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A collaborative effort between experiment and theory towards elucidating the electronic and molecular structures of uranium-gold clusters is presented. Anion photoelectron spectra of UAu_n^–(n=3-7) were taken at third (355 nm) and fourth (266 nm) harmonics of a Nd:YAG laser, as well as excimer (ArF 193nm) photon energies, where the experimental adiabatic electron affinities (AEA) and vertical detachment energies (VDE) values were measured. Complementary first principles calculations were subsequently carried out to corroborate experimentally determined electron detachment energies and to determine the geometry and electronic structure for each cluster. Except for the ring-like neutral isomer of UAu₆ where one unpaired electronmore » is spread over the Au atoms, all other neutral and anionic UAu_n clusters (n = 3 - 7) were calculated to possess open-shell electrons with the unpaired electrons localized on the central U atom. The smaller clusters closely resemble the analogous UF_n species, but significant deviations are seen starting with UAu₅ where a competition between U-Au and Au-Au bonding begins to become apparent. The UAu₆ system appears to mark a transition where Au-Au interactions begin to dominate, where both a ring-like and two heavily distorted octahedral structures around the central U atom are to be nearly isoenergetic. With UAu₇, only ring-like structures are calculated. Altogether, the calculated electron detachment energies are in good agreement with the experimental values« less

The atomization enthalpies of the U(VI) species UF6 and the uranium oxyhalides UO2X2 (X=F, Cl, Br, I, At) were calculated using a composite relativistic Feller-Peterson-Dixon (FPD) approach based on scalar relativistic DKH3-CCSD(T) with extrapolations to the CBS limit. The inherent multideterminantal nature of the U atom was mitigated by utilizing the singly charged atomic cation in all calculations with correction back to the neutral asymptote via the accurate ionization energy of the U atom. The effects of SO coupling were recovered using full 4-component CCSD(T) with contributions due to the Gaunt Hamiltonian calculated using Dirac-Hartree-Fock. The final atomization enthalpy formore » UF6 (752.2 kcal/mol) was within 2.5 kcal/mol of the experimental value, but unfortunately the latter carries a ±2.4 kcal/mol uncertainty that is predominantly due to the experimental uncertainty in the formation enthalpy of U atom. The analogous value for UO2F2 (607.6 kcal/mol) was in nearly exact agreement with experiment, but the latter has a stated experimental uncertainty of ±4.3 kcal/mol. The FPD atomization enthalpy for UO2Cl2 (540.4 kcal/mol) was within the experimental error limits of ±5.5 kcal/mol. FPD atomization energies for the non-U-containing molecules (used for reaction enthalpies) H2O and HX (X=F, Cl, Br, I, At) were within at most 0.3 kcal/mol of their experimental values where available. The FPD atomization enthalpies, together with FPD reaction enthalpies for two different reactions, were used to determine heats of formation for all species of this work with estimated uncertainties of ±4 kcal/mol. The calculated heat of formation for UF6 (-511.0 kcal/mol) is within 2.5 kcal/mol of the accurately-known (±0.45 kcal/mol) experimental value.« less

Abstract Guided ion beam tandem mass spectrometry was used to examine the kinetic energy dependent reactions of U ⁺ with O ₂ and CO. In the reaction of U ⁺ with O ₂ , the UO ⁺ product is formed in a barrierless and exothermic process with a reaction efficiency at low energies of k/k _col =1.1±0.2, but increases at higher collision energies. Formation of both UO ⁺ and UC ⁺ in the reaction of U ⁺ with CO is endothermic. 0 K bond dissociation energies (BDEs) of D ₀ (U ⁺ ‐O)=7.88±0.09 eV and D ₀ (U ⁺ ‐C)=4.03±0.13 eV were determinedmore » by analyzing the kinetic energy dependent cross sections in the latter endothermic reactions. These values are within experimental uncertainty of previously reported experimental values. Additionally, the electronic states of UO ⁺ and UC ⁺ and the potential energy surfaces for the reactions were explored by quantum chemical calculations. The former include a full Feller‐Peterson‐Dixon composite approach with correlation contributions up to CCSDT(Q) for UO and UO ⁺ , yielding D ₀ (U‐O)=7.82 eV and D ₀ (U ⁺ ‐O)=7.99 eV, as well as more approximate CCSD(T) calculations where a semi‐empirical model was used to estimate spin‐orbit energy contributions, which are generally found to improve the agreement with experiment. Both experimental BDEs are observed to be close to those of their transition metal congeners, ScL ⁺ , YL ⁺ , and GdL ⁺ (L=O and C).« less

