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
posted on 2010-11-08, 00:00authored byDennis Sheberla, Boris Tumanskii, Dmitry Bravo-Zhivotovskii, Gregory Molev, Victoria Molev, Vladimir Ya. Lee, Kazunori Takanashi, Akira Sekiguchi, Yitzhak Apeloig
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 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.