A Comparative Study of the Chemical Properties of Element 120 and Its Homologs

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1 ISSN , Radiochemistry, 2013, Vol. 55, No. 5, pp Pleiades Publishing, Inc., Original Russian Text Yu.A. Demidov, A.V. Zaitsevskii, 2013, published in Radiokhimiya, 2013, Vol. 55, No. 5, pp A Comparative Study of the Chemical Properties of Element 120 and Its Homologs Yu. A. Demidov* а and A. V. Zaitsevskii а,b а Petersburg Nuclear Physics Institute, Orlova roshcha 1, Gatchina, Leningrad oblast, Russia; * iurii.demidov@gmail.com b National Research Center Kurchatov Institute, pl. Kurchatova 1, Moscow, Russia Received February 27, 2013 Abstract The electronic structure and energetics were calculated for molecules of binary compounds of element 120 and its homologs with common elements strongly differing in chemical properties. The calculations were performed within the framework of the relativistic density functional theory using the model of precision two-component atomic core pseudopotentials. The results obtained show that all the examined compounds of element 120 should differ from the corresponding compounds of the known heavy metals of the second group (Sr Ra) in lower strengths and larger lengths of chemical bonds. Therefore, it can be anticipated that element 120 will be, on the whole, more inert chemically than its nearest homologs. Keywords: transactinides, electronic structure modeling, relativistic density functional theory, relativistic pseudopotentials DOI: /S ! Recently all the elements of the end of the seventh period of Mendeleev s table (atomic numbers Z 118) have been synthesized [1 3], and synthesis of elements of the beginning of the eighth period has been attempted [4]. The reactions of unique (double magic) 48 Ca nuclei with heavy atoms of the target, with which all the long-lived isotopes with Z from 112 to 118 were prepared [5], are practically unsuitable for such synthesis, because they would require targets from shortlived materials. The use of projectile nuclei that are heavier than 48 Ca seems to be more promising. For example, element 120 (E120) can be prepared by the reactions 238 U + 64 Ni, 244 Pu + 58 Fe, or 248 Cm + 54 Cr. The half-lives of some nuclides in the vicinity of the island of stability [6] appear to be sufficient for studying the chemical properties of these elements. For example, the energies of adsorption of atoms with Z = 112 [7] and 114 [8] on the gold surface were determined by thermochromatography. Elements of the beginning of the eighth period should be less stable [9]; nevertheless, there are certain prospects for studying their chemical properties also. To develop experimental methods for chemical identification of these elements, it is important to have preliminary information on their fundamental chemical properties. Furthermore, such information is necessary for understanding the specific features of the manifestation of the periodic law in the lower part of Mendeleev s table. Among elements of the beginning of the eighth period, E120 is of particular interest. The electronic configuration of the atom of this element, like that of its lighter homologs (Sr, Ba, Ra), is characterized by the filled external s sublevel whose relativistic stabilization and contraction can make E120 relatively inert. Similar effects for E112 and E114 with the filled external 7s and 7p 1/2 subshells, respectively, appear to be so strong that cause essential differences between the properties of these elements and their lighter homologs and give rise to a subperiodic structure specific of the seventh period [10]. Certain characteristics of E120 have already been predicted by first principle based modeling of the electronic structure of the atom, atomic ions, and simplest molecules of its compounds. The excitation energies of the free E120 atom are known with high accuracy (no worse than 1%) from the calculations made by the relativistic configuration interaction method in combination with the multiparticle perturbation theory [11]. The energy of the transition of the E120 atom from the 461

