Direct rate measurements showing negative temperature dependence for reaction of atomic oxygen with cis-2-butene and tetramethylethylene

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Abstract
Reported in this paper are the first direct rate measurements showing a negative temperature dependence for the reaction of ground state atomic oxygen with cis-2-butene and tetramethylethylene. Wide variations made in the experimental conditions (e.g., total pressure, O atom concentration, and olefin concentration) of these two systems have shown that the measured rate constants were uninfluenced by secondary reactions. The absence of any dependence of the measured rate constants on total pressure at several temperatures indicate that the reactions investigated were bimolecular processes. When expressed in the form of an Arrhenius equation, the observed negative temperature dependence results in an apparent negative activation energy, i.e., kcis-2-B=(9.69± 0.96)× 10−12exp (319± 63 cal mol−1/RT) and kTME=(5.58± 1.07)× 10−12 exp (1570± 120 cal mol−1/RT). Units are in cubic centimeters per molecule seconds. If a threshold energy of 0.0 cal mole−1 is assigned to the reaction of O(3P) with TME, the temperature dependence of the pre-exponential term for a rate expression of the form k =A(T)exp(−E0/RT),E≥ O) is calculated to be T−2. These new results are discussed in terms of both collision and transition state theories.