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Theoretical Actinide Chemistry – Methods and Models
Stockholm University, Faculty of Science, Department of Physics. (Kvantkemi)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The chemistry of actinides in aqueous solution is important, and it is essential to build adequate conceptual models and develop methods applicable for actinide systems. The complex electronic structure makes benchmarking necessary. In the thesis a prototype reaction of the water exchange reaction for uranyl(VI), for both ground and luminescent states, described with a six-water model, was used to study the applicability of density functional methods on actinides and different solvation models. An excellent agreement between the wave function methods CCSD(T) and MP2 was obtained in the ground state, implying that near-minimal CASPT2 can be used with confidence for the reaction in the luminescent state of uranyl(VI), while density functionals are not suited to describe energetics for this type of reaction. There was an ambiguity concerning the position of the waters in the second hydration sphere. This issue was resolved by investigating a larger model, and prop- erly used the six-water model was found to adequately describe the water exchange reaction. The effect of solvation was investigated by comparing the results from conductor-like polarizable continuum models using two cavity models. Scattered numbers made it difficult to determine which solvation model to use. The final conclusion was that the water exchange reaction in the luminescent state of uranyl(VI) should be addressed with near-minimal CASPT2 and a solvation model without explicit cavities for hydrogens. Finally it was shown that no new chemistry appears in the luminescent state for this reaction. The thesis includes a methodological investigation of a multi-reference density functional method based on a range separation of the two-electron interaction. The method depends on a universal parameter, which has been determined for lighter elements. It is shown here that the same parameter could be used for actinides, a prerequisite for further development of the method. The results are in that sense promising.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2011. , 89 p.
Keyword [en]
Actinide, Quantum chemistry, Wave function, Density functional theory, Solvent models, Water exchange, Molecular configurations, Multi-reference density functional method
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-54848ISBN: 978-91-7447-232-5OAI: oai:DiVA.org:su-54848DiVA: diva2:398617
Public defence
2011-03-28, FA32, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2011-03-06 Created: 2011-02-18 Last updated: 2011-03-16Bibliographically approved
List of papers
1. An Investigation of the Accuracy of Different DFT Functionals on the Water Exchange Reaction in Hydrated Uranyl(VI) in the Ground State and the First Excited State
Open this publication in new window or tab >>An Investigation of the Accuracy of Different DFT Functionals on the Water Exchange Reaction in Hydrated Uranyl(VI) in the Ground State and the First Excited State
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2008 (English)In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 4, no 4, 569-577 p.Article in journal (Refereed) Published
Abstract [en]

We discuss the accuracy of density functional theory (DFT) in the gas phase for the water-exchange reactions in the uranyl(VI) aqua ion taking place both in the electronic ground state and in the first excited state (the luminescent 3Δg state). The geometries of the reactant and intermediates have been optimized using DFT and the B3LYP functional, with a restricted closed-shell formalism for the electronic ground state and either an unrestricted open-shell formalism or the time-dependent DFT method for the 3Δg state. The relative energies have been computed with wave-function-based methods such as Møller–Plesset second-order perturbation theory, or a minimal multireference perturbative calculation (minimal CASPT2); coupled-cluster method (CCSD(T)); DFT with B3LYP, BLYP, and BHLYP correlation and exchange functionals; and the hybrid DFT−multireference configuration interaction method. The results obtained with second-order perturbative methods are in excellent agreement with those obtained with the CCSD(T) method. However, DFT methods overestimate the energies of low coordination numbers, yielding to too high and too low reaction energies for the associative and dissociative reactions, respectively. Part of the errors appears to be associated with the amount of Hartree–Fock exchange used in the functional; for the dissociative intermediate in the ground state, the pure DFT functionals underestimate the reaction energy by 20 kJ/mol relative to wave-function-based methods, and when the amount of HF exchange is increased to 20% (B3LYP) and to 50% (BHLYP), the error is decreased to 13 and 4 kJ/mol, respectively.