2 462 DEMIDOV, ZAITSEVSKII Table 1. Properties of E120 and Ba, determined by relativistic DFT calculations Parameter M = E120 M = Ba Ionization potential of the atom, kj mol (563 a ) 503 (503 b ) Bond length in MH molecule, Å 2.41 (2.38 a ) 2.22 (2.23 b ) Atomization energy of MH molecule, kj mol (96 a ) 204 (197 b ) Bond length in MAu molecule, Å 3.06 (3.03 a ) 2.92 (2.91 a ) Atomization energy of MAu molecule, kj mol (174 a ) 249 (274 a ) a Results of relativistic ab initio calculations by the coupled cluster method [13]; b experimental data [24]. ground state to the lowest excited state (189 kj mol 1 ), which is commonly identified with the promotion energy, is appreciably higher than the corresponding quantities for Ra and Ba (157 and 104 kj mol 1, respectively [12]). The ionization potential of E120, determined by the relativistic coupled cluster method (565 kj mol 1 [13]), is even higher than that of its nearest homologs (509 kj mol 1 for Ra, 503 kj mol 1 for Ba [12]). Calculations of the electronic structure of the E120Au molecule using precision ab initio methods [13] and relativistic density functional theory [14] show that the chemical bond energy should be lower than that in BaAu by 1/3 and that the bond length should be larger. Also, E120 should differ from its nearest homologs in that it should have low electron affinity, lower polarizability, and small van der Waals radius [15]. Available information, however, concerns only free atoms and very specific compounds of E120, and it is apparently insufficient for gaining even very general concept of its chemistry. This study concerns the simplest compounds of E120 with the most common elements (H, C, O, F, Na). The properties of these compounds, largely characterizing the chemical properties of E120, are predicted on the basis of first principle based relativistic calculation of the electronic structure of their molecules. Comparison of the calculation results with the related data for homologs of E120 (Sr, Ba, Ra) shows how the chemical properties of Group II elements vary with the atomic number and reveals specific features of element 120. METHOD OF MODELING The model of the electronic structure of the compounds under consideration was determined by two- component relativistic pseudopotentials of small cores of Group II elements, with explicit description of both valence (nsnp, where n coincides with the number of the period) and subvalence [(n 1)s(n 1)p] electronic shells [16, 17]. The parameters of these pseudopotentials were optimized to achieve maximum accurate description of the characteristics of specifically the valence shells (in so doing, the properties of subvalence shells are reproduced with somewhat lower accuracy). The model ensures correct consideration of the effects of the finite nucleus size and Breit interactions. The many-electron problem was solved within the framework of two-component noncollinear formulation of the relativistic density functional theory (with the absolute value of the spin magnetization acting as analog of the spin density [18]) with PBE0 ab initio hybrid approximation [19] for the exchange-correlation functional. The components of the auxiliary Kohn Sham spinors were represented as expansions of atom-centered Gaussian functions in terms of basis sets, with unrestricted optimization of the expansion coefficients. The basis sets of the Group II atoms had the dimensions 5s5p4d1f (Sr), 5s4p3d1f (Ba), 8s7p3d2f (Ra), and 8s7p2d1f (E120) and were adapted to calculations with inclusion of the spin-dependent relativistic interactions, which is significant at very large amplitudes of these interactions for the heaviest atoms of the group [20]. The basis sets for the light atoms (H Na) were obtained by augmenting the triple zeta sets of the contracted Gaussians [21] with polarization and diffuse functions. The sufficient saturation of the basis sets chosen is manifested, in particular, in small values of superposition errors. The reliability of the chosen scheme for modeling the structure of compounds of heavy elements in ground electronic states has been demonstrated previously [22, 23]. As applied to the heaviest elements, it is confirmed by comparison of the results obtained with the values known from the experiment or obtained by precision ab initio relativistic calculations (Table 1). The modeling technology described was applied to determination of the equilibrium structures and energies of free molecules of the simplest binary compounds of Group II elements in the ground electronic states: Na 2 M, MH, MC, MO, and MF 2, where M = Sr, Ba, Ra, or E120. The choice of the objects was governed by the traditional views that Group II elements are bivalent; exception was made only for hydrides, because dihydrides are considerably less stable with respect to decomposition to molecular hydrogen and

3 A COMPARATIVE STUDY OF THE CHEMICAL PROPERTIES OF ELEMENT Table 2. Atomization energies of molecules of Group II elements Compound M = E120 M = Ra M = Ba M = Sr Na 2 M 29 а, 74 b 68 а, 80 b 94 а 89 а MH (197) 181 MC MO (561 ± 42) 415 (460 ± 84) MF (1155 ± 84) 1096 (1105 ± 84) а Linear isomer Na M Na; b isomer with the Na Na bond preserved. Experimental data from [24] are given in parentheses. atomic alkaline earth element. The choice between MC and M 2 C was not obvious and was governed by the similar facts. RESULTS AND DISCUSSION The estimates obtained for the atomization energy of the molecules under consideration are given in Table 2. Figure 1 gives the general view on the elementary chemistry of E120 and its homologs, presenting the decomposition energies of molecules of binary compounds with the formation of the gaseous species that are the most stable in a wide range of conditions: Group II metal atoms and diatomic molecules of the selected light elements. Arranging these quantities for a given element M in the order of increasing electronegativity of the selected light elements (Na, H, C, O, and F), we come to the simplest characteristic of the chemistry of M, which can be termed a chemical graph. Such graphs should be essentially different for elements that strongly differ in properties; however, the graphs presented in Fig. 1 for Group II elements, including E120, are similar in shape and change regularly in the series from Sr to E120. It should be noted that the corresponding graphs representing the bond lengths in the molecules (Fig. 2) vary less regularly. The bond of Na with E120 is less strong than with the other heavy elements of Group II. In contrast to Sr and Ba, which form stable Na M Na molecules with linear equilibrium configuration, for E120 and Ra the most stable structures are triangular complexes of C 2v symmetry with the bond between the Na atoms preserved, whereas the linear isomers are unstable. The figures show the characteristics of the linear isomers. Hydride of E120 differs from the corresponding compounds of the homologs in lower bond energy (Table 2) and increased equilibrium internuclear distance in the diatomic molecule (2.41 Å, whereas for, e.g., SrH the bond length is 2.13 Å). Formation of hydride of E120 from molecular hydrogen and atomic Fig. 1. Energies ΔE of decomposition of molecules of binary compounds of heavy Group II elements to form free atom M and diatomic molecules of Na, H, C, O, and F. Fig. 2. Bond lengths R e in molecules of compounds of heavy Group II elements.

