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Since water is both a product and a common reactant impurity in the (partial) methanol oxidation to methyl formate (MeFo) on gold, its effect on the isothermal selectivity to methyl formate was investigated under well-defined single-collision conditions employing pulsed molecular beam experiments and in situ IRAS measurements. Both a flat Au(111) and a stepped Au(332) surface were used as model catalysts to elucidate how water affects the reactivity of low-coordinated step sites as compared to (111) terrace sites employing a range of reaction conditions. The interactions of water with methanol/methoxy as well as with oxygen species are addressed. Theoretical calculations, including static DFT and ab initio molecular dynamics (AIMD) simulations, are employed to enhance the microscopic understanding of water-induced changes in the oxygen species and overoxidation reactivity on gold, which are essential for the selectivity. The results provide not only information on conditions that mitigate the generally negative effect of water on methyl formate formation but also atomic-level insights into water-induced changes in the complex reaction network that governs the reactivity in applied gold catalysts, such as nanoporous gold., Competing Interests: The authors declare no competing financial interest., (© 2025 The Authors. Published by American Chemical Society.)

To elucidate the role of low-coordinated sites in the partial methanol oxidation to methyl formate (MeFo), the isothermal reactivity of flat Au(111) and stepped Au(332) in pulsed molecular beam experiments was compared for a broad range of reaction conditions. Low-coordinated step sites were found to enhance MeFo selectivity, especially at low coverage conditions, as found at higher temperatures. The analysis of the transient kinetics provides evidence for the essential role of Au x O y phases for MeFo formation and the complex interplay of different oxygen species for the observed selectivity. Ab initio molecular dynamic simulations yielded microscopic insights in the formation of Au x O y phases on flat and stepped gold surfaces emphasizing the role of low-coordinated sites in their formation. Moreover, associated surface restructuring provides atomic-scale insights which align with the experimentally observed transient kinetics in MeFo formation., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Partial methanol oxidation proceeds with high selectivity to methyl formate (MeFo) on nanoporous gold (npAu) catalysts. As low-coordinated sites on npAu were suggested to affect the selectivity, we experimentally investigated their role in the isothermal selectivity for flat Au(111) and stepped Au(332) model surfaces using a molecular beam approach under well-defined conditions. Direct comparison shows that steps enhance desired MeFo formation and lower undesired overoxidation. DFT calculations reveal differences in oxygen distribution that enhance the barriers to overoxidation at steps. Thus, these results provide an atomic-level understanding of factors controlling the complex reaction network on gold catalysts, such as npAu., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

The hydroperoxyalkyl radicals (˙QOOH) are known to play a significant role in combustion and tropospheric processes, yet their direct spectroscopic detection remains challenging. In this study, we investigate molecular stereo-electronic effects influencing the kinetic and thermodynamic stability of a ˙QOOH along its formation path from the precursor, alkylperoxyl radical (ROO˙), and the depletion path resulting in the formation of cyclic ether + ˙OH. We focus on reactive intermediates encountered in the oxidation of acyclic hydrocarbon radicals: ethyl, isopropyl, isobutyl, tert -butyl, neopentyl, and their alicyclic counterparts: cyclohexyl, cyclohexenyl, and cyclohexadienyl. We report reaction energies and barriers calculated with the highly accurate method Weizmann-1 (W1) for the channels: ROO˙ ⇌ ˙QOOH, ROO˙ ⇌ alkene + ˙OOH, ˙QOOH ⇌ alkene + ˙OOH, and ˙QOOH ⇌ cyclic ether + ˙OH. Using W1 results as a reference, we have systematically benchmarked the accuracy of popular density functional theory (DFT), composite thermochemistry methods, and an explicitly correlated coupled-cluster method. We ascertain inductive, resonance, and steric effects on the overall stability of ˙QOOH and computationally investigate the possibility of forming more stable species. With new reactions as test cases, we probe the capacity of various ab initio methods to yield quantitative insights on the elementary steps of combustion.

