There is a need to better understand the nature of compounds containing the actinides to aid in the development of new nuclear reactors for efficient energy generation and for cleaning up the environment at nuclear weapons production sites. In addition, the actinides present a less well-understood domain of chemistry and the presence of 5f electrons makes the chemistry significantly different from the rest of the Periodic Table. We will describe how computational electronic structure theory based on correlated molecular orbital theory plays an important role in developing an understanding of bonding in the actinides and the difficulties in employing such methods for the reliable prediction of the properties of the actinides, for example, including the role of relativistic effects. The properties of the AnH as neutrals, cations, and anions for Ac to Pu will be described including the redox properties of these simple actinides and whether the early actinides are behaving like transition metals of actinides. The properties of UB, UC, UN, UO, and UF as neutrals, cations, and anion will be described, again with a focus on the nature of the bonding and the redox properties of the actinide. We will also describe the properties of diatomic molecules containing Ac, Th, and Pa and where actinide behavior begins. This work is supported by the U.S. DOE Office of Science (BES) under the Heavy Element Program, Grant No. DE-SC0018921.

I present benchmark relativistic coupled-cluster calculations of vibrational frequencies for uranium-containing molecules. The exact two-component (X2C) approach has been used to provide accurate treatments of relativistic effects. X2C coupled-cluster singles and doubles augmented with a non-iterative triples correction [CCSD(T)] calculations are shown to provide promising results but still exhibit substantial deviation from measured values in matrix-isolation infrared spectroscopy studies in the literature. High-level correlation contributions, those beyond CCSD(T), as well as matrix shifts are investigated aiming at a quantitative comparison with experimental measurements.

The isoelectronic molecules UN and UO⁺ are known to have Ω=3.5 and Ω=4.5 ground states, respectively (where Ω is the unsigned projection of the electronic angular momentum along the internuclear axis). A ligand field theory model has been proposed to account for the difference (Matthew and Morse, J. Chem. Phys. 138, 184303 (2013)). The ground state of UO⁺ arises from the U³⁺(5f³(⁴I_4.5))O²⁻ configuration. Owing to the higher nominal charge of the N³⁻ ligand, the U³⁺ ion in UN is stabilized by promoting one of the 5f electrons to the more polarizable 7s orbital, reducing the repulsive interaction with the ligand and rendering U³⁺(5f²7s(⁴H_3.5))N³⁻ the lowest energy configuration. In the present work we have advanced the characterization of the UN ground state through studies of two electronic transitions, [18.35]4.5-X(1)3.5 and [18.63]4.5-X(1)3.5, using sub-Doppler laser excitation techniques with fluorescence detection. Spectra were recorded under field-free conditions, and in the presence of static electric or magnetic fields. The ground state electric dipole moment (Ω=4.30(2) D) and magnetic g-factor (2.160(9)) were determined from these data. These values were both consistent with the 5f²7s configurational assignment. Dispersed fluorescence measurements were used to determine vibrational constants for the ground and first electronically excited states. Electric dipole moments and magnetic g-factors are also reported for the higher energy electronically excited states.

We present a relativistic equation-of-motion coupled-cluster with single and double excitations formalism within the exact-two-component framework (X2C-EOM-CCSD), where both scalar relativistic effects and spin-orbit coupling are variationally included at the reference level. Three different molecular mean-field treatments of relativistic corrections, including the one-electron, Dirac-Coulomb, and Dirac-Coulomb-Breit Hamiltonian, are considered in this work. Benchmark calculations include atomic excitations and fine-structure splittings arising from spin-orbit coupling. Comparison with experimental values and relativistic time-dependent density functional theory is also carried out. The computation of the oscillator strength using the relativistic X2C-EOM-CCSD approach allows for studies of spin-orbit-driven processes such as the spontaneous phosphorescence lifetime. We also demonstrate applications of X2C-EOM-CCSD method in simulating X-ray absorption spectra of heavy-element-containing molecules. The implementation supports massively parallel computation on supercomputers with tens of thousands of CPU cores.

The measured isomer shifts in Mössbauer spectra, reflecting the electronic densities at nuclear sites, provide information on the chemical environments at atomic precision. In this study, the contact and effective densities of systems containing heavy elements have been calculated with the recently developed Cholesky decomposition-based relativistic coupled-cluster (CC) methods with analytical energy derivatives. The combination of exact two-component theory with atomic-mean-field spin-orbit integrals and CC methods enables accurate treatment of both electron correlation and relativistic effects. The contributions of electron-correlation, relativistic, and environmental effects to the calculated electronic densities are systematically investigated for gold-containing compounds.

