Excited states of polonium(IV): Electron correlation and spin-orbit coupling in the Po^{4+} free ion and in the bare and solvated [PoCl5]^- and [PoCl6]^{2-} complexes

Polonium (Po, Z = 84) is a main-block element with poorly known physico-chemical properties. Not much information has been firmly acquired since its discovery by Marie and Pierre Curie in 1898, especially regarding its speciation in aqueous solution and spectroscopy. In this work, we revisit the absorption properties of two complexes, [PoCl5]^- and [PoCl6]^{2-}, using quantum mechanical calculations. These complexes have the potential to exhibit a maximum absorption at 418 nm in HCl medium (for 0.5 mol/L concentrations and above). Initially, we examine the electronic spectra of the Po^{4+} free ion and of its isoelectronic analogue, Bi^{3+}. In the spin-orbit configuration interaction (SOCI) framework. Our findings demonstrate that the SOCI matrix should be dressed with correlated electronic energies and that the quality of the spectra is largely improved by decontracting the reference states at the complete active space plus singles (CAS+S) level. Subsequently, we investigate the absorption properties of the [PoCl5]^- and [PoCl6]^{2-} complexes in two stages. Firstly, we perform methodological tests at the MP2/def2-TZVP gas phase geometries, indicating that the decontraction of the reference states can there be skipped without compromising the accuracy significantly. Secondly, we study the solution absorption properties by means of single-point calculations performed at the solvated geometries, obtained by an implicit solvation treatment or a combination of implicit and explicit solvation. Our results highlight the importance of saturating the first coordination sphere of the Po^{IV} ion to obtain a qualitatively correct picture. Finally, we conclude that the known-for-decades 418 nm peak could be attributed to a mixture of both the [PoCl5(H2O)]^- and [PoCl6]^{2-} complexes. This finding not only aligns with the behaviour of the analogous Bi^{III} ion under similar conditions but..

Modélisation de la chimie du polonium en solution

While discovered long ago, the chemistry of the polonium element remains unclear. Since experimental studies are dangerous and expensive, theoretical chemistry may help in selecting wiser experiments. This work presents the a fundamental study of polonium chemistry by means of wave-function based calculations inclusive of relativistic effects. To begin with, a general overview of the current knowledge is given. Then, a description of the used methods and their potential alternatives is given. The first results section presents a systematic study of polonium(IV) complexes, namely hydrated and chlorinated ones. Their geometries and interaction energies are investigated, a bonding analysis is then carried out and the spin-orbit coupling importance is finally probed on certain species. The next part revisits the absorption properties of polonium complexes, namely [PoCl5]- and [PoCl6]2-. Starting with the bare Po4+ ion, the investigation then develops on relevant gas phase and solvated complexes. Finally, we propose a new attribution to an experimentally observed UV-Vis peak, as a mixture of two polonium complexes, [PoCl5(H2O)]- and [PoCl6]2-.Bien que découvert il y a longtemps, la chimie de l’élément polonium reste floue. Les études expérimentales étant dangereuses et coûteuses, la chimie théorique peut aider à sélectionner des expériences plus judicieuses. Ce travail présente une étude fondamentale de la chimie du polonium au moyen de calculs basés sur la fonction d'onde incluant les effets relativistes. Dans un premier temps, un aperçu général des connaissances actuelles est donné. Ensuite, une description des méthodes utilisées et de leurs alternatives potentielles est donnée. La première section de résultats présente une étude systématique de complexes du polonium(IV), notamment hydratés et chlorés. Leurs géométries et énergies d'interaction sont étudiées, une analyse de liaison est ensuite réalisée et l'importance du couplage spin-orbite est enfin sondée sur certaines espèces. La partie suivante revisite les propriétés d'absorption de complexes de polonium, à savoir [PoCl5]- et [PoCl6]2-. En commençant par l'ion libre Po4+, l’étude se poursuit ensuite sur des complexes pertinents en phase gazeuse et en présence de molécules d’eau, le solvant. Enfin, nous proposons une nouvelle attribution à un pic UV-Vis observé expérimentalement, faisant l’hypothèse d’un mélange de deux complexes du polonium, [PoCl5(H2O)]- et [PoCl6]2-