Recent advances in synthesis and characterization methods have enabled the controllable fabrication of atomically precise metal clusters (AMCs) of subnanometer size that possess unique physical and chemical properties, yet to be explored. Such AMCs have potential applications in a wide range of fields, from luminescence and sensing to photocatalysis and bioimaging, making them highly desirable for further research. Therefore, there is a need to develop innovative methods to stabilize AMCs upon surface deposition, as their special properties are lost due to sintering into larger nanoparticles. To this end, dispersion-corrected density functional theory (DFT-D3) and ab initio molecular dynamics (AIMD) simulations have been employed. Benchmarking against high-level post-Hartree-Fock approaches revealed that the DFT-D3 scheme describes very well the lowest-energy states of clusters of five and ten atoms, Cu 5 and Cu 10 . AIMD simulations performed at 400 K illustrate how intrinsic defects of graphene sheets, carbon vacancies, are capable of confining individual Cu 5 clusters, thus allowing for their stabilization. Furthermore, AIMD simulations provide evidence on the dimerization of Cu 5 clusters on defect-free graphene, in agreement with the ab initio predictions of (Cu 5 ) n aggregation in the gas phase. The findings of this study demonstrate the potential of using graphene-based substrates as an effective platform for the stabilization of monodisperse atomically precise Cu 5 clusters.

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

The rich surface chemistry of gold at the nanoscale has made it an important catalyst for low-temperature applications. Recent studies point to the possible role of self-organized structures formed by chemisorbed O atoms on the surface of gold catalysts for their catalytic activity and/or deactivation. In this study, we investigate the reactivity of a double O chain running along a step on a Au(221) surface with oxygen vacancies as a prototypical model of a 1D surface gold oxide. We compare CO and O 2 adsorption on such a chain with the oxygen-free Au(221) surface model. A systematic study of the reactivity of the double chain with O vacancies was done with respect to the regular Au(221) surface using CO as a probe. The CO oxidation was investigated assuming dissociative and associative mechanisms. Remarkably, O 2 adsorbs stronger on the double oxygen vacancy than on the regular Au(221) surface, and its dissociation barrier reduces significantly from 1.84 eV to 0.87 eV, whereas the CO adsorption energy is similar on these surfaces. Calculations suggest that CO oxidation should occur more efficiently on the double O vacancy than on the regular Au(221) surface due to stronger adsorption of O 2 and a low activation barrier for O 2 + CO surface reaction.

Rare earth oxides are attracting increasing interest as a relatively unexplored group of materials with potential applications in heterogeneous catalysis and electrocatalysis; therefore, a credible and universal computational approach is needed for modeling their reactivity. In this work, we systematically assessed the performance of the PBE+U method against the results of the hybrid HSE06 method with respect to the description of structural parameters and energetic properties of the selected hexagonal lanthanide sesquioxides and the cubic fluorite-type cerium dioxide. In addition, we evaluated the performance of PBE+U in describing the electronic structure and adsorption properties of the CeO 2 (111) and Nd 2 O 3 (0001) surfaces. The HSE06 method reproduces rather well the lattice parameters and selected energetic properties with respect to the experimental values. The PBE+U method is able to reproduce the results of HSE06 or the experimental values only if the U parameter is selected from an appropriate range of values. The U value around 3 eV gives the best description of the lattice parameters of most bulk oxides. 2 eV-3 eV is also found to be the optimal range of U for the reaction energies of bulk La 2 O 3 , Ce 2 O 3 , Nd 2 O 3 , Er 2 O 3 , and Ho 2 O 3 . U = 1 eV gives the best results for Pr 2 O 3 , Pm 2 O 3 , Eu 2 O 3 , Tm 2 O 3 , and Lu 2 O 3 , whereas Gd 2 O 3 could not be accurately described by the PBE+U method. The U values (∼3 eV) found optimal for most bulk oxides also work well in the calculations of adsorption of small molecules on Nd 2 O 3 (0001) and CeO 2 (111), although larger U values are required to obtain sufficient localization of 4f electrons.

