessful. For La(III) and, in particular, for Ce(IV), Hartree-Fock, KS, and coupled cluster singles and doubles calculations are in agreement in that excess 4f populations arise from ligand donation, along with donation into the 5d shell. Likewise, KS calculations for all systems with incompletely filled 4f shells, even those with "optimally tuned" functionals affording a small DE, produce varying degrees of excess 4f populations which may be only partially attributed to 5d polarization.