Here, the thermochemistry of halocarbon species containing iodine and bromine is examined through an extensive interplay between new Feller–Peterson–Dixon (FPD) style composite methods and a detailed analysis of all available experimental and theoretical determinations using the thermochemical network that underlies the Active Thermochemical Tables (ATcT). From the computational viewpoint, a slower convergence of the components of composite thermochemistry methods is observed relative to species that solely contain first row elements, leading to a higher computational expense for achieving comparable levels of accuracy. Potential systematic sources of computational uncertainty are investigated, and, not surprisingly, spin-orbit coupling is found to be amore » critical component, particularly for iodine containing molecular species. The ATcT analysis of available experimental and theoretical determinations indicates that prior theoretical determinations have significantly larger uncertainties than originally reported, particularly in cases where molecular spin-orbit effects were ignored. Accurate and reliable heats of formation are reported for 38 halogen containing systems, based on combining the current computations with previous experimental and theoretical work via the ATcT approach.« less

Abstract The impact of complete basis set extrapolation schemes (CBS), diffuse functions, and tight weighted‐core functions on enthalpies of formation predicted via the DLPNO‐CCSD(T1) reduced Feller‐Peterson‐Dixon approach has been examined for neutral H,C,O‐compounds. All tested three‐point (TZ/QZ/5Z) extrapolation schemes result in mean unsigned deviation (MUD) below 2 kJ mol ⁻¹ relative to the experiment. The two‐point QZ/5Z and TZ/QZ CBS extrapolation schemes are inferior to their inverse power counterpart () by 1.3 and 4.3 kJ mol ⁻¹ . The CBS extrapolated frozen core atomization energies are insensitive (within 1 kJ mol ⁻¹ ) to augmentation of the basis set with tight weighted core functions. The core‐valencemore » correlation effects converge already at triple‐ζ, although double‐ζ/triple‐ζ CBS extrapolation performs better and is recommended. The effect of diffuse function augmentation converges slowly, and cannot be reproduced with double‐ ζ or triple‐ ζ calculations as these are plagued with basis set superposition and incompleteness errors.« less

Here, anion photoelectron spectroscopy and first-principles quantum chemistry are used to demonstrate to what degree Au can act as a surrogate for F in UF6 and its anion. Unlike UF6, UAu6 exhibits strong ligand–ligand, i.e., Au–Au, interactions, resulting in three low-lying isomers, two of which are three-dimensional while the third isomer has a ring-like quasi two-dimensional structure. Additionally, all the UAu6 isomers have open-shell electrons, which in nearly all cases are localized on the central U atom. As a result, the adiabatic electron affinity and vertical detachment energy are measured to be 3.05 ± 0.05 and 3.28 ± 0.05 eV,more » respectively, and are in very good agreement with calculations.« less

A combination of high-level ab initio calculations and anion photoelectron detachment (PD) measurements is reported for the UC, UC^–, and UC⁺ molecules. To better compare the theoretical values with the experimental photoelectron spectrum (PES), a value of 1.493 eV for the adiabatic electron affinity (AEA) of UC was calculated at the Feller–Peterson–Dixon (FPD) level. The lowest vertical detachment energy (VDE) is predicted to be 1.500 eV compared to the experimental value of 1.487 ± 0.035 eV. A shoulder to lower energy in the experimental PD spectrum with the 355 nm laser can be assigned to a combination of low-lying excitedmore » states of UC^– and excited vibrational states. The VDEs calculated for the low-lying excited electronic states of UC at the SO-CASPT2 level are consistent with the observed additional electron binding energies at 1.990, 2.112, 2.316, and 3.760 eV. Potential energy curves for the Ω states and the associated spectroscopic properties are also reported. Compared to UN and UN⁺, the bond dissociation energy (BDE) of UC (411.3 kJ/mol) is predicted to be considerably lower. The natural bond orbitals (NBO) calculations show that the UC^0/+/– molecules have a bond order of 2.5 with their ground-state configuration arising from changes in the oxidation state of the U atom in terms of the 7s orbital occupation: UC (5f²7s¹), UC^– (5f²7s²), and UC⁺ (5f²7s⁰). Furthermore, the behavior of the UN and UC sequence of molecules and anions differs from the corresponding sequences for UO and UF.« less

