Sunghwan Kim - Structures, energetics, and aromaticities of the tetrasilacyclobutadiene dianion and related compounds: (Si4H4)2-, (Si4H4)2-·2Li+, [Si4(SiH3)4]2-·2Li+, [Si4(SiH3)4]2-·2Na+

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      Publication Details (including relevant citation   information):

      S. Kim, S. Wang,   and H.F. Schaefer;

      Journal of Physical Chemistry A,   2011115, 5478.



      In light of the important recent synthesis of a stable   tetrasilacyclobutadiene dianion compound by Sekiguchi and   co-workers and the absence of theoretical studies, ab initio   methods have been used to investigate this dianion and a number   of related species. These theoretical methods predict multiple   minima for each compound, and most minima contain folded and   bicyclic silicon rings. For   (Si4H4)2-,   (Si4H4)2-·2Li+,   [Si4(SiH3)4]2-·2Li+,   [Si4(SiH3)4]2-·2Na+,   and   [Si4(SiH3)4]2-·2K+,   respectively, the energetically lowest-lying structures are   designated A-3 (C2v symmetry), B-8 (C1  symmetry), C-1 (C2 symmetry), D-1 (C2  symmetry), and E-1 (C2h symmetry). None of these   structures satisfies both the ring planarity and the cyclic bond   equalization criteria of aromaticity. However, all of the   representative NICS values of these lowest-lying structures are   negative, indicating some aromatic character. Especially,   structures C-1 and D-1 of C2 symmetry effectively   satisfy the criteria of aromaticity due to the slightly   trapezoidal silicon rings, which are nearly planar with nearly   equal bond lengths. SiH3 substitution for hydrogen in   (Si4H4)2-·2Li+  significantly reduces the degree of aromaticity, as reflected in   the substantially smaller NICS absolute values for   [Si4(SiH3)4]2-·2Li+  than those of (Si4H4)2- and   (Si4H4)2-·2Li+. The   aromaticity is further weakened in   [Si4(SiH3)4]2-·2Na+  and   [Si4(SiH3)4]2-·2K+  by replacing lithium with the sodium and potassium cations.


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