CN RADICAL REACTIONS WITH SELECTED OLEFINS IN THE TEMPERATURE-RANGE 174-K-740-K

The rates of CN reactions with C2H2, C2H4, and C4H4 (vinylacetylene) have been measured by the method of laser photolysis/laser-induced fluorescence. These rates were well fit by the following three-parameter Arrhenius expressions (in units of cm3/s): k(C2H2) = 5.67 x 10(-9) T-0.55 exp(-4.0/T), k(C2H4) = 1.72 x 10(-10) T-0.035 esp(160/T), k(C4H4) = 1.07 x 10(-7) T-0.82 exp(-228/T). Though the olefinic and acetylenic C-H bond strengths differ significantly in C2H2 and C2H4, the reaction rates in C2H2 and C2H4 are very similar. This indicates that the reactions most likely occur by addition to unsaturated carbon-carbon bonds. The rate for CN + C4H4 is approximately the sum of k(C2H2) and k(C2H4) indicating that the double and the triple bond in C4H4 are equally reactive as in C2H2 and C2H4. Conventional TST-RRKM results were unable to account for the slight negative temperature dependence of the CN + C2H2 and CN + C2H4 reaction rates. However, the approximate method introduced by Forst for calculating the specific rate constants for the decomposition of the CNC2H2 and CNC2H4 adducts yielded correct temperature dependences.