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

The structure and spectroscopic characteristics of various 4-silamethylenec yclopropene (4-silatriafulvene) derivatives were investigated using ab init io 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 s tructures were found to be the transition states for the inversion motion b etween the two equivalent bent structures. The barriers were very low to an ew facile inversion at room temperature. It was found that silyl substituti on at the unsaturated silicon lowered the barrier height of the inversion m otion. Significant differences were found in the calculated absorption maxi ma and Si-29 NMR chemical shifts between the planar and trans-bent 4-silatr iafulvene derivatives, which suggested significant temperature dependence i n the spectra. All of these features were in good agreement with the experi mental results for a 4-silatriafulvene derivative, 1,2-di-tert-butyl-4,4-bi s(tert-butyldimethylsilyl)-4-silamethylenecyclopropene, that we recently is olated. (C) 2001 John Wiley & Sons, Inc.