Scrutinizing "Invisible" astatine: A challenge for modern density functionals

Uncovering chemical homology of superheavy elements: a close look at astatine

The fascination with superheavy elements (SHE) spans the nuclear physics, astrophysics, and theoretical chemistry communities. Extreme relativistic effects govern these elements' chemistry and challenge the traditional notion of the periodic law. The experimental quest for SHE critically depends on theoretical predictions of these elements' properties, especially chemical homology, which allows for successful prototypical experiments with more readily available lighter homologues of SHE. This work is a comprehensive quantum-chemical investigation into astatine (At) as a non-intuitive homologue of element 113, nihonium (Nh). Combining relativistic coupled-cluster and density functional theory approaches, we model the behaviour of At and AtOH in thermochromatographic experiments on a pristine gold surface. Insights into the electronic structure of AtOH and NhOH and accurate estimates of At-gold and AtOH-gold adsorption energies rationalise recent experimental findings and justify the use of At as a chemical homologue of Nh for the successful design of future experiments on Nh detection and chemical characterisation.

The Chemical Bond at the Bottom of the Periodic Table: The Case of the Heavy Astatine and the Super-Heavy Tennessine

In this work, we study the chemical bond in molecules containing heavy and super-heavy elements according to the current state-of-the-art bonding models. An Energy Decomposition Analysis in combination with Natural Orbital for Chemical Valence (EDA-NOCV) within the relativistic four-component Dirac-Kohn-Sham (DKS) framework is employed, which allows to successfully include the spin-orbit coupling (SOC) effects on the chemical bond description. Simple halogen-bonded adducts ClX⋯L (X=At, Ts; L=NH3, Br-, H2O, CO) of astatine and tennessine have been selected to assess a trend on descending along a group, while modulating the ClX⋯L bond features through the different electronic nature of the ligand L. Interesting effects caused by SOC have been revealed: i) a huge increase of the ClTs dipole moment (which is almost twice as that of ClAt), ii) a lowering of the ClX⋯L bonding energy arising from different contributions to the ClX…L interaction energy strongly depending on the nature of L, iii) a quenching of one of the π back-donation components to the bond. In the ClTs(CO) adduct, the back-donation from ClTs to CO becomes the most important component. The analysis of the electronic structure of the ClX dimers allows for a clear interpretation of the SOC effects in these systems.

Estimation of thermodynamic and physicochemical properties of the alkali astatides: On the bond strength of molecular astatine (At2 ) and the hydration enthalpy of astatide (At- )

The recent accurate and precise determination of the electron affinity (EA) of the astatine atom At0 warrants a re-investigation of the estimated thermodynamic properties of At0 and astatine containing molecules as this EA was found to be much lower (by 0.4 eV) than previous estimated values. In this contribution we estimate, from available data sources, the following thermodynamic and physicochemical properties of the alkali astatides (MAt, M = Li, Na, K, Rb, Cs): their solid and gaseous heats of formation, lattice and gas-phase binding enthalpies, sublimation energies and melting temperatures. Gas-phase charge-transfer dissociation energies for the alkali astatides (the energy requirement for M+ At- ➔ M0  + At0 ) have been obtained and are compared with those for the other alkali halides. Use of Born-Haber cycles together with the new AE (At0 ) value allows the re-evaluation of ΔHf (At0 )g (=56 ± 5 kJ/mol); it is concluded that (At2 )g is a weakly bonded species (bond strength <50 kJ/mol), significantly weaker bonded than previously estimated (116 kJ/mol) and much weaker bonded than I2 (148 kJ/mol), but in agreement with the finding from theory that spin-orbit coupling considerably reduces the bond strength in At2 . The hydration enthalpy (ΔHaq ) of At- is estimated to be -230 ± 2 kJ/mol (using ΔHaq [H+ ] = -1150.1 kJ/mol), in good agreement with molecular dynamics calculations. Arguments are presented that the largest alkali halide, CsAt, like the smallest, LiF, will be only sparingly soluble in water, following the generalization from hard/soft acid/base principles that "small likes small" and "large likes large."

