Stability and electronic structures of substituted tetrahedranes, silicon and germanium parents – a DFT, ADMP, QTAIM and GVB study

Abstract

The electronic structures and the stability of tetrahedrane, substituted tetrahedranes and silicon and germanium parents have been studied at ωB97XD/6-311++G(d,p) level of theory. The quantum theory of atoms in molecules (QTAIM) was used to evaluate the substituent effect on the carbon cage in the tetrahedrane derivatives. The results indicate that electron withdrawing groups (EWGs) have two different behaviors, i.e., a stronger EWG makes the tetrahedrane cage slightly unstable while a weak EWG causes greater instability in the tetrahedrane cage. On the other hand, the sigma electron donating group, σ-EDG, stabilizes the tetrahedrane cage and the π-EDG leads to tetrahedrane disruption. NICS and D3BIA indices were used to evaluate the sigma aromaticity of the studied molecules, where EWGs and EDGs result in the decrease and increase, respectively, of both aromaticity indices, showing that sigma aromaticity plays an important role in the stability of tetrahedrane derivatives. Moreover, for tetra-tert-butyltetrahedrane there is another stability factor: hydrogen–hydrogen bonds which impart a high stabilization in this cage. Generalized valence bond (GVB) theory was also used to explain the stability effect of the substituents directly bonded to the carbon of the tetrahedrane cage. Moreover, the ADMP simulations are in accordance with our thermodynamic results indicating the unstable and stable cages under dynamic simulation.