Probing Antioxidant-Related Properties for Phenolic Compounds.

In this work, properties related to antioxidant-potential mechanisms (such as the bond dissociation enthalpy, BDE, for the homolytic cleavage of the O-H bond and ionization energies, IEs) were determined for phenol, pyrocatechol, and gallic acid (GA). Both the protonated and deprotonated forms of GA were investigated. The Feller-Peterson-Dixon (FPD) composite method was employed with a variety of computational approaches, i.e., density functional theory, Möller-Plesset perturbation theory, and coupled-cluster-based methods, in combination with large correlation consistent basis sets with extrapolation to the complete basis set limit and consideration of core electron correlation effects. FPD results were compared to experimental and computational data available in the literature, presenting good agreement. For example, the FPD BDE (298 K) obtained for phenol, which was based on valence-correlated MP2/CBS calculations with contributions from correlating all electrons, was determined to be 87.56 kcal/mol, a value that is 0.42 kcal/mol lower than the result obtained in the most recent experiments, 87.98 ± 0.62. Calibration against coupled-cluster calculations was also carried out for phenol. We expect that the outcomes gathered here may help in establishing a general protocol for computational chemists that are interested in determining antioxidant-related properties for phenolic compounds with considerable accuracy as well as to motivate future IE measurements (particularly for GA) to be accomplished in the near future.

Influence of the complete basis set approximation, tight weighted-core, and diffuse functions on the DLPNO-CCSD(T1) atomization energies of neutral H,C,O-compounds.

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 1 / l max 3 extrapolation schemes are inferior to their inverse power counterpart ( 1 / l max + 1 / 2 4 ) 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-valence 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.

Coupled Cluster Study of the Heats of Formation of UF6 and the Uranium Oxyhalides, UO2X2 (X = F, Cl, Br, I, and At).

The atomization enthalpies of the U(VI) species UF6 and the uranium oxyhalides UO2X2 (X = F, Cl, Br, I, and 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 multideterminant 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 for 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 the U atom. The analogous value for UO2F2 (607.6 kcal/mol) was in nearly exact agreement with the 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 limit 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, and 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.

Active Thermochemical Tables: Enthalpies of Formation of Bromo- and Iodo-Methanes, Ethenes and Ethynes.

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 a 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.

Electronic Structure and Anion Photoelectron Spectroscopy of Uranium-Gold Clusters UAun-, n = 3-7.

A collaborative effort between experiment and theory toward elucidating the electronic and molecular structures of uranium-gold clusters is presented. Anion photoelectron spectra of UAun-(n = 3-7) were taken at the third (355 nm) and fourth (266 nm) harmonics of a Nd:YAG laser, as well as excimer (ArF 193 nm) photon energies, where the experimental adiabatic electron affinities and vertical detachment energies 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 electron is spread over the Au atoms, all other neutral and anionic UAun 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 UFn 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 calculated to be nearly isoenergetic. With UAu7, only ring-like structures are calculated. Overall, the calculated electron detachment energies are in good agreement with the experimental values.

A high level theory investigation on the lowest-lying ionization potentials of glycine (NH2CH2COOH).

In this work, an investigation on the ionization potentials (IPs) of the glycine molecule (NH2CH2COOH) is presented. IPs ranging up to ∼20 eV were probed for each of the six conformations considered, with the referred threshold being chosen based on both: (i) the observations by recent photoelectron-photoion coincidence (PEPICO) experiments and (ii) the energy range of relevance to the modeling of other photo-induced processes (e.g., photoionization). For computing the IPs, the equation-of-motion ionization potential coupled-cluster with single and double excitations method (EOMIP-CCSD) was employed with large correlation consistent aug-cc-pVXZ and aug-cc-pCVXZ (X = D, T, and Q) basis sets. Extrapolation to the complete basis set limit and consideration of core electron correlation effects were also taken into account. Subsequently, the Feller-Peterson-Dixon (FPD) approach was used for considering all the contributions and to obtain accurate IPs. In addition, coupled-cluster with single and double excitations as well as perturbative triples, CCSD(T), was also used with the aug-cc-pVTZ basis set. When compared to each other, results obtained through the use of these approaches yielded excellent agreement. In general, the outcomes from the present work provide additional information to the insights gathered from the recent PEPICO experiments as well as accurate IPs for all 6 conformations of glycine using an approach based on high levels of theory. Hence, it is expected that other investigations focusing on photo-induced processes originating from NH2CH2COOH (for instance, the computational modeling of its photoionization) will be motivated for study in the future.