A new terbium (III) luminescent compound {[Tb 2 (PDC) 2 (ox)(H 2 O) 4 ](H 2 O) 2 } n was synthesized by the self-assembly of Tb 3+ ions with 3,5-pyridinedicarboxylate (PDC) and oxalate (ox) ligands and characterized by fluorescence spectroscopy and single-crystal X-ray diffraction. The density functional theory (DFT) and high-level correlated ab initio wave function methods with Spin-Orbit Coupling correction (CASSCF/SO and CAS-NEVPT2/SOC) were successfully applied to predict the absorption and emission spectra of this strongly correlated lanthanide system in excellent agreement with the experimental results.

Understanding the chemistry of coordination compounds as lithium storage materials is significant for advancing lithium-ion batteries' technology. Coordination compounds have become a new family of versatile anode materials because the metal center, the ligand, and the nonrigid crystal structure can simultaneously contribute to the lithium storage capacity. However, the capacities and cycling abilities of coordination compounds are relatively low in comparison to inorganic nanomaterials, and the mechanism for lithium storage is unclear. This work reports that linking the mononuclear complex [Ni(PBIM) 2 (HIPA)] ( 1 ), where PBIM = 2-(2-pyridyl)benzimidazole, and HIPA = 5-hydroxyisophthalic acid, to a one-dimensional coordination polymer [Ni(PBIM)(HIPA)] n ( 2 ) via coordination bonds by a facile bottom-up assembly route can significantly enhance the lithium storage capacity from 554 mA h g -1 of 1 to 1025 mA h g -1 of 2 at 100 mA g -1 . A combined experimental and theoretical study shows that the favorable lithium-ion diffusion pathways afforded by the coordination-chain-based structure of 2 are responsible for its superior electrochemical property.

Nonmetal elements strictly govern the electrochemical performance of molybdenum compounds. Yet, the exact role played by nonmetals during electrocatalysis remains largely obscure. With intermetallic MoSi 2 comprising silicene subunits, we present an unprecedented hydrogen evolution reaction (HER) behavior in aqueous alkaline solution. Under continuous operation, the HER activity of MoSi 2 shows a more than one order of magnitude improvement in current density from 1.1 to 21.5 mA cm -2 at 0.4 V overpotential. Meanwhile, this activation behavior is highly reversible, such that voltage withdrawal leads to catalyst inactivation but another operation causes reactivation. Thus, the system shows dynamics strikingly analogous to the legendary Sisyphus' labor, which drops and recovers in a stepwise manner repeatedly, but never succeeds in reaching the top of the mountain. Isomorphic WSi 2 behaves almost the same as MoSi 2 , whereas other metal silicides with silicyne subunits, including CrSi 2 and TaSi 2 , do not exhibit any anomalous behavior. A thin amorphous shell of MoSi 2 is observed after reaction, within which the Si remains partially oxidized while the oxidation state of Mo is basically unchanged. First-principles calculations further reveal that the adsorption of hydroxide ions on silicene subunit edges and the subsequent Si vacancy formation in MoSi 2 jointly lead to the anomalous HER kinetics of the adjacent Mo active centers. This work demonstrates that the role of nonmetal varies dramatically with the electronic and crystallographic structures of silicides and that silicene structural subunit may serve as a promoter for boosting HER in alkaline media., (Copyright © 2019 Science China Press. Published by Elsevier B.V. All rights reserved.)

