The diverse world of silicon chemistry: 14th International Symposium on Organosilicon Chemistry (ISOS XIV) in conjunction with the 3rd European Organosilicon Day, Würzburg, Germany, 31st July to 5th August, 2005

Geometry, Electronic Structure, and Pseudo Jahn-Teller Effect in Tetrasilacyclobutadiene Analogues

We revealed the origin of the structural features of a series of tetrasilacyclobutadiene analogues based on a detailed study of their electronic structure and the pseudo Jahn-Teller effect (PJTE). Starting with the D_4h symmetry of the Si₄R₄ system with a square four-membered silicon ring as a reference geometry, and employing ab initio calculations of energy profiles along lower-symmetry nuclear displacements in the ground and several excited states, we show that the ground-state boat-like and chair-like equilibrium configurations are produced by the PJT interaction with appropriate excited sates. For Si₄F₄ a full two-mode b_1g−b_2g adiabatic potential energy surface is calculated showing explicitly the way of transformation from the unstable D_4h geometry to the two equilibrium C_2h configurations via the D_2h saddle point. The PJTE origin of these structural features is confirmed also by estimates of the vibronic coupling parameters. For Si₄R₄ with large substituents the origin of their structure is revealed by analyzing the PJT interaction between the frontier molecular orbitals. The preferred chair-like structures of Si₄R₄ analogues with amido substituents, and heavier germanium-containing systems Ge₄R₄ (potential precursors for semiconducting materials) are predicted.