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Experimental data on the thermodynamics and reaction mechanism of the inner-sphere. uoride exchange reaction (UO22+)-O-17 + UO2F+ reversible arrow (UO2F+)-O-17 + UO22+ have been compared with different intimate reaction mechanisms using quantum chemical methods. Two models have been tested that start from the outer sphere complexes, (H2O)[UAO2F(OH2)(4)(+)] center dot center dot center dot [UBO2(OH2)(5)(2+)] and [UAO2F(OH2)(4)(+)] center dot center dot center dot [UBO2(OH2)(5)(2+)]; the geometry and energies of the intermediates and transition states along possible reaction pathways have been calculated using different ab initio methods, SCF, B3LYP and MP2. Both the experimental data and the theoretical results suggest that the. uoride exchange takes place via the formation and breaking of a U - F - U bridge that is the rate determining step. The calculated activation enthalpy Delta H-not equal = 30.9 kJ mol(-1) is virtually identical to the experimental value 31 kJ mol(-1); however this agreement may be a coincidence as we do not expect a larger accuracy than 10 kJ mol(-1) with the methods used. The calculations show that the. uoride bridge is formed as an insertion of UAO2F(OH2)(4)(+) into UBO2(OH2)(5)(2+) followed by a subsequent transfer of water from the first to the second coordination sphere of U-B., QC 20110816

The spin-orbit effects were investigated on the complexes involved in the electron self-exchange between Np(V) and Np(VI) in both the outer-sphere and inner-sphere mechanisms, the latter for binuclear complexes containing hydroxide, fluoride, and carbonate as bridging ligands. Results obtained with the variation-perturbation and the multireference single excitation spin-orbit Cl calculations are compared. Both effects due to different relaxations of spinors within a multiplet (spin-orbit relaxation) and scalar (electrostatic) relaxation effects in the excited states are accounted for in the latter scheme. The results show that the scalar (electrostatic) relaxation is well described by the single-excitation spin-orbit CI, and that spin-orbit relaxation effects are small in the Np complexes, as in the lighter d-transition elements but in contrast to the main group elements., QC 20100525

The rates and mechanisms of the electron self-exchange between Np(V) and Np(VI) in solution have been studied with quantum chemical methods and compared with previous results for the U(V)-U(VI) pair. Both outer-sphere and inner-sphere mechanisms have been investigated, the former for the aqua ions, the latter for binuclear complexes containing hydroxide, fluoride, and carbonate as bridging ligand. Solvent effects were calculated using the Marcus equation for the outer-sphere reactions and using a nonequilibrium PCM method for the inner-sphere reactions. The nonequilibrium PCM appeared to overestimate the solvent effect for the outer-sphere reactions. The calculated rate constant for the self-exchange reaction NpO2+(aq)+NpO22+(aq) &REVARR; NPO22+(aq)+NPO2+(aq), at 25° C is k = 67 M-1 s(-1), in fair agreement with the observed rates 0.0063-15 M-1 s-1. The differences between the Np(V)-Np(VI) and the U(V)-U(VI) pairs are minor., QC 20100525

The rates and mechanisms of the electron self-exchange between U(V) and U(VI) in solution have been studied with quantum chemical methods. Both outer-sphere and inner-sphere mechanisms have been investigated; the former for the aqua ions, the latter for binuclear complexes containing hydroxide, fluoride, and carbonate as bridging ligand. The calculated rate constant for the self-exchange reaction UO2+(aq) + UO22+(aq)UO22+(aq) + UO2+(aq), at 25 degreesC, is k = 26 M-1 s(-1). The lower limit of the rate of electron transfer in the inner-sphere complexes is estimated to be in the range 2 x 10(4) to 4 x 10(6) M-1 s(-1), indicating that the rate for the overall exchange reaction may be determined by the rate of formation and dissociation of the binuclear complex. The activation energy for the outer-sphere model calculated from the Marcus model is nearly the same as that obtained by a direct calculation of the precursor- and transition-state energy. A simple model with one water ligand is shown to recover 60% of the reorganization energy. This finding is important because it indicates the possibility to carry out theoretical studies of electron-transfer reactions involving M3+ and M4+ actinide species that have eight or nine water ligands in the first coordination sphere., QC 20100525 QC 20110927

Reaction enthalpies for the reactions 2MO(3)(g) + MF6(g) --> 3MO(2)F(2)(g), MO2F2(9) + 2H(2)O(g) --> MO2(OH)(2)(g) + 2HF(g), MF6(g) + 2H(2)O(g) --> MO2F2(9) + 4HF(g), MO3(g) + H2O(g) --> MO2(OH)(2)(g), and MF6(g) + 3H(2)O (9) --> MO3(g) + 6HF(g) have been calculated at the CCSD(T) level for M = U and Np and at the MP2 level for M = U, Np, and Pu. The results are compared with previous calculated reaction enthalpies for M = U. The errors in the calculated reaction enthalpies are estimated to be below 20 kJ/mol for Np and about 50 kJ/mol for Pu., QC 20100525