In vivo stability of 211At-radiopharmaceuticals: on the impact of halogen bond formation

211At, when coupled to a targeting agent, is one of the most promising radionuclides for therapeutic applications. The main labelling approach consists in the formation of astatoaryl compounds, which often show a lack of in vivo stability. The hypothesis that halogen bond (XB) interactions with protein functional groups initiate a deastatination mechanism is investigated through radiochemical experiments and DFT modelling. Several descriptors agree on the known mechanism of iodoaryl substrates dehalogenation by iodothyronine deiodinases, supporting the higher in vivo dehalogenation of N-succinimidyl 3-[211At]astatobenzoate (SAB) conjugates in comparison with their iodinated counterparts. The guanidinium group in 3-[211At]astato-4-guanidinomethylbenzoate (SAGMB) prevents the formation of At-mediated XBs with the selenocysteine active site in iodothyronine deiodinases. The initial step of At-aryl bond dissociation is inhibited, elucidating the better in vivo stability of SAGMB conjugates compared with those of SAB. The impact of astatine's ability to form XB interactions on radiopharmaceutical degradation may not be limited to the case of aryl radiolabeling.

Rigorous Negative Ion Binding Energies in Low-Energy Electron Elastic Collisions with Heavy Multi-Electron Atoms and Fullerene Molecules: Validation of Electron Affinities

Dramatically sharp resonances manifesting stable negative ion formation characterize Regge pole-calculated low-energy electron elastic total cross sections (TCSs) of heavy multi-electron systems. The novelty of the Regge pole analysis is in the extraction of rigorous and unambiguous negative ion binding energies (BEs), corresponding to the measured electron affinities (EAs) of the investigated multi-electron systems. The measured EAs have engendered the crucial question: is the EA of multi-electron atoms and fullerene molecules identified with the BE of the attached electron in the ground, metastable or excited state of the formed negative ion during a collision? Inconsistencies in the meaning of the measured EAs are elucidated and new EA values for Bk, Cf, Fm, and Lr are presented.

Xenon Derivatives as Aerogen Bond-Donating Catalysts for Organic Transformations: A Theoretical Study on the Metaphorical "Spherical Cow in a Vacuum" Provides Insights into Noncovalent Organocatalysis

Computations indicate that cationic and noncharged xenon derivatives should exhibit higher catalytic activity than their iodine-based noncovalent organocatalytic congeners. Perfluorophenyl xenonium(II) is expected to demonstrate the best balance between catalytic activity and chemical stability for use in organocatalysis. Comparing its catalytic activity with that of isoelectronic perfluoroiodobenzene indicates that the high catalytic activity of cationic noncovalent organocatalysts is predominantly attributed to the electrostatic interactions with the reaction substrates, which cause the polarization of ligated species during the reaction progress. In contrast, the electron transfer and covalent contributions to the bonding between the catalyst and substrate have negligible effects. The dominant effect of electrostatic interactions results in a strong negative correlation between the calculated Gibbs free energies of activation for the modeled reactions and the highest potentials of the σ-holes on the central atoms of the catalysts. No such correlation is observed for noncharged catalysts.

Investigating the Heaviest Halogen: Lessons Learned from Modeling the Electronic Structure of Astatine's Small Molecules

We present a systematic study of electron-correlation and relativistic effects in diatomic molecular species of the heaviest halogen astatine (At) within relativistic single- and multireference coupled-cluster approaches and relativistic density functional theory. We establish revised reference ab initio data for the ground states of At₂, HAt, AtAu, and AtO⁺ using a highly accurate relativistic effective core potential model and in-house basis sets developed for accurate modeling of molecules with large spin-orbit effects. Spin-dependent relativistic effects on chemical bonding in the ground state are comparable to the binding energy or even exceed it in At₂. Electron-correlation effects near the equilibrium internuclear separation are mostly dynamical and can be adequately captured using single-reference CCSD(T). However, bond elongation in At₂ and, especially, AtO⁺ results in rapid manifestation of its multireference character. While useful for evaluating the spin-orbit effects on the ground-state bonding and properties, the two-component density functional theory lacks predictive power, especially in combination with popular empirically adjusted exchange-correlation functionals. This drawback supports the necessity to develop new functionals for reliable quantum-chemical models of heavy-element compounds with strong relativistic effects.