Several studies have previously observed surprisingly low frequencies for the C-H stretching modes of self-assembled monolayers (SAMs) prepared from aromatic thiols. The reason for this property has so far remained elusive. Therefore, we report a novel study of the vibrational spectra of SAMs prepared on Au from two different aromatic thiols, namely, 4'-nitro-1,1'-biphenyl-4-thiol (NBPT) and 4-aminothiophenol (ATP). The SAMs were prepared by vapor deposition (VD) in ultrahigh vacuum (UHV) as well as by the solution method (SM) and their quality was controlled by X-ray photoelectron spectroscopy (XPS). In addition, amino terminated SAMs were also obtained by electron irradiation and by chemical reduction of NBPT SAMs. Beside infrared reflection absorption spectroscopy (IRRAS), we have employed high resolution electron energy loss spectroscopy (HREELS), by which VD SAMs can be studied in situ, i.e. without exposing them to air. Hence, we can exclude possible contributions of solvent molecules to the vibrational spectra. Nonetheless, HREELS in fact reveals the same large red shift of the C-H stretching modes in the SAMs as also observed in ex situ IRRAS experiments. In contrast, HREELS for physisorbed ATP and ATP in a KBr pellet measured by transmission infrared spectroscopy exhibit the expected aromatic bands. Using a computational approach, we can exclude molecular packing effects as origin of this shift. Therefore, we propose chemical changes in the aromatic rings during SAM formation as an alternative explanation for the observed frequency shift. As another striking effect, the N-H stretching vibrational modes of the amino-terminated SAMs are extremely weak in both IRRAS and HREELS despite the fact that XPS confirms the presence of amino groups. A very weak signal is observed only in the case of an electron irradiated NBPT SAM. In contrast, an energy loss ascribed to the N-H stretching vibrations is clearly observed in HREELS of ATP physisorbed on an ATP SAM and on graphite as well as in the transmission infrared spectrum of ATP in KBr. The extremely low intensity of these vibrations in the SAM is traced back to the inherently low transition dipole moment for the excitation of N-H stretching modes in free N-H groups. Furthermore, the calculations suggest that the much stronger signals of N-H stretching modes involved in hydrogen-bonding with adjacent amino groups are suppressed because these vibrations are oriented parallel to the surface.

Gold has many unique properties, some of which continue to be uncovered, such as the rich chemistry of gold at the nanoscale. In this study, gold surprises us again by the unusual stability of one-dimensional gold oxide structures on the surface of gold, which enhances in the presence of silver impurities. We employ first-principles calculations to investigate the surface segregation of silver in the presence of atomic-oxygen adsorbates arranged in chains on the Au(321) surface. Such 1D oxide chains have previously been suggested as the most stable form of adsorbed oxygen on gold. Although Ag-O bonds are expected to be generally stronger than Au-O bonds, we show that this does not hold for 1D oxide chains, where Au-O bonds seem to be at least as strong as Ag-O bonds. Remarkably, we find that up to very high surface concentrations of silver, the Ag atoms do not occupy positions within the oxide chain, but prefer locations next to it. Ab initio molecular dynamics simulations support this picture and reveal how oxide chains and silver atoms rearrange on the surface toward a lower-energy configuration.

The reaction of NCN with H atoms has been investigated by ab initio MO and RRKM theory calculations. The mechanisms for formation of major products on the doublet and quartet potential energy surfaces have been predicted at the CCSD(T) level of theory with the complete basis set limit. In addition, the heat of formation for NCN predicted at this rigorous level and those from five isogyric reactions are in close agreement with the best value based on the isodesmic process, (3)CCO + N2 = (3)NCN + CO, 109.4 kcal/mol, which lies within the two existing experimental values. The rate constants for the three possible reaction channels, H + NCN → CH + N2 (k(P1)), HCN + (4)N (k(QP1)), and HNC + (4)N (k(QP2)), have been calculated in the temperature range 298-3000 K. The results show that k(P1) is significantly higher than k(QP1) and k(QP2) and that the total rate constant agrees well with available experimental values in the whole temperature range studied. The kinetics of the reverse CH + N2 reaction has also been revisited at the CCSD(T)/CBS level; the predicted total rate constants at 760 Torr Ar pressure can be represented by kr = 4.01 × 10(-15) T(0.90) exp(-17.42 kcal mol(-1)/RT) cm(3) molecule(-1) s(-1) at T = 800-4000 K. The result agrees closely with the most recent experimental data and the best theoretical result of Harding et al. (J. Phys. Chem. A 2008, 112, 522) as well as that of Moskaleva and Lin (Proc. Combust. Inst. 2000, 28, 2393) evaluated with a steady-state approximation after a coding error correction made in this study.