Interaction of Th with H0/-/+: Combined Experimental and Theoretical Thermodynamic Properties.

High-level electronic structure calculations of the low-lying 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 2Δ3/2, 3Φ2, and 3Δ1 states, respectively. The adiabatic electron affinity is calculated to be 0.820 eV, and the vertical detachment energy is calculated to be 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.

Theoretical and Experimental Study of the Spectroscopy and Thermochemistry of UC+/0/.

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 excited 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 UC0/+/- 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). The behavior of the UN and UC sequence of molecules and anions differs from the corresponding sequences for UO and UF.

Electronic Properties of UN and UN- from Photoelectron Spectroscopy and Correlated Molecular Orbital Theory.

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 good 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.

ThAu2 -, ThAu2O-, and ThAuOH- anions: Photoelectron spectroscopic and theoretical characterization.

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 reported 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.

Guided Ion Beam Studies of the Thorium Monocarbonyl Cation Bond Dissociation Energy and Theoretical Unveiling of Different Isomers of [Th,O,C]+ and Their Rearrangement Mechanism.

Threshold collision-induced dissociation (TCID) of the thorium monocarbonyl cation, ThCO+, with xenon is performed using a guided ion beam tandem mass spectrometer. The only product observed is Th+ resulting from loss of the CO ligand. Analysis of the kinetic energy-dependent cross sections for this CID reaction yields the first experimental determination of the bond dissociation energy (BDE) of Th+-CO at 0 K as 0.94 ± 0.06 eV. Calculated BDEs at the CCSD(T) level of theory with cc-pVXZ (X = T and Q) basis sets and a complete basis set (CBS) extrapolation are in good agreement with the experimental result. The Feller-Peterson-Dixon composite coupled-cluster methodology was also applied on both ThCO+ and ThCO, with contributions up to CCSDT(Q) and a four-component treatment of spin-orbit coupling effects. The final 0 K Th+-CO BDE of 0.94 ± 0.04 eV is in excellent agreement with the current experimental result. The ionization energy of ThCO, as well as the atomization energies and heats of formation for both ThCO and ThCO+, is reported at this same level of theory. Complete potential energy profiles of both quartet and doublet spin are also constructed to elucidate the mechanism for the formation and interconversion of different isomers of [Th,O,C]+. Chemical bonding patterns in low-lying states of ThCO+ and potential energy curves for ThCO+ dissociation are also investigated.

Improving the theoretical description of Ln(III)/An(III) separation with phosphinic acid ligands: a benchmarking study of structure and selectivity.

The efficient separation of trivalent lanthanides from minor actinides with soft-donor ligands, while showing experimental promise, has theorists continuing to search for suitable approaches for describing and interpreting selectivity. To remedy this, we employ several computational methods in describing the structure of model M(H2PX2)3 complexes, with M = Eu and Am, and X = O, S, Se, and Te, and predicting the selectivity of model phosphinic acid ligands in Eu(III)/Am(III) separation. After first establishing a set of MP2 and CCSD(T)-DKH3 results as benchmarks, we evaluate several density functionals and descriptions of valence shells for their accuracy with respect to metal-ligand bonding and selectivity. We find that commonly employed functionals with a 0-27% range of exact exchange used with small-core effective core potentials or with an explicit treatment of the relativistic effects (DKH2) incorrectly predict a decrease in the metal-ligand bond distance in going from Eu(III) to Am(III) and completely fail to track a selectivity trend, even giving a wrong sign for some or all ligands. Surprisingly, when these functionals are used in conjunction with an f-in-core description of metal ions, the correct trend in selectivity is recovered, though Am-X distances are overestimated in relation to Eu-X. Functionals with high components of exact exchange (50%) and double-hybrid functionals are reasonably aligned with benchmark results, pointing to the problems of DFT with small exact exchange fractions to handle f-electrons. Natural bond orbital analyses reveal that these poorly performing functionals disproportionately overpopulate outer f orbitals in the model complexes. We anticipate that recommendations resulting from this work will lead to more accurate theoretical descriptions of lanthanide/actinide selectivity with soft-donor chalcogen-based ligands in the future.