Modélisation de la chimie du polonium en solution

Bien que découvert il y a longtemps, la chimie de l’élément polonium reste floue. Les études expérimentales étant dangereuses et coûteuses, la chimie théorique peut aider à sélectionner des expériences plus judicieuses. Ce travail présente une étude fondamentale de la chimie du polonium au moyen de calculs basés sur la fonction d'onde incluant les effets relativistes. Dans un premier temps, un aperçu général des connaissances actuelles est donné. Ensuite, une description des méthodes utilisées et de leurs alternatives potentielles est donnée. La première section de résultats présente une étude systématique de complexes du polonium(IV), notamment hydratés et chlorés. Leurs géométries et énergies d'interaction sont étudiées, une analyse de liaison est ensuite réalisée et l'importance du couplage spin-orbite est enfin sondée sur certaines espèces. La partie suivante revisite les propriétés d'absorption de complexes de polonium, à savoir [PoCl5]- et [PoCl6]2-. En commençant par l'ion libre Po4+, l’étude se poursuit ensuite sur des complexes pertinents en phase gazeuse et en présence de molécules d’eau, le solvant. Enfin, nous proposons une nouvelle attribution à un pic UV-Vis observé expérimentalement, faisant l’hypothèse d’un mélange de deux complexes du polonium, [PoCl5(H2O)]- et [PoCl6]2-.While discovered long ago, the chemistry of the polonium element remains unclear. Since experimental studies are dangerous and expensive, theoretical chemistry may help in selecting wiser experiments. This work presents the a fundamental study of polonium chemistry by means of wave-function based calculations inclusive of relativistic effects. To begin with, a general overview of the current knowledge is given. Then, a description of the used methods and their potential alternatives is given. The first results section presents a systematic study of polonium(IV) complexes, namely hydrated and chlorinated ones. Their geometries and interaction energies are investigated, a bonding analysis is then carried out and the spin-orbit coupling importance is finally probed on certain species. The next part revisits the absorption properties of polonium complexes, namely [PoCl5]- and [PoCl6]2-. Starting with the bare Po4+ ion, the investigation then develops on relevant gas phase and solvated complexes. Finally, we propose a new attribution to an experimentally observed UV-Vis peak, as a mixture of two polonium complexes, [PoCl5(H2O)]- and [PoCl6]2-

Modélisation de la chimie du polonium en solution

While discovered long ago, the chemistry of the polonium element remains unclear. Since experimental studies are dangerous and expensive, theoretical chemistry may help in selecting wiser experiments. This work presents the a fundamental study of polonium chemistry by means of wave-function based calculations inclusive of relativistic effects. To begin with, a general overview of the current knowledge is given. Then, a description of the used methods and their potential alternatives is given. The first results section presents a systematic study of polonium(IV) complexes, namely hydrated and chlorinated ones. Their geometries and interaction energies are investigated, a bonding analysis is then carried out and the spin-orbit coupling importance is finally probed on certain species. The next part revisits the absorption properties of polonium complexes, namely [PoCl5]- and [PoCl6]2-. Starting with the bare Po4+ ion, the investigation then develops on relevant gas phase and solvated complexes. Finally, we propose a new attribution to an experimentally observed UV-Vis peak, as a mixture of two polonium complexes, [PoCl5(H2O)]- and [PoCl6]2-.Bien que découvert il y a longtemps, la chimie de l’élément polonium reste floue. Les études expérimentales étant dangereuses et coûteuses, la chimie théorique peut aider à sélectionner des expériences plus judicieuses. Ce travail présente une étude fondamentale de la chimie du polonium au moyen de calculs basés sur la fonction d'onde incluant les effets relativistes. Dans un premier temps, un aperçu général des connaissances actuelles est donné. Ensuite, une description des méthodes utilisées et de leurs alternatives potentielles est donnée. La première section de résultats présente une étude systématique de complexes du polonium(IV), notamment hydratés et chlorés. Leurs géométries et énergies d'interaction sont étudiées, une analyse de liaison est ensuite réalisée et l'importance du couplage spin-orbite est enfin sondée sur certaines espèces. La partie suivante revisite les propriétés d'absorption de complexes de polonium, à savoir [PoCl5]- et [PoCl6]2-. En commençant par l'ion libre Po4+, l’étude se poursuit ensuite sur des complexes pertinents en phase gazeuse et en présence de molécules d’eau, le solvant. Enfin, nous proposons une nouvelle attribution à un pic UV-Vis observé expérimentalement, faisant l’hypothèse d’un mélange de deux complexes du polonium, [PoCl5(H2O)]- et [PoCl6]2-

Geometries, interaction energies and bonding in [Po(H₂O)_n]⁴⁺ and [PoCl_n]^4-n complexes