Recently, several forms of unsupported gold were shown to display a remarkable activity to catalyze oxidation reactions. Experimental evidence points to the crucial role of residual silver present in very small concentrations in these novel catalysts. We focus on the catalytic properties of nanoporous gold (np-Au) foams probed via CO and oxygen adsorption/co-adsorption. Experimental results are analyzed using theoretical models represented by the flat Au(111) and the kinked Au(321) slabs with Ag impurities. We show that Ag atoms incorporated into gold surfaces can facilitate the adsorption and dissociation of molecular oxygen on them. CO adsorbed on top of 6-fold coordinated Au atoms can in turn be stabilized by co-adsorbed atomic oxygen by up to 0.2 eV with respect to the clean unsubstituted gold surface. Our experiments suggest a linking of that most strongly bound CO adsorption state to the catalytic activity of np-Au. Thus, our results shed light on the role of silver admixtures in the striking catalytic activity of unsupported gold nanostructures.

With a relativistic all-electron density functional method, we studied two anionic uranium(VI) carbonate complexes that are important for uranium speciation and transport in aqueous medium, the mononuclear tris(carbonato) complex [UO(2)(CO(3))(3)](4-) and the trinuclear hexa(carbonato) complex [(UO(2))(3)(CO(3))(6)](6-). Focusing on the structures in solution, we applied for the first time a full solvation treatment to these complexes. We approximated short-range effects by explicit aqua ligands and described long-range electrostatic interactions via a polarizable continuum model. Structures and vibrational frequencies of "gas-phase" models with explicit aqua ligands agree best with experiment. This is accidental because the continuum model of the solvent to some extent overestimates the electrostatic interactions of these highly anionic systems with the bulk solvent. The calculated free energy change when three mono-nuclear complexes associate to the trinuclear complex, agrees well with experiment and supports the formation of the latter species upon acidification of a uranyl carbonate solution.

Polarization-resolved optical spectra of coinage metal monomers and dimers Mn (M=Cu, Ag, Au; n=1, 2) at ideal O2- sites of MgO(001) as well as at oxygen vacancies, Fs and Fs+, of that surface were established using a computational approach based on linear response time-dependent density functional theory. Calculations were performed for structures determined by applying a generalized-gradient density functional method to cluster models embedded in an elastic polarizable environment. This embedding scheme provides an accurate description of substrate relaxation and long-range electrostatic interaction. We compared the optical properties of adsorbed atoms and dimers with those of the corresponding gas-phase species and we systematically analyzed trends among congeners.

We review systematic experimental and theoretical efforts that explored formation, structure and reactivity of PdZn catalysts for methanol steam reforming, a material recently proposed to be superior to the industrially used Cu based catalysts. Experimentally, ordered surface alloys with a Pd : Zn ratio of approximately 1 : 1 were prepared by deposition of thin Zn layers on a Pd(111) surface and characterized by photoelectron spectroscopy and low-energy electron diffraction. The valence band spectrum of the PdZn alloy resembles closely the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1 : 1 stoichiometry. Among the issues studied with the help of density functional calculations are surface structure and stability of PdZn alloys and effects of Zn segregation in them, and the nature of the most likely water-related surface species present under the conditions of methanol steam reforming. Furthermore, a series of elementary reactions starting with the decomposition of methoxide, CH(3)O, along both C-H and C-O bond scission channels, on various surfaces of the 1 : 1 PdZn alloy [planar (111), (100) and stepped (221)] were quantified in detail thermodynamically and kinetically in comparison with the corresponding reactions on the surfaces Pd(111) and Cu(111). The overall surface reactivity of PdZn alloy was found to be similar to that of metallic Cu. Reactive methanol adsorption was also investigated by in situ X-ray photoelectron spectroscopy for pressures between 3 x 10(-8) and 0.3 mbar.

