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<p>The symmetry breaking in octahedral silsesquioxane and its germanium analogues (Si<sub>8</sub>O<sub>12</sub>H<sub>8</sub> and Ge<sub>8</sub>O<sub>12</sub>H<sub>8</sub>) has been investigated using the M06-2X/6-31++G(3df, 3pd) method and group theory. Both structures undergo <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10904_2025_3970_Article_IEq1.gif" Format="GIF" Height="17" Rendition="HTML" Resolution="72" Type="Linedraw" Width="59" /> </InlineMediaObject> <EquationSource Format="TEX">\({O}_{h}\downarrow {T}_{h}\)</EquationSource> </InlineEquation> symmetry breaking, characterized by pseudo-Jahn−Teller stabilization energies of 0.22&#xa0;kcal/mol for Si-POSS and 9.82&#xa0;kcal/mol for Ge-POSS. Under the influence of the pseudo-Jahn–Teller effect, the distortion vector involves the vibrational a<sub>2g</sub> mode with imaginary frequency. The distortion forces in O<sub>h</sub>-POSS are predominantly localized on the oxygen atoms and driven by the coupling between the lowest unoccupied molecular orbital (a<sub>1g</sub>) and the highest occupied molecular orbital (a<sub>2g</sub>). The symmetry breaking is attributed to a pseudo-Jahn–Teller mechanism of type (a<sub>2g</sub> x a<sub>1g</sub>) = a<sub>2g</sub>. The symmetrical substitution of oxygen atoms by X (where X = C, N, P) results in viable T<sub>h</sub>-Si<sub>8</sub>X<sub>12</sub>H<sub>8</sub> and T<sub>h</sub>-Ge<sub>8</sub>X<sub>12</sub>H<sub>8</sub> 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<sub>h</sub>-symmetrical POSS is sufficiently large to accommodate small ions.</p>

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

  • 期刊论文

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  • 发表日期：

    2025年8月18日

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  • Jules Tshishimbi Muya,
  • Arnout Ceulemans,
  • Carol Parish

摘要

The symmetry breaking in octahedral silsesquioxane and its germanium analogues (Si8O12H8 and Ge8O12H8) 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 a2g mode with imaginary frequency. The distortion forces in Oh-POSS are predominantly localized on the oxygen atoms and driven by the coupling between the lowest unoccupied molecular orbital (a1g) and the highest occupied molecular orbital (a2g). The symmetry breaking is attributed to a pseudo-Jahn–Teller mechanism of type (a2g x a1g) = a2g. The symmetrical substitution of oxygen atoms by X (where X = C, N, P) results in viable Th-Si8X12H8 and Th-Ge8X12H8 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 Th-symmetrical POSS is sufficiently large to accommodate small ions.