THE DYNAMICS OF H-2 ELIMINATION FROM ETHYLENE

The dynamics of H-2 elimination in the photodissociation of ethylene at 193 nm were investigated through measurements of the translational energy distribution and rovibrational state distribution of H-2 products. Using 1,1-D2C-CH2 and 1,2-HDC-CDH, it was shown that both four-centered and three-centered elimination of H-2 could take place with an acetylene to vinylidene formation ratio of approximately two-thirds. Limited 1,2 hydrogen atom migration occurred during the fragmentation, presumably through an ethylidene-type structure. The relatively high rotational excitation of the H-2 fragment suggests that the transition is not symmetric; as the two ethylene hydrogen atoms approach each other and reach the transition state, one C-H bond should be significantly longer than the other. The vibrational energy distributions can be roughly characterized by a vibrational temperature of 4800 K and an average translational energy, which is dependent on the rovibrational state of H-2, exceeding 20 kcal mol-1.