title: The Chemistry of Copernicium: Superheavy Element and Homolog Studies
Creators: name: Ionescu, Paul
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 540 Chemistry
subject: 570 Life sciences; biology

description: In this work, chemical experiments on mercury are described which serve as a foundation for the ongoing research on the transactinide copernicium (Cn, element 112). A first indication of a Cn reaction is reported. The reactivity is higher than expected, as Cn until now has been shown to behave as an inert element resembling a noble gas.
The Periodic Table serves chemists as a fundamental reference tool, providing an accessible framework that reveals information about the elements. Even the chemical behaviour of unfamiliar elements can be deduced and approximated well from the periodicity of elements in a common group due to the same number of valence electrons. On this basis, the chemistry of Cn as a member of group 12 is related to its homolog Hg. However, the relativistic effects on electron structure increase with the nuclear charge and it follows that the chemistry of Cn is more affected by relativity than that of its lighter homologs. Predictions, as well as the first series of chemical experiments on Cn both pointed towards an increasingly inert nature of the transactinide largely owing to the influence of relativistic effects and therefore deviating from its homologs. The experimental method of gas chromatography is central to the investigation of short-lived transactinides and used extensively throughout this work. A second generation gas chromatographic experiment was designed to investigate the reactivity of Cn towards the chalcogen Se in comparison to Hg, but preliminary results were inconclusive. This work continues by building on the preliminary findings.
First, a uniform and stable trigonal selenium stationary phase was developed for use in chromatography. As a chalcogen, Se forms a number of allotropes. Some of these allotropes are unstable and change properties over time. To ensure long-term stability, the thermodynamically most stable state at standard temperature and pressure was chosen, the trigonal allotrope of selenium, t-Se. An additional condition for the chromatographic t-Se surface was producing a uniform, thin film, as the Se covered α-particle and spontaneous fission fragment detectors. A thinner, uniform film minimizes interference with the energy resolution of the spectrometry. Multiple methods for the production of such t-Se films were explored and have been characterized and assessed for the feasibility in use for transactinide gas chromatographic investigation.
A novel selenium casting method, photo-induced chemical vapour deposition and physical vapour deposition were the techniques explored here. Allotropy of the Se films and the conversion to the stable t-Se form is documented. For the purpose of α- and spontaneous fission detector manufacturing, physical vapour deposition was ultimately chosen. Isothermal gas chromatography with carrier-free 197m, 203Hg radioisotopes was used to evaluate the long-term stability and degree of conversion of various selenium surfaces. It was discovered that despite complete conversion to the t-Se allotrope, reactive sites in the Se were present. It was found that due to crystallization, approximately 1% of the selenium atoms on the surface are monovalent and provide sites with increased reactivity for the Hg tracer. These findings are to be submitted for publication to Thin Solid Films.
A first use of the technique of inverse thermochromatography is presented here, where the stable t-Se stationary phase was placed in a chromatographic column with positive temperature gradient to increase the reaction rate for the carrier-free 197m, 203Hg tracer gradually. The Eyring-Polanyi equation was used in a microscopic Monte-Carlo simulation to describe the forward reaction rate of the HgSe formation, and a first report of the Gibbs free energy of activation was obtained at ΔG‡(HgSe) = 95 ± 12 kJ/mol. Additionally, using the Arrhenius equation, a first report of the activation energy for the HgSe formation of Ea(HgSe) = 100 ± 10 kJ/mol with pre-exponential factor of A = 4:3 ∙ 1013 ± 2:6 ∙ 1013 was made. The activation energy of group 12 elements was used to make an extrapolation to the value of Cn, which was predicted at Ea(CnSe) = 106 ± 22 kJ/mol, higher than all of the group 12 metals, and therefore supporting the postulate that Cn is more inert than its lighter homologs. The novel method of inverse chromatography and the thermochemical data is to be submitted for publication to the Journal of Radioanalytical and Nuclear Chemistry.
The discoveries made based on the reactivity of Hg toward t-Se and the optimized t-Se chromatographic surface culminated in a new experimental campaign in 2018 at Flerov Laboratory for Nuclear Reactions in Dubna, Russian Federation, with the transactinide Cn to expand on earlier preliminary findings. Contrary to expectation, it became evident from the 8 atoms observed that the superheavy element Cn shows much greater affinity toward t-Se than anticipated.
The adsorption enthalpy was determined to be -ΔHt-Se/ads(Cn) = 62 ±5 kJ/mol, greater than that of Hg at -ΔHt-Se/ads(Hg) = 60 kJ/mol. Such a strong value points toward chemisorption, where the Cn atom not only adsorbs to the t-Se surface, but forms a chemical bond. Given the experimental results, an upper limit of the activation energy of Ea(CnSe) < 90 kJ/mol was deduced, significantly lower than that of Hg at Ea(HgSe) = 100 ± 10 kJ/mol. As a result the surprising conclusion was reached that Cn exhibits increased reactivity toward t-Se than its lighter homolog Hg. Therefore a reversal of the established adsorption trend on gold of -ΔHAu/ads(Hg) > -ΔHAu/ads(Cn) > -ΔHAu/ads(Rn) is presented here, with -ΔHt-Se/ads(Cn) > -ΔHt-Se/ads(Hg) > -ΔHt-Se/ads(Rn). These findings contradict the previous assumptions that Cn resembles a noble gas and suggest a first observed reaction of the transactinide.
The ambient conditions of the gas chromatographic investigation of Cn were closely monitored and two contaminations interfering with experimental results were identified. One was attributed to the presence of oxygen in the system when Ta getters were operating below temperatures of 900°C. The oxygen presence was shown to influence the radionuclide transport yield of Hg by a factor of 3. Additionally, an unknown contamination was observed to interfere with Au-covered detectors in the array as temperatures decreased below room temperature level. Se detectors were unaffected. The most likely source was identified to be left-over organic materials from the plutonium oxide target production evaporating during irradiation of the target.
To investigate the effects of contaminant gases such as the yield fluctuation by the presence of oxygen in the gas chromatography, a new experiment was designed, built and calibrated with the possibility of purposely functionalizing the chromatographic carrier gas. The Trace-gas Reaction Analyzer for Chemistry (TRACY) is a compact modular gas chromatography easily adapted for a variety of experiments both on- and off-line.