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 CH(2)CHCH(2), 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 + CH(2)D and DCCH + CH(3). In one mechanism a 1,3 hydrogen shift takes place to given the CH(2)CDCH intermediate, which then dissociates to yield CH(3) + DCCH, whereas in the second, one of two branches occurs after a 1,2 hydrogen shift. The shift forms CH(2)D-C-CH(2), which either dissociates directly to CH(2)D + CCH(2) (vinlyidene) or undergoes a second 1, 2 shift to give CH(2)D-CH-CH, which subsequently dissociates to CH(2)D + HCCH.