INFRARED-ABSORPTION PROBING OF THE CL- DIRECT MEASUREMENT OF ARRHENIUS PARAMETERS FOR HYDROGEN ABSTRACTION(C2H4 REACTION )

The laser photolysis/infrared long path absorption technique has been used to study the reaction of Cl atoms with C2H4. The time-resolved po pulation of HCl produced in the reaction is monitored by continuous in frared absorption after pulsed photolytic generation of Cl atoms. At r oom temperature, the reaction proceeds principally through addition to form a chloroethyl radical, and the HCl production at 10 Torr is almo st exclusively via secondary reactions. Above 500 K the abstraction ch annel dominates the HCl production, allowing direct extraction of Arrh enius parameters for the hydrogen abstraction, The abstraction rates a re extracted by correcting the total rate by the HCl yield, which is d etermined by comparison with the Cl + C3H8 reaction. The abstraction r eaction is well-described between 500 and 800 K by the simple Arrheniu s expression k(T) (6.2 +/- 1.4) x 10(-11)e(-(3400+/-450/T) cm(3) molec ule(-1) s(-1) (2 sigma). The activation energy determined in the prese nt work would yield a heat of formation of the vinyl radical of Delta H degrees(f,298)(C2H3) = 71.3 +/- 1.3 kcal mol(-1) when combined with an extrapolation of published 298-495 K measurements of the HCl + C2H3 activation energy. A third-law analysis al 500 K yields Delta H degre es(f,298)(C2H3) = 71.6 +/- 0.3 kcal mol(-1).