Lanthanide-bearing molecules are promising platforms to search for new physics beyond the Standard Model. Ytterbium, the heaviest f-block lanthanide, has a closed-shell 4f¹⁴6s² valence electron configuration and forms ionic metal-ligand complexes whose electronic structure is similar to that of alkaline earth metals, M, with valence ns² configurations. Like many other monovalent M⁺L⁻ molecules, Yb⁺L⁻ compounds have a quasi-atomic, metal-centered unpaired σ electron that is amenable to efficient optical photon cycling - a requirement for laser cooling and magneto-optical trapping techniques, which provide important sensitivity advantages for precision molecular spectroscopy experiments. These electronic properties in Yb-bearing molecules are, however, profoundly influenced by the presence of low-lying excited electronic states arising from the promotion of a 4f electron to the metal-centered σ orbital, an effect absent in alkaline earth metals. Such f-hole states not only open new loss pathways detrimental for photon cycling, but also modify the electronic character of the nominal Yb⁺[(4f)¹⁴6s] ground state via configuration interactions and second-order spin-orbit coupling. In this talk, we present high-resolution cavity Fourier transform microwave measurements of the rotational spectra of 11 isotopologues of YbCCH. Using the well-determined rotation, spin-rotation, and nuclear hyperfine constants of this extensive isotopic catalog, we have derived the precise equilibrium structure, unpaired spin distribution, and other electronic properties in the ²Σ⁺ electronic ground state. We investigate how these properties are influenced by ligand substitution and inner-shell 4f electrons by direct comparisons with other monovalent Yb-ligand molecules (YbOH and YbF) and the alkaline earth analogues (MgCCH, CaCCH, and SrCCH). These results elucidate general principles for chemically tuning the electronic structure and properties of laser-coolable Yb-bearing molecules.

Rotational spectra were recorded on a chirp-pulsed Fourier transform microwave (CP-FTMW) spectrometer with laser ablation sourcing at the Missouri University of Science and Technology. A thorium rod was struck by a Nd:YAG laser outputting the 1064 nm fundamental to generate a plasma, utilizing a carbonyl sulfide (OCS) in an argon carrier gas. The spectra of ThS and OThS were collected and assigned in the 8 to 18 GHz region of the electromagnetic spectrum. Rotational constants of OThS have been determined, yielding initial values of A = 13555.4230 MHz, B = 2633.1716 MHz, and C = 2200.00178 MHz. Structural determinations, their comparisons to theoretical valuesHuang, T., Wang, Q., Yu, W., Wang, X., & Andrews, L. (2018). OMS, OM(η²-SO), and OM(η²-SO)(η²-O₂S) Molecules (M = Ce, Th) with Chiral Structure: Matrix Infrared Spectra and Theoretical Calculations. The Journal of Physical Chemistry. A, 122(24), 5391–5400. https://doi.org/10.1021/acs.jpca.8b03731 and an expansion on previous experiments of ThSSteimle, T., Zhang, R., & Heaven, M. (2015). The pure rotational spectrum of thorium monosulfide, ThS. Chemical Physics Letters, 639, 304-306. https://doi.org/10.1016/j.cplett.2015.09.048ill be discussed.

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Footnotes:

Huang, T., Wang, Q., Yu, W., Wang, X., & Andrews, L. (2018). OMS, OM(η²-SO), and OM(η²-SO)(η²-O₂S) Molecules (M = Ce, Th) with Chiral Structure: Matrix Infrared Spectra and Theoretical Calculations. The Journal of Physical Chemistry. A, 122(24), 5391–5400. https://doi.org/10.1021/acs.jpca.8b03731, Steimle, T., Zhang, R., & Heaven, M. (2015). The pure rotational spectrum of thorium monosulfide, ThS. Chemical Physics Letters, 639, 304-306. https://doi.org/10.1016/j.cplett.2015.09.048w

Rotational spectra were recorded on a chirp-pulsed Fourier transform microwave (CP-FTMW) spectrometer with laser ablation sourcing at the Missouri University of Science and Technology. The cerium or hafnium rods were struck by a Nd:YAG laser outputting the 1064 nm fundamental to generate a plasma, and utilized carbonyl sulfide (OCS) in an argon carrier gas. When compared to the rotational spectra of OCS and its isotopologues, there are many transitions that are present when the laser ablation source is initiated. An expansion of previous work on HfSCooke, S. & Gerry, M. (2002). The Pure Rotational Spectrum, Geometry, and Hyperfine Constants of Hafnium Monosulfide, HfS. The Journal of Molecular Spectroscopy, 216(1), 122-133. https://doi.org/10.1006/jmsp.2002.8678ill be discussed. However, no such study for a cerium comparison was found. A discussion of the spectra collected in the 8 to 18 GHz region of the electromagnetic spectrum, along with the differences between them and that of pure OCS, will be presented.

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Footnotes:

Cooke, S. & Gerry, M. (2002). The Pure Rotational Spectrum, Geometry, and Hyperfine Constants of Hafnium Monosulfide, HfS. The Journal of Molecular Spectroscopy, 216(1), 122-133. https://doi.org/10.1006/jmsp.2002.8678w

Rotational spectra were recorded on a chirp-pulsed Fourier transform microwave (CP-FTMW) spectrometer with laser ablation sourcing at the Missouri University of Science and Technology. A uranium rod was struck by a Nd:YAG laser outputting the 1064 nm fundamental to generate a plasma, utilizing carbonyl sulfide (OCS) in an argon carrier gas. When compared to a rotational experiment of OCS, there are many transitions that are present when the laser is initiated. A discussion of the spectra collected in the 8 to 18 GHz region of the electromagnetic spectrum, along with the differences between it and that of pure OCS, will be presented.