Jahn-Teller distortion in polyoligomeric silsesquioxane (POSS) cations.

Area of Science:

• Computational Chemistry
• Materials Science
• Quantum Chemistry

Background:

• Silsesquioxanes are cage-like silicon-oxygen compounds with potential applications in materials science.
• Understanding the electronic structure and symmetry of silsesquioxane monocations is crucial for predicting their behavior and properties.
• Jahn-Teller and pseudo-Jahn-Teller effects are known mechanisms for symmetry breaking in molecules with degenerate electronic states.

Purpose of the Study:

• To investigate and elucidate the symmetry breaking mechanisms in cubic octa-tert-butyl silsesquioxane and octachloro silsesquioxane monocations.
• To determine the electronic states and ground group symmetries of these silsesquioxane monocations.
• To quantify the Jahn-Teller stabilization energies and analyze the nature of symmetry distortions.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model the electronic structure.
• Group theory principles were utilized to analyze molecular symmetry and electronic states.
• Calculations were performed at the B3LYP/def2-SVP level of theory.

Main Results:

• Under Oh symmetry, the monocations possess (2)T2g and (2)Eg electronic states, leading to different symmetry breaking pathways.
• The ground states adopt C3v and D4h symmetries for octa-tert-butyl and octachloro silsesquioxane, respectively.
• Jahn-Teller stabilization energies were calculated as 3959 cm(-1) for octa-tert-butyl and 1328 cm(-1) for octachloro silsesquioxane.
• Octa-tert-butyl silsesquioxane displays a combined Jahn-Teller and pseudo-Jahn-Teller distortion, localized on a Si-(tert-butyl) group.
• Octachloro silsesquioxane exhibits a Jahn-Teller distortion distributed over oxygen atoms.

Conclusions:

• The study reveals distinct symmetry breaking mechanisms in the two silsesquioxane monocations due to differences in their substituents.
• The findings provide insights into the electronic and structural properties of silsesquioxane monocations, relevant for molecular design.
• The identified distortion forces and coupling between electronic states and vibrational modes are key to understanding molecular stability.