Cu(4), Ag(4), and Au(4) species adsorbed on an MgO(001) surface that exhibits neutral (F(s)) and charged (F(s) (+)) oxygen vacancies have been studied using a density functional approach and advanced embedding models. The gas-phase rhombic-planar structure of the coinage metal tetramers is only moderately affected by adsorption. In the most stable surface configuration, the plane of the tetramers is oriented perpendicular to the MgO(001) surface; one metal atom is attached to an oxygen vacancy and another one is bound to a nearby surface oxygen anion. A very similar structural motif was recently found on defect-free MgO(001), where two O(2-) ions serve as adsorption sites. Following the trend of the interactions with the regular MgO(001) surface, Au(4) and Cu(4) bind substantially stronger to F(s) and F(s) (+) sites than Ag(4). This stronger adsorption interaction at oxygen vacancies, in particular at F(s), is partly due to a notable accumulation of electron density on the adsorbates. We also examined the propensity of small supported metal species to aggregate to adsorbed di-, tri- and tetramers. Furthermore, we demonstrated that core-level ionization potentials offer the possibility for detecting experimentally supported metal tetramers and characterizing them structurally with the help of calculated data.

Using a set of model reactions, we estimated the heat of formation of gaseous PuO2(2+) from quantum-chemical reaction enthalpies and experimental heats of formation of reference species. To this end, we carried out relativistic density functional calculations on the molecules PuO(2)2+, PuO2, PuF6, and PuF4. We used a revised variant (PBEN) of the Perdew-Burke-Ernzerhof gradient-corrected exchange-correlation functional, and we accounted for spin-orbit interaction in a self-consistent fashion. As open-shell Pu species with two or more unpaired 5f electrons are involved, spin-orbit interaction significantly affects the energies of the model reactions. Our theoretical estimate for the heat of formation DeltafH degree 0(PuO2(2+),g), 418+/-15 kcal mol-1, evaluated using plutonium fluorides as references, is in good agreement with a recent experimental result, 413+/-16 kcal mol-1. The theoretical value connected to the experimental heat of formation of PuO2(g) has a notably higher uncertainty and therefore was not included in the final result.

PdZn (1:1) alloy is assumed to be the active component of a promising catalyst for methanol steam reforming. Using density functional calculations on periodic supercell slab models, followed by atomistic thermodynamics modeling, we study the chemical composition of the surfaces PdZn(111) and, as a reference, Cu(111) in contact with water and hydrogen at conditions relevant to methanol steam reforming. For the two surfaces, we determine similar maximum adsorption energies for the dissociative adsorption of H(2), O(2), and the molecular adsorption of H(2)O. These reactions are calculated to be exothermic by about -40, -320, and -20 kJ mol(-1), respectively. Using a thermodynamic analysis based on theoretically predicted adsorption energies and vibrational frequencies, we determine the most favorable surface compositions for given pressure windows. However, surface energy plots alone cannot provide quantitative information on individual coverages in a system of coupled adsorption reactions. To overcome this limitation, we employ a kinetic model, from which equilibrium surface coverages of H, O, OH, and H(2)O are derived. We also discuss the sensitivity of our results and the ensuing conclusions with regard to the model surfaces employed and the inaccuracies of our computational method. Our kinetic model predicts surfaces of both materials, PdZn and Cu, to be essentially adsorbate-free already from very low values of the partial pressure of H(2). The model surfaces PdZn(111) and Cu(111) are predicted to be free of water-related adsorbates for a partial H(2) pressure greater than 10(-8) and 10(-5) atm, respectively.

