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 Si(4)R(4) 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 Si(4)F(4) 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 Si(4)R(4) 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 Si(4)R(4) analogues with amido substituents, and heavier germanium-containing systems Ge(4)R(4) (potential precursors for semiconducting materials) are predicted.