Chemistry of Superheavy Elements

Sulfate complexation of element 104, Rf, in H2SO4/HNO3 mixed solution

The cation-exchange behavior of 261Rf (T 1/2= 78 s) produced in the 248Cm(18O, 5n) reaction was studied on a “one-atom-at-a-time” scale in 0.15–0.69 M H2SO4/HNO3 mixed solutions ([H+]=1.0 M) using an automated ion-exchange separation apparatus coupled with the detection system for alpha-spectroscopy (AIDA). It was found that adsorption probabilities ( decrease with an increase of [HSO4 −], showing a successive formation of Rf sulfate complexes. Rf exhibits a weaker complex formation tendency compared to the lighter homologues Zr and Hf. This is in good agreement with theoretical predictions including relativistic effects.

Adsorption interaction of astatine species with quartz and gold surfaces

The adsorption interaction of various astatine species with quartz and gold surfaces was investigated by gas chromatography methods. Due to variations of the redox potential of the carrier gas elemental astatine, astatine oxide and hypo-astatic acid have been produced. The identification of the astatine compounds is based on the analogy assumption to the gas phase chemistry of the closest homologues in group 17 of the periodic table, iodine and bromine. The deposition temperatures as well as enthalpies of adsorption have been determined for the astatine species. The enhancement of the metallic character within group 17 towards higher Z is clearly confirmed. Macroscopic properties (sublimation enthalpy) of previously unstudied AtO2 and HAtO were estimated. The determined data for elemental astatine were compared to available literature data. Based on the obtained experimental results possible designs of experiments for studying of chemical properties of the recently discovered element 117 can be suggested.

Superheavy element studies with preseparated isotopes

In recent years, significant progress in the field of superheavy element research has been achieved thanks to a novel combination of techniques from different fields. This “physical preseparation” approach includes the coupling of an ancillary setup – typically a chemistry apparatus or a counting setup – to a physical recoil separator. This latter preseparator removes unwanted nuclear reaction products as well as the intense heavy-ion beam associated with superheavy element experiments and thus isolates the evaporation residues of the nuclear fusion reactions. These are guided to the separators's focal plane, where they are extracted and available for further transport to external setups, e.g., by a gas-jet. In this overview, the development of physical preseparation is described, and experimental results from nuclear chemistry and physics that were achieved with “preseparated” isotopes are summarized, with an emphasis on results relevant for superheavy element research. The covered topics range from chemical studies in the liquid as well as in the gas phase, the measurement of nuclear decay properties and of atomic masses. Preseparation was already shown to be a very powerful approach in these studies and promises to allow further progress in superheavy element research.

Aqueous-phase chemistry of the transactinides

The experimental techniques developed to perform rapid chemical separations of the heaviest elements in the aqueous phase are presented. In general, these include transport of the nuclear reaction products to a separation device by the gas-jet technique and dissolution in an aqueous solution containing inorganic ligands for complex formation. The complexes are chemically characterized by a partition method which can be liquid–liquid extraction, ion-exchange- or reversed-phase extraction chromatography. The separated fractions are quickly evaporated to dryness for the preparation of samples for α-particle spectroscopy. Comments are given on the special situation in which chemistry has to be studied with single atoms. Theoretical predictions of chemical properties are compared to the presently known chemical behaviour of rutherfordium, Rf (element 104), dubnium, Db (element 105), seaborgium, Sg (element 106), and hassium, Hs (element 108) and to that of their lighter homologs in the Periodic Table in order to assess the role of relativistic effects in the chemistry of the heaviest elements.

Superheavy elements at GSI: a broad research program with element 114 in the focus of physics and chemistry

Studies of the superheavy elements form one of the pillars of the GSI research program. A unique combination of experimental facilities is installed at the GSI. Various topics, ranging from the synthesis of new elements, spectroscopy experiments to study the nuclear structures of the heaviest nuclei, highly accurate mass measurements beyond uranium, to chemical investigations of elements around element 114 and the synthesis of novel chemical superheavy element compound classes are being studied. This is complemented by fully relativistic quantum chemical calculations. As a recent highlight, the 244Pu(48Ca,3–4n)288,289114 reaction was studied, leading to the observation of element 114 at the new gas-filled recoil separator TASCA.

Gas chromatography of indium in macroscopic and carrier-free amounts using quartz and gold as stationary phases

The chemical investigation of E113 is likely to become soon feasible. The determination of chemical properties of carrier-free amounts of the lighter homologues of element 113, especially indium and thallium, allows designing experimental set-ups and selecting experimental conditions suitable for performing these studies. Here, we present investigations of the interaction of indium species with quartz and gold surfaces. Deposition temperatures as well as enthalpies of adsorption were determined for indium T dep=739±20 °C (−ΔH ads(In)=227±10 kJ mol−1) and for indium hydroxide T dep=250±20 °C (−ΔH ads(InOH)= 124±10 kJ mol−1) respectively, on quartz. In case of adsorption of indium on a gold surface only a lower limit of the deposition temperature was established T dep>980 °C (−ΔH ads(In)≥315±10 kJ mol−1). Investigations of macroscopic amounts of indium in thermosublimation experiments at similar experimental conditions were instrumental to establish a tentative speciation of the observed indium species.