Benchmarking Antioxidant-Related Properties for Gallic Acid through the Use of DFT, MP2, CCSD, and CCSD(T) Approaches.

We present a benchmark investigation on the O-H bond dissociation enthalpies (BDEs) and ionization potential (IP) for gallic acid (GA), a widely known polyphenolic antioxidant. These properties were determined in the gas-phase and in water through the use of density functional theory (DFT), second-order Møller-Plesset perturbation theory (MP2), coupled-cluster with single and double excitations (CCSD), and coupled-cluster with single and double excitations as well as perturbative inclusion of triples (CCSD(T)). The 6-311++G(df,p), cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets were used. Regarding DFT functionals, the M06-2X provided the best agreement for the BDEs when compared to the corresponding CCSD(T)/aug-cc-pVTZ results; M06-2X was also found to be the most suitable for probing the IP for the protonated forms of GA while LC-ωPBE was the most reliable in the case of deprotonated GA. Given that these properties represent important descriptors for examining mechanisms related to the antioxidant potential of a given polyphenol, we hope that the present work can serve as a guide for computational chemists venturing in the field.

Coupled Cluster Studies of Platinum-Actinide Interactions. Thermochemistry of PtAnOn+ (n = 0-2 and An = U, Np, Pu).

Accurate Pt-An bond dissociation enthalpies (BDEs) for PtAnOn+ (An = U, Np, Pu and n = 0-2) and the corresponding enthalpies for the Pt + OAnOn+ substitution reactions have been studied for the first time using an accurate composite coupled cluster approach. Analogous O-AnOn+ bond dissociation enthalpies are also presented. To make the study possible, new correlation consistent basis sets optimized using the all-electron third-order Douglas-Kroll-Hess (DKH3) scalar relativistic Hamiltonian are developed and reported for Pt and Au, with accompanying benchmark calculations of their atomic ionization potentials to demonstrate the effectiveness of the new basis sets. For the charged PtAnOn+ species (n = 1, 2), a low-spin state (LSS) for which the Pt-An σ bond is doubly occupied is studied together with a high-spin state (HSS) obtained by unpairing the σ bond orbital and placing one electron into the An 5f shell. The relative energies of the two spin states have been compared and qualitatively assessed via natural population and natural bond analyses. The enthalpies for the Pt substitution reactions, i.e., Pt + OAnOn+ → PtAnOn+ + O, are calculated to range from about 14-62 kcal/mol, and the Pt-AnOn+ bond dissociation enthalpies range from about 78-149 kcal/mol for the ground electronic states. For the PtAnO+ species, the LSSs were all predicted to be the ground state, whereas the PtAnO2+ molecules all favored the HSSs. The prediction for PtUO2+ is consistent with previous theoretical findings. The natural bond orbital analyses indicate a triple bond between An and O, with a double to quadruple bond between the An and Pt.

Bond Dissociation Energies in Heavy Element Chalcogen and Halogen Small Molecules.