Polonium (Z = 84) is one of the rarest elements on Earth. More than a century after its discovery, its chemistry remains poorly known and even basic questions are not yet satisfactorily addressed. In this work, we perform a systematic study of the geometries, interactions energies and bonding in basic polonium(IV) species, namely the hydrated [Po(H2O)N]4+ and chlorinated [PoCl N]4-n complexes by means of gas-phase electronic structure calculations. We show that while up to nine water molecules can fit in the first coordination sphere of the polonium(IV) ion, its coordination sphere can already be filled with eight chloride ligands. Capitalising on previous theoretical studies, a focused methodological study based on interaction energies and bond distances allows us to validate the MP2/def2- TZVP level of theory for future ground-state studies. After discussing similarities and differences between complexes with the same number of ligands, we perform topological analyses of the MP2 electron densities in the quantum theory of atoms in molecules (QTAIM) fashion. While the water complexes display typical signatures of closed-shell interactions, we reveal large Po-Cl delocalisation indices, especially in the hypothetical [PoCl]3+ complex. This "enhanced" covalency opens the way for a significant spin-orbit coupling (SOC) effect on the corresponding bond distance, which has been studied by two independent approaches (i.e. one a priori and one a posteriori). We finally conclude by stressing that while the SOC may not affect much the geometries of high-coordinated polonium(IV) complexes, it should definitely not be neglected in the case of low-coordinated ones

Geometries, interaction energies and bonding in [Po(H₂O)_n]⁴⁺ and [PoCl_n]^4-n complexes

Polonium (Z = 84) is one of the rarest elements on Earth. More than a century after its discovery, its chemistry remains poorly known and even basic questions are not yet satisfactorily addressed. In this work, we perform a systematic study of the geometries, interactions energies and bonding in basic polonium(IV) species, namely the hydrated [Po(H2O)N]4+ and chlorinated [PoCl N]4-n complexes by means of gas-phase electronic structure calculations. We show that while up to nine water molecules can fit in the first coordination sphere of the polonium(IV) ion, its coordination sphere can already be filled with eight chloride ligands. Capitalising on previous theoretical studies, a focused methodological study based on interaction energies and bond distances allows us to validate the MP2/def2- TZVP level of theory for future ground-state studies. After discussing similarities and differences between complexes with the same number of ligands, we perform topological analyses of the MP2 electron densities in the quantum theory of atoms in molecules (QTAIM) fashion. While the water complexes display typical signatures of closed-shell interactions, we reveal large Po-Cl delocalisation indices, especially in the hypothetical [PoCl]3+ complex. This "enhanced" covalency opens the way for a significant spin-orbit coupling (SOC) effect on the corresponding bond distance, which has been studied by two independent approaches (i.e. one a priori and one a posteriori). We finally conclude by stressing that while the SOC may not affect much the geometries of high-coordinated polonium(IV) complexes, it should definitely not be neglected in the case of low-coordinated ones

Geometries, interaction energies and bonding in [Po(H₂O)_n]⁴⁺ and [PoCl_n]^4-n complexes

Polonium (Z = 84) is one of the rarest elements on Earth. More than a century after its discovery, its chemistry remains poorly known and even basic questions are not yet satisfactorily addressed. In this work, we perform a systematic study of the geometries, interactions energies and bonding in basic polonium(IV) species, namely the hydrated [Po(H2O)N]4+ and chlorinated [PoCl N]4-n complexes by means of gas-phase electronic structure calculations. We show that while up to nine water molecules can fit in the first coordination sphere of the polonium(IV) ion, its coordination sphere can already be filled with eight chloride ligands. Capitalising on previous theoretical studies, a focused methodological study based on interaction energies and bond distances allows us to validate the MP2/def2- TZVP level of theory for future ground-state studies. After discussing similarities and differences between complexes with the same number of ligands, we perform topological analyses of the MP2 electron densities in the quantum theory of atoms in molecules (QTAIM) fashion. While the water complexes display typical signatures of closed-shell interactions, we reveal large Po-Cl delocalisation indices, especially in the hypothetical [PoCl]3+ complex. This "enhanced" covalency opens the way for a significant spin-orbit coupling (SOC) effect on the corresponding bond distance, which has been studied by two independent approaches (i.e. one a priori and one a posteriori). We finally conclude by stressing that while the SOC may not affect much the geometries of high-coordinated polonium(IV) complexes, it should definitely not be neglected in the case of low-coordinated ones