A suggested periodic table up to Z≤ 172, based on Dirac-Fock calculations on atoms and ions

Extended Average Level (EAL) Dirac-Fock calculations on atoms and ions agree with earlier work in that a rough shell-filling order for the elements 119-172 is 8s < 5g≤ 8p(1/2) < 6f < 7d < 9s < 9p(1/2) < 8p(3/2). The present Periodic Table develops further that of Fricke, Greiner and Waber [Theor. Chim. Acta 1971, 21, 235] by formally assigning the elements 121-164 to (nlj) slots on the basis of the electron configurations of their ions. Simple estimates are made for likely maximum oxidation states, i, of these elements M in their MX(i) compounds, such as i = 6 for UF(6). Particularly high i are predicted for the 6f elements.

Gas-phase isothermal chromatography with short-lived technetium isotopes

In order to study the gas-phase chemical behavior of transactinides, an on-line isothermal chromatography apparatus has been developed and applied to separate short-lived technetium isotopes in the form of TcO3 from fission products. The fission products from a 252Cf source were continuously and rapidly transported through the capillary to the isothermal chromatography apparatus using the N2/KBr gas-jet techniques. Volatile oxide molecules were formed at the reaction zone kept at 900 °C since a trace amount of oxygen existed in the N2 carrier gas. With the new developed isothermal chromatography apparatus, a selective separation of Tc from fission products was achieved. After isothermal chromatographic separation, 101,103,104,105,106,107,108Tc were dominantly observed together with their Ru daughters in the γ-spectrum, The chemical yields of 101Tc, 104Tc and 105Tc isotopes with longer half-lives are about 55−57%, and those of 103Tc, 106Tc, 107Tc and 108Tc isotopes with shorter half-lives dropped down to 25−28%. The adsorption enthalpy of the investigated compounds on quartz surfaces was determined to be −150±5 kJ/mol by fitting the measured retention curves with a Monte Carlo model. The observed species of technetium oxide is attributed to TcO3, which is in good agreement with previous experimental results. That means our system worked properly and it can be used to investigate the gas-phase chemical behavior of transactinides.

Search for the “missing” α-decay branch in 239Cm

A search for the α-decay of 239Cm was carried out via the fusion reaction 232Th(12C,5n) 239Cm at GSI, applying nuclear chemistry techniques. The 239Cm α-decay would provide a link between known nuclear masses and α-decay energies measured in the region of the superheavy elements. Three different 12C beam energies were chosen in the present experiments. The α peaks of 238,240Cm were observed clearly at a 12C energy of 74 MeV, while 238Cm disappeared at 70 MeV to provide an undisturbed α-spectrum for a sensitive search of 239Cm decay. No α-events between 6.34 and 6.50 MeV, which could be assigned to 239Cm with confidence, were observed in this experiment. The upper limit of the α-branch of 239Cm was less than 1.0×10−5.

Development of an electrochemistry apparatus for the heaviest elements

We developed a new apparatus for the study of electrochemical properties of the heaviest elements. The apparatus is based on a flow electrolytic cell combined with column chromatography. Glassy-carbon fibers modified with Nafion perfluorinated cation-exchange resin are used as a working electrode as well as a cation-exchanger. The elution behavior of 139Ce with the number of 1010 atoms in 0.1 M ammonium α-hydroxyisobutyric acid solution from the column electrode was investigated at the applied potentials of 0.2–1.0 V versus the Ag/AgCl reference electrode in 1.0 M LiCl. It was found that 139Ce3+ is successfully oxidized to 139Ce4+ even with tracer concentration at around the redox potential determined by cyclic voltammetry for the macro amounts of Ce with 1017 atoms (10−3 M). The present oxidation reaction and separation of Ce4+ was accomplished within a few minutes.

Oxidation of element 102, nobelium, with flow electrolytic column chromatography on an atom-at-a-time scale

We report here on the successful oxidation of element 102, nobelium (No), on an atom-at-a-time scale in 0.1 M alpha-hydroxyisobutyric acid (alpha-HIB) solution using a newly developed technique, flow electrolytic column chromatography. It is found that the most stable ion, No(2+), is oxidized to No(3+) within 3 min and that the oxidized No complex with alpha-HIB holds the trivalent state in the column above an applied potential of 1.0 V.