An expanded halogen bonding scale using astatine

As a non-covalent interaction, halogen bonding is now acknowledged to be useful in all fields where the control of intermolecular recognition plays a pivotal role. Halogen-bond basicity scales allow quantification of the halogen bonding of referential donors with organic functional groups from a thermodynamic point of view. Herein we present the pK BAtI basicity scale to provide the community an overview of halogen-bond acceptor strength towards astatine, the most potent halogen-bond donor element. This experimental scale is erected on the basis of complexation constants measured between astatine monoiodide (AtI) and sixteen selected Lewis bases. It spans over 6 log units and culminates with a value of 5.69 ± 0.32 for N,N,N',N'-tetramethylthiourea. On this scale, the carbon π-bases are the weakest acceptors, the oxygen derivatives cover almost two-thirds of the scale, and sulphur bases exhibit the highest AtI basicity. Regarding the applications of 211At in targeted radionuclide therapy, stronger labelling of carrier agents could be envisaged on the basis of the pK BAtI scale.

Astatine Facing Janus: Halogen Bonding vs. Charge-Shift Bonding

The nature of halogen-bond interactions was scrutinized from the perspective of astatine, potentially the strongest halogen-bond donor atom. In addition to its remarkable electronic properties (e.g., its higher aromaticity compared to benzene), C₆At₆ can be involved as a halogen-bond donor and acceptor. Two-component relativistic calculations and quantum chemical topology analyses were performed on C₆At₆ and its complexes as well as on their iodinated analogues for comparative purposes. The relativistic spin–orbit interaction was used as a tool to disclose the bonding patterns and the mechanisms that contribute to halogen-bond interactions. Despite the stronger polarizability of astatine, halogen bonds formed by C₆At₆ can be comparable or weaker than those of C₆I₆. This unexpected finding comes from the charge-shift bonding character of the C–At bonds. Because charge-shift bonding is connected to the Pauli repulsion between the bonding σ electrons and the σ lone-pair of astatine, it weakens the astatine electrophilicity at its σ-hole (reducing the charge transfer contribution to halogen bonding). These two antinomic characters, charge-shift bonding and halogen bonding, can result in weaker At-mediated interactions than their iodinated counterparts.

Delocalized relativistic effects, from the viewpoint of halogen bonding

The ability of organic and inorganic compounds bearing both iodine and astatine atoms to form halogen-bond interactions is theoretically investigated. Upon inclusion of the relativistic spin-orbit interaction, the I-mediated halogen bonds are more affected than the At-mediated ones in many cases. This unusual outcome is disconnected from the behavior of iodine's electrons. The significant decrease of astatine electronegativity with the spin-orbit coupling triggers a redistribution of the electron density, which propagates relativistic effects toward the distant iodine atom. This mechanism can be controlled by introducing suitable substituents and, in particular, strengthened by taking advantage of electron-withdrawing inductive and mesomeric effects. Noticeable relativistic effects can actually be transferred to light atoms properties, e.g., the halogen-bond basicity of bridgehead carbon atoms doubled in propellane derivatives.

The electron affinity of astatine

One of the most important properties influencing the chemical behavior of an element is the electron affinity (EA). Among the remaining elements with unknown EA is astatine, where one of its isotopes, 211At, is remarkably well suited for targeted radionuclide therapy of cancer. With the At- anion being involved in many aspects of current astatine labeling protocols, the knowledge of the electron affinity of this element is of prime importance. Here we report the measured value of the EA of astatine to be 2.41578(7) eV. This result is compared to state-of-the-art relativistic quantum mechanical calculations that incorporate both the Breit and the quantum electrodynamics (QED) corrections and the electron-electron correlation effects on the highest level that can be currently achieved for many-electron systems. The developed technique of laser-photodetachment spectroscopy of radioisotopes opens the path for future EA measurements of other radioelements such as polonium, and eventually super-heavy elements.

Astatine partitioning between nitric acid and conventional solvents: indication of covalency in ketone complexation of AtO. Chem Commun (Camb) 2020;56:9004-9007. [PMID: 32638758 DOI: 10.1039/d0cc03804k]

