Strain in nonclassical silicon hydrides: An insight into the "ultrastability" of sila-bi[6]prismane (Si18 H12) cluster with the endohedrally trapped silicon atom, Si19 H12.

The recently postulated concept of "ultrastability" and "electron-deficient aromaticity" (Vach, Nano Lett 2011, 11, 5477; Vach, J Chem Theory Comput 2012, 8, 2088) in a sila-bi[6]prismane having an additional entrapped silicon atom, Si19 H12 , has been disproved on the basis of a careful analysis of the energetic characteristics related to the formation of this and other silicon hydrides. The central silicon atom in Si19 H12 is weaker bound to other silicon atoms than in conventional tetrahedral silanes; moreover, Si19 H12 possesses a significant amount of strain. The role of strain in the formation of the title compounds has been further rationalized by calculating the relative energies for the transformation to a half-planar conformation in methane and in silane and by calculating the respective strain energies. The strain energy value in Si18 H12 is equal to 9.93 eV whereas the same property for Si19 H12 lies in range of 6.42-8.85 eV. Two low-energy isomers of Si19 H12 which lie by 2.77 and 3.42 eV (!) lower in energy than the originally considered sila-bi[6]prismane-based structure have been proposed.