Vibronic interactions in silicon polyhedra of the Si₄₆ clathrate compound

The vibronic (vibrational - electronic) interactions in the Si₄₆ clathrate compound are discussed using the highly symmetric (SiH)₂₀ and (SiH)₂₄ cluster models to look at a possible correlation between the Jahn - Teller effect and the superconductivity. The vibronic coupling constants for six H_g modes in the mono-, tri-, and pentaanions of (SiH)₂₀ and for twelve E₄ modes in the tri- and pentaanions of (SiH)₂₄ are calculated using the MNDO-PM3 method, one of the most reliable semiempirical molecular orbital methods, to know which mode of vibration plays a governing role in the Jahn-Teller distortions of these interesting hypothetical multianions. The lowest mode of about 100 cm^-1 appears to significantly couple with their degenerate electronic states to exhibit a large coupling constant in both silicon clusters. This type of low-frequency mode is characteristic of nanosized molecular systems and would have relevance to acoustic phonon modes in the solid. If the Jahn - Teller distortions play a role in the appearance of the molecular superconductivity in the silicon clathrate compound as suggested in the fullerene A₃C₆₀ complexes, the lowest mode of vibration in the Si₂₀ moiety should have a significant effect on the interesting solid-state properties.