Astatine-211 has been produced at Texas A&M University on the K150 cyclotron, with a yield of 890 ± 80 MBq through the ²⁰⁹Bi(α,2n)²¹¹At reaction via an 8 h bombardment with a beam current of 4-8 μA and an α-particle beam energy of 28.8 MeV. The target was then dissolved in HNO₃ and the extraction of ²¹¹At was investigated into a variety of organic solvents in 1-3 M HNO₃. Extraction of ²¹¹At with distribution ratios as high as 11.3 ± 0.6, 12.3 ± 0.8, 42.2 ± 2.2, 69 ± 4, and 95 ± 6 were observed for diisopropyl ether, 1-decanol, 1-octanol, 3-octanone, and methyl isobutyl ketone, respectively, while the distribution ratios for ²⁰⁷Bi were ≤0.05 in all cases. The extraction of ²¹¹At into both methyl isobutyl ketone and 3-octanone showed a strong, linear dependence on the HNO₃ initial aqueous concentration and better extraction than other solvents. DFT calculations show stronger binding between the carbonyl oxygen of the ketone and the At metal center.

Towards a Stronger Halogen Bond Involving Astatine: Unexpected Adduct with Bu3 PO Stabilized by Hydrogen Bonding

The halogen bond is a powerful tool for the molecular design and pushing the limits of its strength is of major interest. Bearing the most potent halogen-bond donor atom, astatine monoiodide (AtI) was recently successfully probed [Nat. Chem. 2018, 10, 428-434]. In this work, we continue the exploration of adducts between AtI and Lewis bases with the tributylphosphine oxide (Bu₃ PO) ligand, revealing the unexpected experimental occurrence of two distinct chemical species with 1:1 and 2:1 stoichiometries. The 1:1 Bu₃ PO⋅⋅⋅AtI complex is found to exhibit the strongest astatine-mediated halogen bond so far (with a formation constant of 10^(4.24±0.35) ). Quantum chemical calculations unveil the intriguing nature of the 2:1 2Bu₃ PO⋅⋅⋅AtI adduct, involving a halogen bond between AtI and one Bu₃ PO molecular unit plus CH⋅⋅⋅O hydrogen bonds chelating the second Bu₃ PO unit.

Spin-orbit coupling is the key to unraveling intriguing features of the halogen bond involving astatine

The effect of spin-orbit coupling (SOC) on the halogen bond involving astatine has been investigated using state-of-the-art two- and four-component relativistic calculations. Adducts between Cl-X (X = Cl, Br, I and At) and ammonia have been selected to establish a trend on going down the periodic table. The SOC influence has been explored not only on the geometric and energetic features that can be used to characterize the halogen bond strength but also on the three main contributions to it that are the charge transfer, the "σ-hole" (i.e. the localized region with a net positive electrostatic potential at the halogen site) and the "polar flattening" (which is related to the effective shape of the halogen site). A surprisingly large increase of the Cl-At dipole moment, due to the inclusion of SOC, has been worked out using four-component CCSD(T) reference calculations, indicating that this bond is significantly more ionic than one may predict. Due to the SOC effect, which induces a peculiar charge accumulation on the At side in the Cl-At dimer, a weakening of the astatine-mediated halogen bond occurs arising from the (i) reduced amount of charge transfer, (ii) decrease of the polar flattening and (iii) lowering of the short-range Coulomb potential. The analysis of the electronic structure of the Cl-At moiety allows for a rationalization of the SOC effects on all the considered features of the halogen bond, including an unprecedented unsymmetrical charge back-donation from Cl-At to ammonia.

On the Interplay between Charge-Shift Bonding and Halogen Bonding

The nature of halogen-bond interactions has been analysed from the perspective of the astatine element, which is potentially the strongest halogen-bond donor. Relativistic quantum calculations on complexes formed between halide anions and a series of Y₃ C-X (Y=F to X, X=I, At) halogen-bond donors disclosed unexpected trends, e. g., At₃ C-At revealing a weaker donating ability than I₃ C-I despite a stronger polarizability. All the observed peculiarities have their origin in a specific component of C-Y bonds: the charge-shift bonding. Descriptors of the Quantum Chemical Topology show that the halogen-bond strength can be quantitatively anticipated from the magnitude of charge-shift bonding operating in Y₃ C-X. The charge-shift mechanism weakens the ability of the halogen atom X to engage in halogen bonds. This outcome provides rationales for outlier halogen-bond complexes, which are at variance with the consensus that the halogen-bond strength scales with the polarizability of the halogen atom.

