Evidence for Vinylidene Production in the Photodissociation of the Allyl Radical

A combination of experimental and theoretical work strongly implicates the production of vinylidene in the photodissociation of the allyl radical CH2CHCH2, an important intermediate in hydrocarbon combustion. The evidence comes from a study of the dissociation of 2-d(1)-allyl, which yields distinctly different, translational energy distributions and mechanisms for the products HCCH + CH2D and DCCH + CH3. In one mechanism a 1,3 hydrogen shift takes place to given the CH2CDCH intermediate, which then dissociates to yield CH3 + DCCH, whereas in the second, one of two branches occurs after a 1,2 hydrogen shift. The shift forms CH2D-C-CH2, which either dissociates directly to CH2D + CCH2 (vinlyidene) or undergoes a second 1, 2 shift to give CH2D-CH-CH, which subsequently dissociates to CH2D + HCCH.