Gas-phase chemistry of Sg(CO)6 – Measurements of the stability ofnovel superheavy element compounds
Ch.E. Düllmann1,2,3, R. Eichler4,5,M. Asai6 H. Brand2, N.M. Chiera4,5, A. Di Nitto1,2, R.
Dressler4, J. Even2,3, R. Eichler4,5, F. Fangli7, M. Götz1,2,3, H. Haba8, W. Hartmann2, E. Jäger2,D. Kaji8, J. Kanaya8, Y. Kaneya6, J. Khuyagbaatar2,3, B. Kindler2, Y. Komori8, B. Kraus4,5, J.V.
Kratz1, J.Krier2, Y. Kudou8, N. Kurz2, S. Miyashita6,9, K. Morimoto8, K. Morita8,10, M.
Murakami8,11, Y. Nagame6, K. Ooe11, D. Piguet4, N. Sato8, T.K. Sato6, J. Steiner2, P.
Steinegger4,5, T. Sumita8, M. Takeyama8, K. Tanaka8, T. Tomitsuka11, A. Toyoshima6, K.
Tsukada6, A. Türler4,5, I. Usoltsev4,5, Y. Wakabayashi8, Y. Wang7, N. Wiehl1,3, V. Wolter1, A.
Yakushev2,3, S. Yamaki8, S. Yano8, S. Yamaki8, Z. Qin7
1Johannes Gutenberg-Universität Mainz, Mainz, Germany.
2GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany.
3Helmholtz-Institut Mainz, Mainz, Germany.
4Paul Scherrer Institute, Villigen, Switzerland.
5University of Bern, Bern, Switzerland.
6Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki, Japan.
7Institute of Modern Physics Lanzhou; Chinese Academy of Sciences, Lanzhou, China.
8Nishina Center for Accelerator-Based Science, RIKEN, Wako, Saitama, Japan.
9Hiroshima University, Kagamiyama, Higashi-Hiroshima, Japan.
10Kyushu University, Higashi-Ku, Fukuoka, Japan.
11Niigata University, Niigata, Niigata, Japan.

Abstract
Experiments aimed at studying the chemical stability of the novel Sg(CO)6 molecule, firstsynthesized in 2013, have been performed. Comparison studies with the stabilities of the lighter
homolog molecules Mo(CO)6 and W(CO)6 were also performed. In separate experiments,
exploratory studies with the goal to identify chemical synthesis routes for these compounds thatfeature higher chemical yields than previous approaches were carried out.

