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

Abstract

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 1Hα and 29Siβ 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 1Hα and 29Siβ 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 t...