In this work, the results of calculations of the properties of the anion UN^– including electron detachment are described, which further expand our knowledge of this diatomic molecule. High-level electronic structure calculations were conducted for the UN and UN^– diatomic molecules and compared to photoelectron spectroscopy measurements. The low-lying Ω states were obtained using multireference CASPT2 including spin-orbit effects up to ~20,000 cm^–1 . At the Feller–Peterson–Dixon (FPD) level, the adiabatic electron affinity (AEA) of UN is estimated to be 1.402 eV and the vertical detachment energy (VDE) is 1.423 eV. The assignment of the UN excited states shows goodmore » agreement with the experimental results with a VDE of 1.424 eV. An Ω = 4 ground state was obtained for UN^– which is mainly associated with the ³H ΛS state. Thermochemical calculations estimate a bond dissociation energy (BDE) for UN^– (U^– + N) of 665.9 kJ/mol, ~15% larger than that of UN and UN⁺. The NBO analysis reveals U–N triple bonds for the UN, UN^–, and UN⁺ species.« less

The thorium–gold negative ions ThAu₂^–, ThAu₂O^–, and ThAuOH^– have been observed and experimentally characterized by anion photoelectron spectroscopy. These experiments are accompanied by extensive ab initio electronic structure calculations using a relativistic composite methodology based primarily on coupled cluster singles and doubles with perturbative triples calculations. The theoretical electron affinities (EAs) at 0 K agree with the experimental adiabatic EAs to within 0.02 eV for all species. Two separate isomers were located in the calculations for ThAuOH^–, and detachment from both of these appears to be present in the photoelectron spectrum. Excited electronic states of the neutral molecules are reportedmore » at the equation of motion-coupled cluster singles and doubles level of theory. Atomization energies and heats of formation are also calculated for each neutral species and have expected uncertainties of 3 and 4 kcal/mol, respectively. The σ bonds between Th and Au are determined by natural bond orbital analysis to consist of predominately sd hybrids on Th bonding with the Au 6s orbital. In order to investigate the correspondence between the bonding in Th–Au and Th–F molecules, a limited number of calculations were also carried out on most of the F-analogs of this study. These results demonstrate that Au does behave like F in these cases, although the Th–F σ bonds are much more ionic compared to Th–Au. This results in an EA for ThF₂ that is 10 kcal/mol smaller than that of ThAu₂. The EA values for the Th(IV) species, i.e., ThX₂O and ThXOH, only differed, however, by 3–4 kcal/mol.« less

In this work, high-level electronic structure calculations of the lowlying energy electronic states for ThH, ThH^–, and ThH⁺ are reported and compared to experimental measurements. The inclusion of spin–orbit coupling is critical to predict the ground-state ordering as inclusion of spin–orbit switches the coupled-cluster CCSD(T) ordering of the two lowest energy states for ThH and ThH⁺. At the multireference spin–orbit SO-CASPT2 level, the ground states of ThH, ThH^–, and ThH⁺ are predicted to be the ²Δ_3/2, ³Φ₂, and ³Δ₁ states, respectively. The adiabatic electron affinity is calculated to be 0.820 eV, and the vertical detachment energy is calculated to bemore » 0.832 eV in comparison to an experimental value of 0.87 ± 0.02 eV. The observed ThH^– photoelectron spectrum has many transitions, which approximately correlate with excitations of Th⁺ and/or Th. The adiabatic ionization energy of ThH including spin–orbit corrections is calculated to be 6.181 eV. The natural bond orbital results are consistent with a significant contribution of the Th⁺H^– ionic configuration to the bonding in ThH. The bond dissociation energies for ThH, ThH^–, and ThH⁺ using the Feller–Peterson–Dixon approach were calculated to be similar for all three molecules and lie between 259 and 280 kJ/mol.« less