Photodissociation of Vinyl Bromide and the Heat of Formation of the Vinyl Radical

We have performed measurements of the translational energy distributions and anisotropy parameters of the photodissociation products of vinyl bromide at 193 nm. Br‐atom and HBr elimination were observed with a branching ratio of 1.28 ± 0.05 between the two channels. Both processes occurred with a large release of translational energy, an average of about 2.0 eV for the Br‐atom channel and somewhat less for HBr elimination. The maximum release of translational energy for Br + C 2 H 3 formation led to an upper limit of 77 ± 3 kcal/mol to D 0 (C 2 H 3 ‐Br), from which an upper limit to the heat of formation of the vinyl radical, 71 ± 3 kcal/mol, was derived. This result was used to reexamine crossed molecular beams data relevant to the determination of the heat of formation of the vinyl radical. A metastable state of C 2 H 3 observed in the photodissociation of vinyl bromide was interpreted as the formation of C 2 H 3 (Ã 2 A″).