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Zusammenfassung:
We investigated the symmetry breaking mechanism in cubic octa-tert-butyl silsesquioxane and octachloro silsesquioxane monocations (Si8O12(C(CH3)3)8+ and Si8O12Cl8+) using density functional theory (DFT) and group theory. Under Oh symmetry, these ions possess 2T2g and 2Eg electronic states and undergo different symmetry breaking mechanisms. The ground states of Si8O12(C(CH3)3)8+ and Si8O12Cl8+ belong to the C3v and D4h point groups and are characterized by Jahn–Teller stabilization energies of 3959 and 1328 cm–1, respectively, at the B3LYP/def2-SVP level of theory. The symmetry distortion mechanism in Si8O12Cl8+ is Jahn–Teller type, whereas in Si8O12(C(CH3)3)8+ the distortion is a combination of both Jahn–Teller and pseudo-Jahn–Teller effects. The distortion force acting in Si8O12(C(CH3)3)8+ is mainly localized on one Si–(tert-butyl) group, while in Si8O12Cl3+ it is distributed over the oxygen atoms. The main distortion forces acting on the Si8O12 core arise from the coupling between the electronic state and the vibrational modes, identified as 9t2g + 1eg + 3a2u for the Si8O12(C(CH3)3)8+ and 1eg + 2eg for Si8O12Cl8+.
