Generation and EPR Spectroscopy of the First Silenyl Radicals,R2C=Si.−R: Experiment and Theory

The first two persistent silenyl radicals (R₂C=Si^.?R), with a half‐life (t_1/2) of about 30 min, were generated and characterized by electron paramagnetic resonance (EPR) spectroscopy. The large hyperfine coupling constants (hfccs) (a(²⁹Si_α)=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, R₂C=C^.?R (t_1/2≈3 h), exhibits a small hfcc (a(¹³C_α)=26.6 G) and has a nearly linear geometry (?C=C?R=168.7°).