As a first step toward modeling the interaction of dissolved actinide contaminants with mineral surfaces, we studied low-coverage adsorption of aqueous uranyl, UO2(2+), on the hydroxylated alpha-Al2O3(0001) surface. We carried out density functional periodic slab model calculations and modeled solvation effects by explicit aqua ligands. We explored the formation of both inner- and outer-sphere complexes and estimated the corresponding adsorption energies. Effects of solvation were accounted for by explicit consideration of the first hydration shell of uranyl and by means of a posteriori corrections for long-range solvent effect. With energetics described at the GGA-PW91 level and under the assumption of a fully protonated ideal surface, we predict a weakly bound outer-sphere adsorption complex.

The six-valent uranyl and neptunyl complexes [An(VI)O2X4]n (An = U, Np; X = F, Cl, OH, n = -2; X = H2O, n = +2) have been studied within the framework of density functional theory. The relative stabilities of the cis and trans isomers, structural properties, charge distribution, and ligand binding energies have been determined using the modified Perdew-Burke-Ernzerhof functional at the all-electron scalar relativistic level. Uranyl and neptunyl complexes with different ligands have been compared in a systematic fashion, demonstrating close similarity of these actinides in oxidation state VI. In addition, the effect of an aqueous solution has been taken into account with the polarizable continuum model COSMO. Computed averaged ligand binding energies permit one to rationalize the observed different stabilities of the title species in aqueous media.

By using a set of model reactions, we estimated the heat of formation of gaseous UO2(2+) from quantum-chemical reaction enthalpies and experimental heats of formation of reference species. For this purpose, we performed relativistic density functional calculations for the molecules UO2(2+), UO2, UF6, and UF5. We used two gradient-corrected exchange-correlation functionals (revised Perdew-Burke-Ernzerhof (PBEN) and Becke-Perdew (BP)) and we accounted for spin-orbit interaction in a self-consistent fashion. Indeed, spin-orbit interaction notably affects the energies of the model reactions, especially if compounds of U(IV) are involved. Our resulting theoretical estimates for delta fH(o)0 (UO2(2+)), 365+/-10 kcal mol(-1) (PBEN) and 370+/-12 kcal mol(-1) (BP), are in quantitative agreement with a recent experimental result, 364+/-15 kcal mol(-1). Agreement between the results of the two different exchange-correlation functionals PBEN and BP supports the reliability of our approach. The procedure applied offers a general means to derive unknown enthalpies of formation of actinide species based on the available well-established data for other compounds of the element in question.

We have studied the solvation of uranyl, UO(2)(2+), and the reduced species UO(OH)(2+) and U(OH)(2)(2+) systematically using three levels of approximation: direct application of a continuum model (M1); explicit quantum-chemical treatment of the first hydration sphere (M2); a combined quantum-chemical/continuum model approach (M3). We have optimized complexes with varying numbers of aquo ligands (n = 4-6) and compared their free energies of solvation. Models M1 and M2 have been found to recover the solvation energy only partially, underestimating it by approximately 100 kcal/mol or more. With our best model M3, the calculated hydration free energy Delta(h)G degrees of UO(2)(2+) is about -420 kcal/mol, which shifts to about -370 kcal/mol when corrected for the expected error of the model. This value agrees well with the experimentally determined interval, -437 kcal/mol < Delta(h)G degrees < -318 kcal/mol. Complexes with 5 and 6 aquo ligands have been found to be about equally favored with models M2 and M3. The same solvation models have been applied to a two-step reduction of UO(2)(2+) by water, previously theoretically studied in the gas phase. Our results show that the solvation contribution to the reaction free energy, about 60 kcal/mol, dominates the endoergicity of the reduction.