4 464 DEMIDOV, ZAITSEVSKII E120 would require energy consumption (121 kj mol 1, according to our estimates). The bond energy of carbon with E120 is somewhat lower than with Ra. Strontium and barium form stronger bonds with carbon. In all the cases, the energy of the decomposition of the monocarbide to form C 2 is negative. It should be noted that E120, like all the other Group II elements, can form a fairly stable acetylenide molecule (the bond energy of E120 with C 2 molecule is 394 kj mol 1 ). In going from the known Group II elements to E120, the bond energy in oxide molecules should strongly decrease. The predicted atomization energy of the molecule of (E120)O is only 60% of that of RaO and less than 50% of that of BaO. As a result, the formation of (E120)O molecules from molecular oxygen and atomic E120 should involve almost zero energy change, which is quite unexpected for a homolog of alkaline earth metals. All the Group II elements form stable compounds with fluorine, and E120 should not become an exception. The atomization energy of E120F 2 is only 16% lower than that of RaF 2. Similarly to molecules of difluorides of the known alkaline earth metals, the E120F 2 molecule is nonlinear (bond angle about 103 ). An important characteristic of any element is the bond energy in its dimer. The results of our calculations for E120 dimer can only confirm the small value of this energy (4 kj mol 1 at the equilibrium distance of 5.32 Å), already known from the results of modeling by the relativistic DFT method using the generalized gradient approximation for the exchange-correlation functional (2 kj mol 1 and 5.65 Å, respectively [14]). In this respect, E120 is a typical Group II element. However, the approach that we used (as well as the approach used in [14]) is a fortiori unsuitable for quantitative estimation of the characteristics of such weakly bonded van der Waals complexes in which the dispersion interactions play a major role [25]. Thus, we have calculated the electronic structure and energy characteristics for molecules of binary compounds of superheavy element 120 and its homologs with common elements essentially differing in the chemical properties. All the E120 compounds studied should differ from the corresponding compounds of the known heavy elements of Group II (Sr Ra) in lower strengths and larger lengths of the chemical bonds. Therefore, it can be anticipated that E120, on the whole, will be more inert chemically than its nearest homologs. In most cases, with an increase in the atomic number within Group II, the chemical bond energy first increases, reaching a maximum for Ba, and then decreases because of rapid increase in the relativistic contraction. The characteristics vary regularly without sharp jumps in going from one element to another, which indicates that the formal place of element 120 at the end of Group II quite agrees with its chemical properties. ACKNOWLEDGMENTS The authors are grateful to Prof. C. van Wüllen for placing at their disposal the complex of programs for modeling the electronic structure of molecules using the relativistic density functional theory [18] and to A.V. Titov, L.V. Skripnikov, N.S. Mosyagin, and A.A. Rusakov for useful discussions. The calculations were performed with the Multipurpose Computation Complex of the National Research Center Kurchatov Institute. The study was supported by the Russian Foundation for Basic Research (project no ofi-m-2011). REFERENCES 1. Oganessian, Y.T., Eur. Phys. J. A, 2009, vol. 42, pp Oganessian, Y.T., Abdullin, F.S., Bailey, P.D., et al., Phys. Rev. Lett., 2010, vol. 104, pp Popeko, A., Proc. DAE Symp. on Nucl. Phys., Chatterjee, A., Biswas, D.C., Shukla, P., and Visakhapatnam, A.P., Eds., Andhra Univ., 2011, vol. 56, pp Oganessian, Y.T., Utyonkov, V.K., Lobanov, Yu.V., et al., Phys. Rev. C, 2009, vol. 79, pp Oganessian, Y., J. Phys. G: Nucl. Part. Phys., 2007, vol. 34, pp. R165 R Oganessian, Y., Radiochim. Acta, 2011, vol. 99, pp Eichler, R., Aksenov, N.V., Belozerov, A.V., et al., Nature, 2007, vol. 447, no. 7140, pp Eichler, R., Aksenov, N.V., Albin, Yu.V., et al., Radiochim. Acta, 2010, vol. 98, no. 3, pp Sobiczewski, A., Acta Phys. Pol. B, 2011, vol. 42, no. 8, pp Zaitsevskii, A.V., van Wüllen, C., and Titov, A.V., Russ. Chem. Rev., 2009, vol. 78, no. 12, pp Dinh, T.H., Dzuba, V.A., Flambaum, V.V., and Ginges, J.S.M., Phys. Rev. A, 2008, vol. 78, pp

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