National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-54856 (URN)10.1021/ct700062x (DOI)000254710700003 ()
Available from: 2011-02-18 Created: 2011-02-18 Last updated: 2015-07-29Bibliographically approved
2. On the combined use of discrete solvent models and continuum descriptions of solvent effects in ligand exchange reactions: a case study of the uranyl(VI) aquo ion
Open this publication in new window or tab >>On the combined use of discrete solvent models and continuum descriptions of solvent effects in ligand exchange reactions: a case study of the uranyl(VI) aquo ion
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2009 (English)In: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 124, no 5-6, 377-384 p.Article in journal (Refereed) Published
Abstract [en]

Modeling of the solvent is important when using quantum chemical methods for the assignment of mechanisms from experimental studies of the exchange of water between metal aquo ions and the bulk solvent. In the present study, we have investigated if and how the mechanisms for water exchange in the UO2(OH2)52+–H2O system is affected by the choice of chemical models for the second coordination sphere and physical models for describing the cavity in conductor-like polarizable continuum (CPCM) models. In the first case, we have compared models with one and five waters in the second coordination sphere. For both models, we have compared cavities in which each atom is assigned one spherical cavity and one in which the water molecules are described by a single spherical cavity (the United Atom model). There are significant differences in the relative energy of dissociative and associative intermediates; however, they are not large enough to affect the conclusion that the water exchange proceeds through an associative/interchange mechanism.

Keyword
Actinide, Quantum chemistry, Solvent models, Water exchange
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-54864 (URN)10.1007/s00214-009-0627-8 (DOI)000271539400008 ()
Available from: 2011-02-18 Created: 2011-02-18 Last updated: 2015-07-29Bibliographically approved
3. Water Exchange Mechanism in the First Excited State of Hydrated Uranyl(VI)
Open this publication in new window or tab >>Water Exchange Mechanism in the First Excited State of Hydrated Uranyl(VI)
2009 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 48, no 23, 11310-11313 p.Article in journal (Refereed) Published
Abstract [en]

The water exchange mechanism of the uranyl(VI) aquo ion in the luminescent state, 3Δg in the spin-orbit free nomenclature, has been investigated using quantum chemical methods and compared to the corresponding reaction in the electronic ground state. The reaction mechanism was studied by calculation of the enthalpy of reaction of the A- and D- intermediates relative to the reactant, using a penta-aquo ion model with one additional water molecule in the second hydration sphere. The reaction barriers around the intermediates are low, and they are therefore a good approximation for the activation enthalpy. The energy of the D-intermediate is significantly larger than that of the A-intermediate both in the luminescent and in the ground states, suggesting that the water exchange is the same in both states. This suggestion is supported by the experimental rate constants for luminescence decay and water exchange in the electronic ground state that are 0.5 x 106 s-1 and 1.3 x 106 s-1, respectively.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-54865 (URN)10.1021/ic9017689 (DOI)000272037500062 ()
Available from: 2011-02-18 Created: 2011-02-18 Last updated: 2015-07-29Bibliographically approved
4. On the universality of the long-/short-range separation in multiconfigurational density-functional theory. II. Investigating f0 actinide species
Open this publication in new window or tab >>On the universality of the long-/short-range separation in multiconfigurational density-functional theory. II. Investigating f0 actinide species
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2009 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, 054107Article in journal (Refereed) Published
Abstract [en]

In a previous paper [ Fromager et al., J. Chem. Phys. 126, 074111 (2007) ], some of the authors proposed a recipe for choosing the optimal value of the μ parameter that controls the long-range/short-range separation of the two-electron interaction in hybrid multiconfigurational self-consistent field short-range density-functional theory (MC-srDFT) methods. For general modeling with MC-srDFT methods, it is clearly desirable that the same universal value of μ can be used for any molecule. Their calculations on neutral light element compounds all yielded μopt = 0.4 a.u. In this work the authors investigate the universality of this value by considering “extreme” study cases, namely, neutral and charged isoelectronic f0 actinide compounds (ThO2, PaO2+, UO22+, UN2, CUO, and NpO23+). We find for these compounds that μopt = 0.3 a.u. but show that 0.4 a.u. is still acceptable. This is a promising result in the investigation of a universal range separation. The accuracy of the currently best MC-srDFT (μ = 0.3 a.u.) approach has also been tested for equilibrium geometries. Though it performs as well as wave function theory and DFT for static-correlation-free systems, it fails in describing the neptunyl (VII) ion NpO23+ where static correlation is significant; bending is preferred at the MC-srDFT (μ = 0.3 a.u.) level, whereas the molecule is known to be linear. This clearly shows the need for better short-range functionals, especially for the description of the short-range exchange. It also suggests that the bending tendencies observed in DFT for NpO23+ cannot be fully explained by the bad description of static correlation effects by standard functionals. A better description of the exchange seems to be essential too.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-54866 (URN)10.1063/1.3187032 (DOI)
Available from: 2011-02-18 Created: 2011-02-18 Last updated: 2015-07-29Bibliographically approved

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