Instability of the Octahedral Symmetry in Si8O12H8 and Ge8O12H8: A Consequence of the Pseudo-Jahn–Teller Effect

Abstract The symmetry breaking in octahedral silsesquioxane and its germanium analogues (Si 8 O 12 H 8 and Ge 8 O 12 H 8 ) has been investigated using the M06-2X/6-31++G(3df, 3pd) method and group theory. Both structures undergo $${O}_{h}\downarrow {T}_{h}$$ symmetry breaking, characterized by pseudo-Jahn−Teller stabilization energies of 0.22 kcal/mol for Si-POSS and 9.82 kcal/mol for Ge-POSS. Under the influence of the pseudo-Jahn–Teller effect, the distortion vector involves the vibrational a 2g mode with imaginary frequency. The distortion forces in O h -POSS are predominantly localized on the oxygen atoms and driven by the coupling between the lowest unoccupied molecular orbital (a 1g ) and the highest occupied molecular orbital (a 2g ). The symmetry breaking is attributed to a pseudo-Jahn–Teller mechanism of type (a 2g x a 1g ) = a 2g . The symmetrical substitution of oxygen atoms by X (where X = C, N, P) results in viable T h -Si 8 X 12 H 8 and T h -Ge 8 X 12 H 8 structures. The observed pseudo-Jahn–Teller distortion and substitutional symmetry breaking caused by X indicates a consistent electronic relaxation mechanism, characterized by the formation of C=C, N=N and P=P bonds on the POSS cubic faces, which serves as hallmarks of stability. Additionally, we find that the volume of substituted T h -symmetrical POSS is sufficiently large to accommodate small ions.