A theoretical study of trimethylsilyl radical and related species

Abstract

Energies and structures for a series of silicon-centered radicals were calculated by ab initio methods. The lowest energy conformation of trimethylsilyl radical is found to be pyramidal. The angle formed by an S i 4 bond and the plane formed by Si with the other two carbons is found to be 51.7O. The inversion barrier through the higher energy planar C3" structure is calculated to be 13.3 kcal/mol. The planar CSh structure lies still higher in energy. In the planar forms of the methyl-substituted radicals, the methyl groups are slightly distorted so that the CH bond more eclipsed with the SOMO is bent away from this MO. The results are compared to the tert-butyl radical and interpretations for the various conformations offered. Substituent effects on the degree of pyramidalization and on the barrier to inversion in silyl radicals are substantial. Substitution of three fluorines for hydrogens in .SM3 raises the barrier from 5 to 68 kcal/mol. Successive substitution of methyl groups for hydrogens also increases the pyramidal-planar barrier.

Theoretical design of the biradical character in 1,3-diphosphacyclobutanediyl and homologous structures

“…40 This might change for the higher homologues in the series where the unpaired electrons are located at the silicon or germanium centers and pyramidalization at these atomic centers is to be expected. 41,42 Whether parent 1 possesses a closed shell system, with c 1 c c 2 , or is biradical in nature, with c 1 E c 2 , depends on the ability of the neighbouring phosphorus atoms to p-overlap with the p-orbitals at the carbon centers. Two extreme cases can be recognized: (a) the phosphorus atoms are strongly pyramidalized, the destabilizing interaction of the phosphorus lone pairs will be small, the HOMO is energetically close to the LUMO and a biradical nature prevails.…”

Heavy Analogs of Organic Free Radicals: Si‐, Ge‐, Sn‐ and Pb‐Centered Radicals

“…Thus, the Me 3 Si • radical was found to be pyramidal at the UHF/6-21G level with an inversion barrier of 13.3 kcal/mol, 62 whereas the carbon analog Me 3 C • was essentially planar at the UHF/4-31G level with a negligible inversion barrier of 1.2 kcal/mol. Thus, the Me 3 Si • radical was found to be pyramidal at the UHF/6-21G level with an inversion barrier of 13.3 kcal/mol, 62 whereas the carbon analog Me 3 C • was essentially planar at the UHF/4-31G level with a negligible inversion barrier of 1.2 kcal/mol.…”

“…Thus, the Me 3 Si • radical was found to be pyramidal at the UHF/6-21G level with an inversion barrier of 13.3 kcal/mol, 62 whereas the carbon analog Me 3 C • was essentially planar at the UHF/4-31G level with a negligible inversion barrier of 1.2 kcal/mol. 62 Replacement of hydrogen atoms in the H 3 Si • radical successively with Me and F substituents results in the progressive increase in the degree of pyramidalization and inversion barriers calculated at the UHF/3-21G level: 5.1 kcal/mol (H 3 Si • ), 6.3 kcal/mol (H 2 MeSi • ), 13.4 kcal/mol (H 2 FSi • ). 62 Replacement of hydrogen atoms in the H 3 Si • radical successively with Me and F substituents results in the progressive increase in the degree of pyramidalization and inversion barriers calculated at the UHF/3-21G level: 5.1 kcal/mol (H 3 Si • ), 6.3 kcal/mol (H 2 MeSi • ), 13.4 kcal/mol (H 2 FSi • ).…”

“…63 Two reasons are responsible for this striking difference: the marked decrease in the SOMO-LUMO energy gap for the Me 3 Si • radical stabilizing its SOMO upon pyramidalization (electronic effect), and a decrease of the van der Waals repulsion of Me substituents on going from Me 3 C • to Me 3 Si • radicals because of the longer Si-C bonds (steric effect). 62 Further increase in the degree of halogen substitution at the radical center of R 3 E • causes even more severe pyramidalization and a sharp rise in the inversion barriers (see also Section 2.1). 62 Further increase in the degree of halogen substitution at the radical center of R 3 E • causes even more severe pyramidalization and a sharp rise in the inversion barriers (see also Section 2.1).…”

On the Mechanism of the Reductive Metallation of Asymmetrically Substituted Silyl Chlorides

“…Radical intermediate B with silicon-centred chirality is either prone to another electron capture forming D (B Ǟ D) or to racemisation via the vertex mechanism [14] (B Ǟ C Ǟ ent-B). Theoretical data predict a pyramidal configuration for silyl radicals with high inversion barriers [15] indicating that the second electron transfer (B Ǟ D) might occur prior to enantiomerisation (B Ǟ ent-B). Serious experimental data suggest the configurational stability of silyl radicals.…”