THEORETICAL-STUDY ON THE STRUCTURAL CORRELATIONS BETWEEN SILYL RADICALS AND THEIR PARENT SILANES

The geometries of Y(n)SiMe(3-n) radicals (Y = Me(3)Si, Cl; n=0-3) and of their parent silanes have been determined by ab initio calculations at the UHF/TZP level. It is shown that the correlation between the Si-3s character in the singly occupied molecular orbital (SOMO) and in the sigma(Si-H) bonding MO of the parent silanes does not lead to any structural information. The experimental Si-29 hyperfine splitting (hfs) constants are fairly well reproduced at the UMP2/DZP//TZP level. The large increments in a(Si-29) observed experimentally when methyl groups are replaced successively by chlorine atoms are mainly due to the electronic influence of the substituent rather than to structural changes at the radical centre as previously suggested. This finding confirms that in localized radicals the relationship between the magnitude of the isotropic hfs constant for the central atom and the structure at the radical centre does not, in general, hold.