Characterization of a Triplet Vinylidene.

Singlet vinylidenes (R 2 C═C:) are proposed as intermediates in a series of organic reactions, and very few have been studied by matrix isolation or gas-phase spectroscopy. Triplet vinylidenes, however, featuring two unpaired electrons at a monosubstituted carbon atom are thus far only predicted as electronically excited-state species and represent an unexplored class of carbon-centered diradicals. We report the photochemical generation and low-temperature EPR/ENDOR characterization of the first ground-state high-spin (triplet) vinylidene. The zero-field splitting parameters ( D = 0.377 cm -1 and | E| / D = 0.028) were determined, and the 13 C hyperfine coupling tensor was obtained by 13 C-ENDOR measurements. Most strikingly, the isotropic 13 C hyperfine coupling constant (50 MHz) is far smaller than the characteristic values of triplet carbenes, demonstrating a unique electronic structure which is supported by quantum chemical calculations.