Modeling polonium complexes in solution

International audiencePolonium is a heavy and rare element standing under the number 84 on the Periodic Table. It is a naturally occurring element, though its abundance is among the weakest ones. It is highly toxic, which hinders experimental studies aiming at revealing its properties. Nevertheless, polonium was used as a heat source in the space industry, as an alpha-emission source, or as a static eliminator, before cheaper and safer alternatives replaced it. It is also a famous poison, with currently no good enough antidote. A better understanding of its chemistry could potentially help in better detecting it and also characterizing its speciation in natural media and industry and also in designing better cure in the case of ingestion. However, the available quantities and the complex manipulation of this element make its characterization arduous. In this context, we perform quantum chemistry calculations to predict the properties of polonium species. In our project, we study polonium complexes in the gas phase and in solution. We consider its most stable oxidation state in solution, i.e. +IV. Following previous work [1,2], two families of complexes were first chosen for performing a methodological study in the gas phase. While the [Po(H2O)n]4+ (n = 1–9) series was selected to get fundamental knowledge of hydration, the chlorinated [PoClm]4–m (m = 1–8) complexes were studied because of experimental evidence for the formation of such species in solution. As an extension to a benchmark study in the gas phase on geometries and interaction energies, ground state bonding descriptors were obtained [3]. Therefore, information on coordination numbers and chemical bonding were both derived. Following the idea to connect with established experimental data, state-of-the-art excited-state calculations have been performed on two potentially occurring chlorinated complexes, revealing once more the difficulty of interpreting from scratch the few spectra that are available for polonium species. We thus emphasize the need for performing more calculations of predictive quality to unravel this enigmatic chemistry. References:[1] R. Ayala, J. M. Martinez, R. R. Pappalardo, A. Muñoz-Paez and E. Marcos Sánchez, J. Phys. Chem. B, 2008, 112, 5416–5422.[2] A. Stoanov, J. Champion and R. Maurice, Inorg. Chem., 2019, 58, 7036-7043.[3] N. Zhutova, F. Réal, V. Vallet and R. Maurice, Phys. Chem. Chem. Phys., 2022, 24, 26180-26189

Modelling polonium complexes in solution

National audiencePolonium is one of the rarest and most toxic elements found in nature. Despite being discovered in 1898, it is still scarcely studied due to its dangerous properties. As far as one can understand, conducting experiments is not the safest way to shed light on it- even at trace concentrations, its most widely available isotope is lethal by ingestion or inhalation. Thus, knowledge of polonium chemistry is pretty narrow.The goal of this study is to predict the theoretical chemical behavior of polonium in solution without performing any experimental work, thus saving the experimenter’s health, lab equipment, and money. This is achieved by doing theoretical chemical calculations using methods of quantum mechanics.By using these, we investigate the behavior of polonium with two different ligands- water and chlorine ions, since the first type of complexes should be common in nature [1] and the second one has been experimentally reported [2]. Here we present our preliminary results regarding geometry, energies, and bond properties of the complexes calculated in the gas phase. This data is a base for further analysis of polonium chemistry in the solvent.[1] R. Ayala et al., J. Phys. Chem. B., 112 (2008): 5416-5422.[2] A. Stoïanov et al., Inorg. Chem., 58 (2019): 7036-7043

Modeling polonium complexes in solution

International audiencePolonium is a heavy and rare element standing under the number 84 on the Periodic Table. It is a naturally occurring element, though its abundance is among the weakest ones. It is highly toxic, which hinders experimental studies aiming at revealing its properties. Nevertheless, polonium was used as a heat source in the space industry, as an alpha-emission source, or as a static eliminator, before cheaper and safer alternatives replaced it. It is also a famous poison, with currently no good enough antidote. A better understanding of its chemistry could potentially help in better detecting it and also characterizing its speciation in natural media and industry and also in designing better cure in the case of ingestion. However, the available quantities and the complex manipulation of this element make its characterization arduous. In this context, we perform quantum chemistry calculations to predict the properties of polonium species. In our project, we study polonium complexes in the gas phase and in solution. We consider its most stable oxidation state in solution, i.e. +IV. Following previous work [1,2], two families of complexes were first chosen for performing a methodological study in the gas phase. While the [Po(H2O)n]4+ (n = 1–9) series was selected to get fundamental knowledge of hydration, the chlorinated [PoClm]4–m (m = 1–8) complexes were studied because of experimental evidence for the formation of such species in solution. As an extension to a benchmark study in the gas phase on geometries and interaction energies, ground state bonding descriptors were obtained [3]. Therefore, information on coordination numbers and chemical bonding were both derived. Following the idea to connect with established experimental data, state-of-the-art excited-state calculations have been performed on two potentially occurring chlorinated complexes, revealing once more the difficulty of interpreting from scratch the few spectra that are available for polonium species. We thus emphasize the need for performing more calculations of predictive quality to unravel this enigmatic chemistry. References:[1] R. Ayala, J. M. Martinez, R. R. Pappalardo, A. Muñoz-Paez and E. Marcos Sánchez, J. Phys. Chem. B, 2008, 112, 5416–5422.[2] A. Stoanov, J. Champion and R. Maurice, Inorg. Chem., 2019, 58, 7036-7043.[3] N. Zhutova, F. Réal, V. Vallet and R. Maurice, Phys. Chem. Chem. Phys., 2022, 24, 26180-26189