Thermodynamic properties including bond dissociation energies (BDEs), heats of formation, and gas-phase acidities for the hydrides and dimers of chalcogens and halogens, H2Y, HX, Y2, and X2 for Y = Se, Te, and At and X = Br, I, and At, have been predicted using the Feller-Peterson-Dixon composite-correlated molecular orbital theory approach. A full four-component CCSD(T) approach was used to calculate the spin-orbit effects on thermodynamic properties, except for Se2, where the AoC-DHF value was used due to strong multireference effects in Se2 for the SO calculations. The calculated results show that the At2 BDE is quite small, 19.5 kcal/mol, with much of the low bond energy due to spin-orbit effects. H2Po is not predicted to be stable to dehydrogenation to Po + H2 in terms of the free energy at 298 K. In the gas phase, HAt is predicted to be a stronger acid than H2SO4. The current results provide insights into potential difficulties in the actual experimental observation of such species for heavy elements.

Ionization potentials for the H2CO trimer.

In this work, a computational study on the ionization potentials (IPs) of the formaldehyde trimer, (H2CO)3, is presented. Twelve lowest-lying vertical IPs were determined through the use of the coupled-cluster level of theory using correlation consistent basis sets with extrapolation to the complete basis set limit and consideration of core electron correlation effects. Specifically, the equation-of-motion ionization potential coupled-cluster with single and double excitations method with the aug-cc-pVnZ and aug-cc-pCVnZ (n = D and T) basis sets was used. The Feller-Peterson-Dixon (FPD) composite approach was employed to provide accurate IPs, and eight conformations of (H2CO)3 were considered. The FPD IPs determined for (H2CO)3 were found to be systematically lower than those computed for the dimer and monomer of H2CO in the pattern IP(monomer) > IP(dimer) > IP(trimer) for a given IP. In addition, the IPs calculated when considering only the more stable conformation (C0) are in good agreement with those obtained using the eight conformations of the H2CO trimer, and thus, the actual conformation played only a minor role in determining such properties in the present case. By providing first accurate IP results for the H2CO trimer, we hope to motivate future experimental and computational investigations (e.g., studies involving photoionization) that rely on such quantities.

The electron affinity of the uranium atom.

The results of a combined experimental and computational study of the uranium atom are presented with the aim of determining its electron affinity. Experimentally, the electron affinity of uranium was measured via negative ion photoelectron spectroscopy of the uranium atomic anion, U-. Computationally, the electron affinities of both thorium and uranium were calculated by conducting relativistic coupled-cluster and multi-reference configuration interaction calculations. The experimentally determined value of the electron affinity of the uranium atom was determined to be 0.309 ± 0.025 eV. The computationally predicted electron affinity of uranium based on composite coupled cluster calculations and full four-component spin-orbit coupling was found to be 0.232 eV. Predominately due to a better convergence of the coupled cluster sequence for Th and Th-, the final calculated electron affinity of Th, 0.565 eV, was in much better agreement with the accurate experimental value of 0.608 eV. In both cases, the ground state of the anion corresponds to electron attachment to the 6d orbital.

Guided Ion Beam and Quantum Chemical Investigation of the Thermochemistry of Thorium Dioxide Cations: Thermodynamic Evidence for Participation of f Orbitals in Bonding.

Kinetic energy dependent reactions of ThO+ with O2 are studied using a guided ion beam tandem mass spectrometer. The formation of ThO2+ in the reaction of ThO+ with O2 is observed to be slightly endothermic and also exhibits two obvious features in the cross section. These kinetic energy dependent cross sections were modeled to determine a 0 K bond dissociation energy of D0(OTh+-O) = 4.94 ± 0.06 eV. This value is slightly larger but within experimental uncertainty of less precise previously reported experimental values. The higher energy feature in the ThO2+ cross section was also analyzed and suggests formation of an excited state of the product ion lying 3.1 ± 0.2 eV above the ground state. Additionally, the thermochemistry of ThO2+ was explored by quantum chemical calculations, including a full Feller-Peterson-Dixon (FPD) composite approach with correlation contributions up to CCSDT(Q) and four-component spin-orbit corrections, as well as more approximate CCSD(T) calculations including semiempirical estimates of spin-orbit energy contributions. The FPD approach predicts D0(OTh+-O) = 4.87 ± 0.04 eV, in good agreement with the experimental value. Analogous FPD results for ThO+, ThO, and ThO2 are also presented, including ionization energies for both ThO and ThO2. The ThO2+ bond energy is larger than those of its transition metal congeners, TiO2+ and ZrO2+, which can be attributed partially to an actinide contraction, but also to contributions from the participation of f orbitals on thorium that are unavailable to the transition metal systems.

