Abstract
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 D4h 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 b1g−b2g adiabatic potential energy surface is calculated showing explicitly the way of transformation from the unstable D4h geometry to the two equilibrium C2h configurations via the D2h 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.
Highlights
Cyclobutadiene and its derivatives have attracted extensive attention of researchers for many years[1,2,3,4,5,6,7,8,9]
According to the general procedure, the pseudo Jahn-Teller effect (PJTE) formulation in this case starts from the highest symmetry D4h configuration at which the molecule is square-planar with the central Si4 ring forming a square and the four ligands symmetrically bonded
Its transformations to the lower symmetry configurations is realized by symmetrized b2u type displacements toward the boat-like geometry and via combined b1g + b2g displacements toward the chair-like configuration (Fig. 2). [Hereafter we employ small letters to denote the symmetry representations of both vibrational modes and molecular orbitals, and capital letters for electronic states]
Summary
Cyclobutadiene and its derivatives have attracted extensive attention of researchers for many years[1,2,3,4,5,6,7,8,9]. From the simplest Si4H4 to the complicated Si4(EMind)[4] compound the central silicon skeleton undergoes several low-symmetry configuration changes of the reference square-planar geometry The origin of this variety of the molecular geometries in the Si4R4 and Ge4R4 series can be rationalized by employing the vibronic coupling theory in the form of the pseudo Jahn-Teller effect (PJTE)[17,18]. Applying the PJTE theory, we reveal the lowest excited states that cause the distortion (puckering) of the high-symmetry planar configuration and estimate the vibronic coupling constants that control these distortions This information provides some clues for manipulation of the structure by means of external perturbations or substitutions, similar to the recently www.nature.com/scientificreports/. Our theoretical prediction of stable structures of Si4R4 analogues with different substituents and Ge4R4 with bulky substituents is expected to provide information for synthesizing new silicon or germanium four-member ring compounds and exploring their potential applications as semiconducting materials
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