Electronic Structure of Bis(silyl)carbon-, Bis(silyl)silicon-, and Bis(silyl)germanium-Centered Radicals (R₃Si)₂XE^• (E = C, Si, Ge; X = H, Re(CO)₅, F): EPR and DFT Studies

Group 14 element bis(silyl)-substituted radicals (R₃Si)₂XE^• (E = C, Si, Ge; X = H, Re(CO)₅, F) and (R₃Si)(1-Ad)HC^• have been studied by EPR spectroscopy and DFT calculations. The significant difference in the kinetic stability at 240 K of the hydrogen-substituted persistent C-centered and analogous short-lived Si- and Ge-centered radicals is explained by different decay mechanisms: H abstraction for E = C and dimerization for E = Si, Ge. The ¹H_α and ²⁹Si_β hyperfine coupling constants (hfcc) in these radicals have dominating negative spin-polarization (SP) contribution; thus, they have a negative sign. In contrast, in the F-substituted radical, where a(F) results from spin delocalization and positive SP contribution, it has a positive sign. For Si-centered radicals it has been shown by calculations that the ¹H_α and ²⁹Si_β hfcc's result from a combination of direct and spin-polarization mechanisms, which vary as a function of the degree of pyramidality around E. As the geometry around E changes from planar to pyramidal, the contribution of the direct mechanism increases and the contribution of spin polarization decreases. The hydrogen-substituted C radicals are planar (∑θ(C) = 360.0°), in contrast to the analogous Si and Ge radicals, which are slightly pyramidal (∑θ(Si) = 354.1° and ∑θ(Ge) = 355.5°). Both (R₃Si)₂XE^• species (E = Si, Ge; X = Re(CO)₅) are planar around E.