Generation and EPR Spectroscopy of the First Silenyl Radicals, R2 C=Si. -R: Experiment and Theory.

The first two persistent silenyl radicals (R2 C=Si. -R), with a half-life (t1/2 ) of about 30 min, were generated and characterized by electron paramagnetic resonance (EPR) spectroscopy. The large hyperfine coupling constants (hfccs) (a(29 Siα )=137.5-148.0 G) indicate that the unpaired electron has substantial s character. DFT calculations, which are in good agreement with the experimentally observed hfccs, predict a strongly bent structure (∡C=Si-R=134.7-140.7°). In contrast, the analogous vinyl radical, R2 C=C. -R (t1/2 ≈3 h), exhibits a small hfcc (a(13 Cα )=26.6 G) and has a nearly linear geometry (∡C=C-R=168.7°).