The reduction of uranyl U(VI) by Fe(II) in solution has been studied by quantum chemical methods, where the pH dependence of the reaction was simulated by using different numbers of coordinated hydroxide ions. The geometries for the binuclear U(VI)-Fe(II) precursor and the U(V)-Fe(III) successor complexes were optimized at the SCF level, and the reaction energies were calculated at the correlated level using the MP2 method. Effective core potentials were used throughout. Solvent effects were obtained by the polarizable continuum model. The accuracy of the solvent model was investigated for the binuclear complexes with two hydroxide bridges, and the accuracy of the MP2 method was assessed by comparing with CASPT2 and CCSD(T) calculations on the smallest complexes. The general trends in geometry and reaction energy are consistent with experiment., QC 20100525

The thermodynamics and the reaction mechanism for the reaction UO22+(aq) + HF(aq) --> UO2F+(aq) + H+(aq) in water solution has been studied using quantum chemical methods. The solvent was modeled using the polarized medium method (CPCM) with additional water molecules in the second coordination sphere of the complexes studied. The overall reaction was divided into three steps that were analyzed separately. The quantum chemical study was made on the reaction step [UO2(H2O)(5)(2+)],HF(H2O)(n) --> [UO2F (H2O)(4)(+)],H3O+ (H2O)(n), with n = 1 and 2, where the species in the second coordination sphere are located outside the square brackets. The formation of the precursor complex and dissociation of the successor complex were described by the Fuoss equation. The geometry of the different precursor and successor complexes was in good agreement with known bond distances, and strong F---H---O, and/or O---H---O hydrogen bonds are an important structure element in all of them. The Gibbs energy, enthalpy, and entropy of reaction was calculated using the electronic energy at the MP2 level in the solvent, with thermal functions calculated at the SCF/B3LYP levels using the gas-phase geometry. The calculated Gibbs energy of reaction for n = 2 at 298.15 K was -35 kJ/moI at the HF and -25 kJ/moI at the B3LYP level after correction for a known systematic error in the HF bond energy; this compares favorably with the experimental value, -11 kJ/mol. The ligand exchange mechanism was explored by identification of a transition state where HF from the second sphere enters the first coordination sphere in an associative reaction. It was not possible to identify the same transition state from the successor side, indicating that the reaction mechanism consists of at least two steps. We suggest that the rate determining step is the entry of HF from the second to the first coordination sphere, with practically no bond-breaking as indicated by the small change in the H-F distance between precurs, QC 20100525

Ab initio methods were applied for the calculation of the total energy and the molar entropy and heat capacity of the compounds UO2F2, UO2(OH)(2), UF6, and UO3 in the gas phase with the purpose to obtain thermodynamic data for reactions that can be compared with experimental values. The total energy, geometry, and vibration frequencies were calculated at different levels of accuracy: second-order perturbation theory (MP2), coupled cluster theory (CCSD(T)), and density functional theory (B3LYP). Our results agree well with experimental values and previous theoretical results. Additionally, the transition state Of UO2F2(g) was studied and the value of the barrier for the inversion of the fluoride atoms was calculated., QC 20100525

The structures of the complexes UO2Fn(H2O)(5-n)(2-n), n = 3-5, have been studied by EXAFS. All have pentagonal bipyramid geometry with U-F of and U-H2O distances equal to 2.26 and 2.48 Angstrom, respectively. On the other hand the complex UO2(OH)(4)(2-) has a square bipyramid geometry both in the solid state and in solution. The structures of hydroxide and fluoride complexes have also been investigated with wave function based and DFT methods in order to explore the possible reasons for the observed structural differences. These studies include models that describe the solvent by using a discrete second coordination sphere, a model with a spherical, or shape-adapted cavity in a conductor-like polarizable continuum medium (CPCM), or a combination of the two. Solvent effects were shown to give the main contribution to the observed structure variations between the uranium(VI) tetrahydroxide and the tetrafluoride complexes. Without a solvent model both UO2(OH)(4)(H2O)(2-) and UO2F4(H2O)(2-) have the same square bipyramid geometry, with the water molecule located at a distance of more than 4 Angstrom from uranium and with a charge distribution that is very near identical in the two complexes. Of the models tested, only the CPCM ones are able to describe the experimentally observed square and pentagonal bipyramid geometry in the tetrahydroxide and tetrafluoride complexes. The geometry and the relative energy of different isomers of UO2F3(H2O)(2)(-) are very similar, indicating that they are present in comparable amounts in solution. All calculated bond distances are in good agreement with the experimental observations, provided that a proper model of the solvent is used., QC 20100525Correction in: Inorganic Chemistry, Volume 48, Issue2, Page781-781DOI 10.1021/ic801638p, WOS:000262430800046