A new type of halogen bond involving multivalent astatine: an ab initio study

Theoretical studies on the dimers formed by CO with the halides of multivalent astatine as a Lewis-acid center are carried out to examine the typical characteristics of supervalent halogen bonds. Calculations at the MP2/aug-cc-pVTZ level reveal that the multiple nucleophilic sites of multivalent halide monomers can promote the formation of various types of halogen bonds, among which the most stable ones are At-halogen bond complexes with multivalent astatine as a Lewis acid center, followed by the π-halogen bond dimers, and the weakest ones are the X-halogen bonds. Compared with multivalent Cl-, Br-, and I-centers, At, as the heaviest halogen, exhibits the highest halogen-bond donating ability. We found that the electrostatic term and the dispersion term play an important role in the overall attractive interaction energy, and the smallest attraction term for all complexes is the polarization term (ΔEpol). Moreover, the tri and pentavalent halides analyzed here possess very "flexible" tautomerism in which the transformation occurs during the formation of the dimers. AIM theory and NBO analysis are also employed here.

Spin-orbit coupling as a probe to decipher halogen bonding

The nature of halogen-bond interactions is scrutinized from the perspective of astatine, the heaviest halogen element. Potentially the strongest halogen-bond donor, its ability is shown to be deeply affected by relativistic effects and especially by the spin-orbit coupling. Complexes between a series of XY dihalogens (X, Y = At, I, Br, Cl and F) and ammonia are studied with two-component relativistic quantum calculations, revealing that the spin-orbit interaction leads to a weaker halogen-bond donating ability of the diastatine species with respect to diiodine. In addition, the donating ability of the lighter halogen elements, iodine and bromine, in the AtI and AtBr species is more decreased by the spin-orbit coupling than that of astatine. This can only be rationalized from the evolution of a charge-transfer descriptor, the local electrophilicity ω+S,max, determined for the pre-reactive XY species. Finally, the investigation of the spin-orbit coupling effects by means of quantum chemical topology methods allows us to unveil the connection between the astatine propensity to form charge-shift bonds and the astatine ability to engage in halogen bonds.

Halogen-like properties of the Al13 cluster mimicking astatine

Astatine-211 is considered to be one of the most promising alpha-emitters in targeted alpha therapy (TAT). However, its development has been hindered by its not so well understood chemistry. Hence, any attempt to understanding it better is imperative. Here, we show through DFT calculations that the neutral endohedral Al13 cluster follows a reactivity pattern very similar to At, more than any other halogen. The halogen and alkali bonding with Al13 and At presented a strong similarity in their charge transfer processes. Moreover, upon the interchange of At with Al13, we found a stable configuration of the compound corresponding to the astatination of aryliodonium salts, which have been considered as possible precursors for the synthesis of 211At-labeled tracers. Our results provide promising proof of concept that the Al13 cluster mimics the behaviour of At and might help in better understanding its chemistry and in streamlining the chemical processes of radiolabeling biomolecules.

Experimental and computational evidence of halogen bonds involving astatine

The importance of halogen bonds-highly directional interactions between an electron-deficient σ-hole moiety in a halogenated compound and an acceptor such as a Lewis base-is being increasingly recognized in a wide variety of fields from biomedicinal chemistry to materials science. The heaviest halogens are known to form stronger halogen bonds, implying that if this trend continues down the periodic table, astatine should exhibit the highest halogen-bond donating ability. This may be mitigated, however, by the relativistic effects undergone by heavy elements, as illustrated by the metallic character of astatine. Here, the occurrence of halogen-bonding interactions involving astatine is experimentally evidenced. The complexation constants of astatine monoiodide with a series of organic ligands in cyclohexane solution were derived from distribution coefficient measurements and supported by relativistic quantum mechanical calculations. Taken together, the results show that astatine indeed behaves as a halogen-bond donor-a stronger one than iodine-owing to its much more electrophilic σ-hole.

Quantum calculations of At-mediated halogen bonds: on the influence of relativistic effects

If astatine is generally a stronger halogen-bond donor than iodine, an inversion is sometimes observed owing to the spin–orbit coupling.