Coupled Cluster Study of the Interactions of AnO2, AnO2+, and AnO22+ (An = U, Np) with N2 and CO.

Thermochemical and spectroscopic properties for actinyl complexes involving UO22+/1+/0 and NpO22+/1+/0 with N2 and CO, together with the UO2-O2, UO2+-O2, and UO2+-NO complexes, have been studied for the first time using an accurate composite coupled cluster approach. Two general bonding motifs were investigated, end-on (η1) and side-on (η2) relative to the metal center of the actinyls. For end-on CO complexes, both C-coordinated (An-C) and O-coordinated (An-O) structures were examined, with the former always being lower in energy. All of the η1 complexes were calculated to be stable, with dissociation energies ranging from 2 to 36 kcal/mol, except for that of UO2+-O2 (the η1 orientation for UO2+-NO was not amenable to single reference coupled cluster). In agreement with a previous study, the η2 structure for UO2+-O2 was calculated to be relatively strongly bound, by 22.3 kcal/mol in this work. The closely related NO complex, however, had a calculated dissociation energy of just 4.0 kcal/mol. The binding energy of O2 to neutral UO2 in a η2 orientation was calculated to be very strong, 75.4 kcal/mol, and strongly resembled a UO2+(O2-) complex at equilibrium. The N-N and C-O bonds were found to be somewhat activated for all the side-on (η2) neutral An(IV) complexes, with stretching frequencies of N2 or CO being red-shifted by as much as 480 cm-1 with a 0.06 Šbond length elongation. Dissociation energies for the η1 complexes are strongly correlated with the extent of electron transfer from ligand to actinyl. The nature of bonding in the actinyl complexes is examined using natural resonance theory (NRT). The correlation between bonding motif and small molecule activation is in agreement with experiments in condensed phases.

A Computational Assessment of Actinide Dioxide Cations AnO22+ for An = U to Lr: The Limited Stability Range of the Hexavalent Actinyl Moiety, [O═An═O]2.

The isolated gas-phase actinide dioxide dications, AnO22+, were evaluated by DFT and coupled cluster CCSD(T) calculations for 12 actinides, An = U-Lr. CASSCF calculations were used to define the orbitals for the CCSD(T) calculations. The characteristic linear [O═An═O]2+ hexavalent actinyl(VI) was found to be the lowest energy structure for An = U, Np, and Pu, which also form stable actinyl(VI) species in solution and possibly for Am when spin-orbit effects are included. For Am, there is a divalent [AnII(O2)]2+ structure where the dioxygen is an end-on physisorbed η1-3O2 2 kcal/mol above the actinyl when spin-orbit effects are included which lower the energy of the actinyl structure. For An = Cm, Bk, and Lr, the lowest energy structure is trivalent [AnIII(O2-)]2+ where the dioxygen is a side-on superoxide, η2-O2-. For Cm, the actinyl is close in energy to the ground state when spin-orbit effects are included. For An = Cf, Es, Fm, Md, and No, the lowest energy structure is divalent [AnII(O2)]2+ where the dioxygen is an end-on physisorbed η1-3O2. The relative energies suggest that curyl(VI) and berkelyl(VI), like well-known americyl(VI), might be stabilized by coordinating ligands in condensed phases. The results further indicate that for californyl and beyond, the actinyl(VI) moieties will probably be elusive even using strong donor ligands. The prevalence of low oxidation states (OSs) An(II) and An(III) for transplutonium actinides reflects stabilization of the 5f orbitals and validates established trends, including the remarkably high stability of divalent No. Bond distances and other parameters suggest maximum bond covalency around plutonyl(VI), with a particularly substantial decrease in bond strength between americyl(VI) and curyl(VI).