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Background

Investigations of the chemical properties of the heaviest elements are among the most fundamental. They seek to probe the uppermost reaches of the Periodic Table of the elements, where the nuclei become extremely unstable and relativistic effects on electron shells are rather significant. Due to developments in experimental techniques, elements as heavy as 108 and 112 have been studied, while experiments on elements beyond 112 are planned.

Chemical theory becomes very important in this area of research, since it helps in planning sophisticated and expensive experiments with single atoms. It also helps in understanding the nature of chemical interactions and trends in properties governed by relativistic effects. However, the only property of these superheavy compounds that can presently be studied experimentally is the volatility, or adsorption on surfaces. This is done using gas-phase chromatography techniques, such as isothermal chromatography and thermochromatography. In the latter, e.g., the adsorption temperature T_ads is measured for single species of the heaviest elements adsorbed on detectors (inert or covered with a metal layer) of the chromatography column. Adsorption enthalpies H_ads are then deduced from T_ads using statistical thermodynamic models and Monte Carlo simulations.

In this context, the aim of this project is to evaluate the properties as well as the adsorption behavior of these superheavy elements on the detector surface. Due to the weakness of these interactions, predictions of physisorption of molecular species still remain a challenge for theoretical chemistry. Using our fully relativistic (four-component) density functional theory (4c-DFT) program in combination with adsorption models, we aim to predict adsorption energies H_ads and temperatures T_ads of group 4-8 halides and oxyhalides of the heaviest elements.

Hard sphere model of MO4 adsorption.	Relativistic (rel.) and nonrelativistic (nonrel.) adsorption enthalpies -Hads of MO4 (M=Ru, Os, and Hs) on quartz.

For instance, with calculated properties and the model of physisorption, we recently could determine adsorption enthalpies of different MO₄ (M=Ru, Os, and Hs) tetraoxides on quartz. The obtained value for the adsorption of HsO₄ of -H_ads(HsO₄)=45.1 kJ/mol is in excellent agreement with experimental measurements (at the GSI in Darmstadt) of 46 kJ/mol. Furthermore, we find the following trend in the volatility of the group 8 tetroxides: RuO₄ < OsO₄ > HsO₄. This inversion in the trend was also observed for other properties, which is also in agreement with corresponding experimental results.