The bonding picture in hypervalent XF3 (X = Cl, Br, I, At) fluorides revisited with quantum chemical topology

Hypervalent XF₃ (X = Cl, Br, I, At) fluorides exhibit T-shaped C_2V equilibrium structures with the heavier of them, AtF₃ , also revealing an almost isoenergetic planar D_3h structure. Factors explaining this behavior based on simple "chemical intuition" are currently missing. In this work, we combine non-relativistic (ClF₃ ), scalar-relativistic and two-component (X = Br - At) density functional theory calculations, and bonding analyses based on the electron localization function and the quantum theory of atoms in molecules. Typical signatures of charge-shift bonding have been identified at the bent T-shaped structures of ClF₃ and BrF₃ , while the bonds of the other structures exhibit a dominant ionic character. With the aim of explaining the D_3h structure of AtF₃ , we extend the multipole expansion analysis to the framework of two-component single-reference calculations. This methodological advance enables us to rationalize the relative stability of the T-shaped C_2v and the planar D_3h structures: the Coulomb repulsions between the two lone-pairs of the central atom and between each lone-pair and each fluorine ligand are found significantly larger at the D_3h structures than at the C_2v ones for X = Cl - I, but not with X = At. This comes with the increasing stabilization, along the XF₃ series, of the planar D_3h structure with respect to the global T-shaped C_2v minima. Hence, we show that the careful use of principles that are at the heart of the valence shell electron pair repulsion model provides reasonable justifications for stable planar D_3h structures in AX₃ E₂ systems. © 2017 Wiley Periodicals, Inc.

Targeted radionuclide therapy with astatine-211: Oxidative dehalogenation of astatobenzoate conjugates

²¹¹At is a most promising radionuclide for targeted alpha therapy. However, its limited availability and poorly known basic chemistry hamper its use. Based on the analogy with iodine, labelling is performed via astatobenzoate conjugates, but in vivo deastatination occurs, particularly when the conjugates are internalized in cells. Actually, the chemical or biological mechanism responsible for deastatination is unknown. In this work, we show that the C−At “organometalloid” bond can be cleaved by oxidative dehalogenation induced by oxidants such as permanganates, peroxides or hydroxyl radicals. Quantum mechanical calculations demonstrate that astatobenzoates are more sensitive to oxidation than iodobenzoates, and the oxidative deastatination rate is estimated to be about 6 × 10⁶ faster at 37 °C than the oxidative deiodination one. Therefore, we attribute the “internal” deastatination mechanism to oxidative dehalogenation in biological compartments, in particular lysosomes.

Radiopharmaceutical chemistry of targeted radiotherapeutics, part 4: Strategies for 211At labeling at high activities and radiation doses of 211At α-particles

INTRODUCTION

Alpha particles are radiation of high energy and short range, properties that can lead to radiolysis-mediated complications in labeling chemistry at the high radioactivity levels required for clinical application. In previous papers in this series, we have shown that radiation dose has a profound effect on the astatine species that are present in the labeling reaction and their suitability for the synthesis of N-succinimidyl 3-[211At]astatobenzoate. The purpose of this study was to evaluate the effects of adding N-chlorosuccinimide (NCS) to the methanol solution used for initial isolation of 211At after distillation, a process referred to as 211At stabilization, on 211At chemistry after exposure to high radiation doses.

METHODS

High performance liquid chromatography was used to evaluate the distribution of 211At species present in methanol in the 500-65,000Gy radiation dose range and the synthesis of SAB from N-succinimidyl 3-(tri-n-butylstannyl)benzoate in the 500-120,000Gy radiation dose range using different 211At timeactivity combinations under conditions with/without 211At stabilization.

RESULTS

In the absence of NCS stabilization, a reduced form of astatine, At(2), increased with increasing radiation dose, accounting for about half the total activity by about 15,000Gy, while with stabilization, At(2) accounted for <10% of 211At activity even at doses >60,000Gy. SAB yields without stabilization rapidly declined with increasing dose, falling to ~20% at about 5000Gy while with stabilization, yields >80% were obtained with 211At solutions stored for more than 23h and receiving radiation doses >100,000Gy.

CONCLUSIONS

Adding NCS to the methanol solution used for initial isolation of 211At is a promising strategy for countering the deleterious effects of radiolysis on 211At chemistry.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

This strategy could facilitate the ability to perform 211At labeling at sites remote from its production and at the high activity levels required for clinical applications.

Unraveling the hydration-induced ground-state change of AtO+ by relativistic and multiconfigurational wave-function-based methods

The hydration-induced ground-state change of AtO⁺ is confirmed by means of multiconfigurational wave-function-based calculations. The involved states are identified for the first time.