²⁹Si hyperfine splittings in the EPR spectra of silyl‐substituted alkyl and propynyl radicals

The ²⁹Si satellites were observed in the EPR spectra of 3‐trimethylsilylprop‐2‐ynyl, 1‐trimethylsiloxyprop‐2‐ynyl and trimethylsiloxymethyl radicals. Comparison of the experimental a(²⁹Si) values with spin densities calculated by semi‐empirical MO methods led to a value of 810 G for Q^si. The magnitude and postive temperature dependence of the a(²⁹Si) values in the siloxy‐substitued radicals showed that their preferred conformations are staggered with the silicon atom in the nodal plane of the orbital containing the unpaired electron. This contrasts with the analogous β‐trialkylsilkylethyl radicals, which adopt eclipsed confromations. The rotaton barrier baout the C‐αO bond in trimethylsiloxymethyl radicals was estimated to be 3.8 keal mol⁻¹. The preference of β‐oxamethyl radicals for the staggered conformation was attributed to the comparatively large stablization due to the interacton of the unpaired electron with the adjacent oxygen, and an attempt was made to quantify this effect.