Substituent effects on inversion motion of 4-silatriafulvene derivatives: An ab initio MO study

The structure and spectroscopic characteristics of various 4-silamethylenecyclopropene (4-silatriafulvene) derivatives were investigated using ab initio MO calculations. The minimum energy geometries of all of the derivatives studied here were trans-bent around the Si=C double bond, and the planar structures were found to be the transition states for the inversion motion between the two equivalent bent structures. The barriers were very low to anew facile inversion at room temperature. It was found that silyl substitution at the unsaturated silicon lowered the barrier height of the inversion motion. Significant differences were found in the calculated absorption maxima and Si-29 NMR chemical shifts between the planar and trans-bent 4-silatriafulvene derivatives, which suggested significant temperature dependence in the spectra. All of these features were in good agreement with the experimental results for a 4-silatriafulvene derivative, 1,2-di-tert-butyl-4,4-bis(tert-butyldimethylsilyl)-4-silamethylenecyclopropene, that we recently isolated. (C) 2001 John Wiley & Sons, Inc.