Theoretical prediction of stable structures of lithiosilicon species based on planar double, triple, and quadruple silicon rings
Graphical abstract
Some stable structures containing planar double, triple, quadruple Si-ring are theoretically predicted. Three of them are shown below.
Introduction
Some organic molecules with a π-conjugated aromatic carbon backbone can act as semiconductors in opto-electronic devices because their aromatic carbon backbones can transport charge and interact efficiently with light [1], [2], [3]. Therefore, such structures have been the focus of intense investigation for years [4], [5].
Silicon and carbon are both in the fourth column of the periodic table and they share many similarities. It is very attractive to investigate whether aromatic silicon rings, in place of aromatic carbon rings, could offer new perspectives in the field of organic semiconductors. In view of its numerous applications in chemistry and inorganic semiconductor materials, silicon has been the subject of many experimental and theoretical investigations. However, some studies have indicated that silicon clusters preferentially adopt three-dimensional polyhedral structures because silicon favors σ-bonding in contrast to the dominating π-bonds for carbon [6], [7], [8], [9].
Santos et al. analyzed the electronic structures and properties of a silabenzene series through the topological analysis of ELF [10]. They predicted theoretically a stable aromatic D6h symmetry cluster Si6Li6, where every lithium atom positions half-way between adjacent silicon atoms outside of the planar six-membered silicon ring. Almost at the same time, Zdetsis [11] theoretically sought planar aromatic silicon structures by checking the stabilization of planar Si6 through the reduction of the Si66− anion in the presence of lithium cations (in the ‘proper’ geometry) and relaxing the symmetry to the subgroup of D6h. The resulting D2h symmetry Si6Li6 was predicted to be stable global minimum [11] where a pair of non-planar Li atoms are above and below the six-membered silicon ring and other four planar Li atoms are positioned between adjacent silicon atoms. Moreover, Zdetsis et al. corroborated the attribution of aromaticity to this silicon analogue of benzene using two different methods [11], [12]. It was also found theoretically that the global minimum of stable planar silicon ring indicates its aromaticity for the first time. However, Zdetsis [11] pointed out that the planar double ring formed by combining two six-membered rings has an imaginary frequency and is thus unstable. The question then arises as to whether all structures containing a planar polycyclic silicon backbone are unstable. A further question is whether Li atoms are effective in stabilizing a polycyclic planar silicon ring. Motivated by these issues, we were prompted to study the feasibility of stabilization of polycyclic planar silicon rings by Li and to study the structures of new polycyclic aromatic molecules. In this work, we focus on a theoretical investigation of the structures of molecules containing a polycyclic planar silicon ring, the results of which suggest that these molecules could be stable at the studied level.
Section snippets
Calculation
Some rules, similar to the method used in Ref. [11], were applied to simplify the search for the stable structures of the polycyclic aromatic hexalithiosilicons: first, a planar polycyclic silicon ring was selected as a backbone, as shown in Fig. 1. These planar rings, composed of several six-membered silicon rings, include all possible structures of double, triple, and quadruple hexatomic silicon rings. For a convenient discussion, a single silicon ring is also shown in Fig. 1a. Second, for
Results and discussion
As shown in Fig. 1, similar to the double carbon ring of naphthalene, a planar double silicon ring could be envisaged as being obtained by combining two single rings to form a chain. This backbone could be used to form three isomers with the same Si10Li8 molecular formula, designated as D-1, D-2, and D-3 in Fig. 2. D-1 and D-2 have similar structures. They may be translated into one another by rotating four planar Li atoms about the D2 axis while not moving the remaining two pairs of Li atoms.
Acknowledgements
The authors wish to thank the reviewers for helpful suggestion. This research was supported by NKBRSF (2009CB220010), 863 (No. 2006AA01A119), and NSFC (Grant No. 20633070).
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