Development of an online preparation system for multitracer solutions

We have developed a new target-irradiation system for the online preparation of multitracer solutions, where the nuclear-reaction products recoiling out of the target are directly implanted in a solvent as a liquid catcher. A rapid online transportation of the solution has enabled highly efficient recovery of the multitracer solutions having even short-lived radioactive isotopes without any chemical treatments. It has been suggested that the collection efficiency depends on the chemical properties of the recoil elements.

Relativistic effective core potential calculations of Hg and eka-Hg (E112) interactions with gold: Spin-orbit density functional theory modeling of Hg–Aun and E112–Aun systems

Interactions of eka-Hg (E112) and Hg atoms with small gold clusters were studied in the frame of the relativistic effective core potential model using the density functional theory (DFT) approach incorporating spin-dependent (magnetic) interactions. The choice of the exchange-correlation functional was based on a comparison of the results of DFT and large-scale coupled cluster calculations for E112Au and HgAu at the scalar relativistic level. A close similarity between the E112Aun and HgAun equilibrium structures was observed. The E112 binding energies on Aun are typically smaller than those for Hg by ca. 25%-32% and the equilibrium E112-Au separations are always slightly larger than their Hg-Au counterparts.

Density Functional Theory-Based Prediction of Some Aqueous-Phase Chemistry of Superheavy Element 111. Roentgenium(I) Is the ‘Softest' Metal Ion

A previous approach (Hancock, R. D.; Bartolotti, L. J. Inorg. Chem. 2005, 44, 7175) using DFT calculations to predict log K1 (formation constant) values for complexes of NH3 in aqueous solution was used to examine the solution chemistry of Rg(I) (element 111), which is a congener of Cu(I), Ag(I), and Au(I) in Group 1B. Rg(I) has as its most stable presently known isotope a t(1/2) of 3.6 s, so that its solution chemistry is not easily accessible. LFER (Linear free energy relationships) were established between DeltaE(g) calculated by DFT for the formation of monoamine complexes from the aquo ions in the gas phase, and DeltaG(aq) for the formation of the corresponding complexes in aqueous solution. For M2+, M3+, and M4+ ions, the gas-phase reaction was [M(H2O)6]n+(g) + NH3(g) = [M(H2O)5NH3]n+(g) + H2O(g) (1), while for M+ ions, the reaction was [M(H2O)2]+(g) + NH3(g) = [M(H2O)NH3]+(g) + H2O(g) (2). A value for DeltaG(aq) and for DeltaE for the formation of M = Cu2+ in reaction 1, not obtained previously, was calculated by DFT and shown to correlate well with the LFER obtained previously for other M2+ ions, supporting the LFER approach used here. The simpler use of DeltaE values instead of DeltaG(aq) values calculated by DFT for formation of monoamine complexes in the gas phase leads to LFER as good as the DeltaG-based correlations. Values of DeltaE were calculated by DFT to construct LFER with M+ = H+, and the Group 1B metal ions Cu+, Ag+, Au+, and Rg+, and with L = NH3, H2S, and PH3 in reaction 3: [M(H2O)2]+(g) + L(g) = [M(H2O)L]+g) + H2O(g) (3). Correlations involving DeltaE calculated by DMol3 for H+, Cu+, Ag+, and Au+ could reliably be used to construct LFER and estimate unknown log K1 values for Rg(I) complexes of NH3, PH3, and H2S calculated using the ADF (Amsterdam Density Functional) code. Log K1 values for Rg(I) complexes are predicted that suggest the Rg(I) ion to be a very strong Lewis acid that is extremely "soft" in the Pearson hard and soft acids and bases sense.

Is E112 a relatively inert element? Benchmark relativistic correlation study of spectroscopic constants in E112H and its cation

We report the first results of relativistic correlation calculation of the spectroscopic properties for the ground state of E112H and its cation in which spin-orbit interaction is taken into account non-perturbatively. Studying the properties of E112 (eka-Hg) is required for chemical identification of its long-lived isotope, (283)112. It is shown that appropriate accounting for spin-orbit effects leads to dramatic impact on the properties of E112H whereas they are not so important for E112H(+). The calculated equilibrium distance, R(e) (calc)=1.662 Angstrom, in E112H is notably smaller than R(e) (expt)=(1.738+/-0.003) Angstrom and R(e) (calc)=1.738 Angstrom in HgH, whereas the dissociation energy, D(e) (calc)=0.42 eV, in E112H is close to D(e) (expt)=0.46 eV and D(e) (calc)=0.41 eV in HgH. These data are quite different from R(e) (NH)=1.829 Angstrom and D(e) (NH)=0.06 eV obtained for E112H within the scalar-relativistic Douglas-Kroll approximation. Our results indicate that E112 should not be expected to behave like a noble gas in contrast to the results by other authors.