1. Research Objectives
Studying fundamental chemical properties of such heaviest elements at the extremes of theperiodic table is an exciting perspective for the chemistry community. These elements'electronic shell structure is strongly influenced by relativistic effects induced by the highnuclear charge. Therefore, the prediction of chemical properties of new elements at the veryedge of the periodic table is not straightforward. Highly sensitive chemical investigations arerequired at the single-atom-at-a-time level to benchmark our theoretical understanding ofthose effects. The low production rates and short half-lives complicate and limit theapplicability of standard chemical procedures and the selection of accessible chemical systems.
Gas phase chemical studies on single atomic or single molecular species and their adsorption
interaction with stationary surfaces have proven very successful [1,2]. Increasing relativistic
effects seem to govern the chemistry of Cn [3,4] and Fl [5,6]. Such effects can also be probed in
low oxidation state compounds of lighter transition metal transactinides, Rf, Db, Sg, Bh, Hs,Mt, and Ds, but such compounds were previously out of reach. Only in 2014, our observed
formation and high volatility of group 6 hexacarbonyls including Sg(CO)6 [7] opened the doorfor such studies. As a next step, the study of the compounds thermal stability appears uniquely
possible based on our preparatory work with lighter homologs [8].
2. Research Contents
Relativistic density functional theory indicates the hexacarbonyl species of Mo, W, and Sg as themost stable ones and suggests an increase in stability (as expressed in the first bond dissociationenergy, FBDE) towards the heavier elements [9,10]. Model experiments with isotopes of Mo andW revealed [8,11] that thermal decomposition studies on a silver surface assess the FBDE on amolecular level. A typical decomposition setup used for this purpose comprised a tubular flow
reactor of 1 m length and 4 mm inner diameter entirely covered inside with thin silver foils. The
reactor could be heated to defined temperatures between room temperature and 600°C. The
amount of hexacarbonyl species able to pass this reactor in dependence of the selected isothermal
temperature is measured in comparison to the total amount produced. A first experimental study
on Sg(CO)6 stability was carried out at the RILAC-GARIS facility at the RIKEN Nishina Centerin FY16. The unique combination of the production of 265Sg at the RIKEN RILAC accelerator, its
efficient isolation in the RIKEN GARIS separator, together with our thermal stability
experimental setup [8] was used. The aim of the experiment was to directly determine a chemicalbond stability within a transactinide molecule.
A separate set of experiments was performed at the research reactor TRIGA Mainz with themain scientific goal being to overcome the up to 90% losses of evaporation residues in thephysical preseparator. As a beam-free environment is necessary for carbonyl-compound
synthesis, a new two-chamber design was tested, whereby the beam is stopped in a first chamber,from which reaction products are flushed into a second, directly connected chamber with a rapid
inert-gas flow. Such a design has first been tested previously in an exploratory study performed
at JAEA Tokai. Similarly to those studies, in the TRIGA Mainz experiments, the chemical
reaction chamber, directly connected to the recoil chamber, was flushed with carbon-monoxide
containing gas, facilitating the synthesis of the carbonyl compounds [12]. These studies will
guide the way to experiments with superheavy elements beyond Sg, which can only be produced
at rates yet lower than those of Sg due to decreasing cross sections with increasing atomic
number Z. Hence, highly efficient systems are required.
3. Research results
First decomposition experiments with 265Sg(CO)6 have been performed successfully at theRILAC-GARIS facility. Preliminary results are shown in Figure 1, where panel a) gives anoverview of decay chains associated with 265Sg. In panel b), red arrows indicate the temperatures,at which the stability of Sg(CO)6 was studied. The results obtained previously [8,11] for Mo(CO)6(blue symbols) and W(CO)6 (green symbols) are shown. A kinetic decomposition model wasdeveloped superimposing gas chromatographic transport of species through the open tubular
reactor with heterogeneous decomposition of the species upon adsorption on the inner surfacesof the reactor [11]. This model is applied to relate the decomposition observations to the FBDE
given for the hexacarbonyl complexes and was used in [11] to formulate an expectation for thebehaviour of Sg(CO)6 based on the predicted stability from [9] (red shaded area) and [10] (grayshaded area). The final data analysis is ongoing.
a)b)Fig 1：a) Decay sequences observed in the decomposition experiment with 265Sg.
b) Measured survival probability of Mo(CO)6 and W(CO)6. The red arrows indicate
temperatures at which the Sg-decompositon was measured. For details, see text.
At TRIGA Mainz, overall yields for the two steps “flushing out of the recoil chamber” and“chemical yield for carbonyl compound formation” of about 40% have been reached, supporting
the effectiveness of this novel approach for experiments with higher yields.
4. Conclusion
First steps towards the determination of bond stability in a SHE-containing molecule havebeen successfully taken. Novel approaches for increased yield appear promising, supporting theexciting program which is available for carbonyl chemical studies of the superheavy transition
metals.
5. References[1] A. Türler, V. Pershina, Chem. Rev., 113, 1237 (2013).

[2] The chemistry of superheavy elements. M. Schädel, D. Shaughnessy (Eds.) 2nd ed., 2014.

[3] R. Eichler et al., Nature 447, 72 (2007).
[4] R. Eichler et al., Angew. Chem. Int. Ed. 47, 3262 (2008).
[5] R. Eichler et al., Radiochim. Acta 98, 133 (2010).
[6] A.Yakushev et al., Inorg. Chem., 53, 1624 (2014).
[7] J. Even et al., Science 345, 1491 (2014).
[8] I. Usoltsev et al., Radiochim. Acta 104, 141 (2016).
[9] C.S. Nash et al., J. Am. Chem. Soc. 121, 10830 (1999).
[10] M. Ilias et al., Inorg. Chem. 56, 1638 (2017).
[11] I. Usoltsev et al., Radiochim. Acta 104, 531 (2016).
[12] V. Wolter, Master’s thesis, Johannes Gutenberg University Mainz, Germany