A flash photolysis–resonance fluorescence kinetics study of the reactions of ground‐state sulfur atoms. V. Rate parameters for the reaction of S(3P)with cis‐2‐butene and tetramethylethylene

Rate parameters for the reaction of ground‐state atomic sulfur, S(3P), with the olefins cis‐2‐butene and tetramethylethylene have been determined over a temperature range of ∽280°K. A major finding of this study was that the rate constants for both reactions showed negative temperature dependencies. When k is expressed in the form of an Arrhenius equation, this necessarily leads to negative activation energies: k1 = (4.68 ± 0.70) × 10−12 exp (+0.23 ± 0.09 kcal/mole)/RT (219°‐500°K) k2 = (4.68 ± 1.70) × 10−12 exp (+1.29 ± 0.23 kcal/mole)/RT (252°‐500°K) Units are cm3 molec−1s−1. When a threshold energy of 0.0 kcal/mole is assumed for reaction (2), the temperature dependence of the preexponential term has a value of T−2. Making the usual simplifying assumptions, neither collision theory nor transition state theory leads to a preexponential factor with a strong enough negative temperature dependence. A comparison of these results with those derived from studies of the reactions of atomic oxygen, O(3P), with the same olefins shows that in both studies simple bimolecular processes were being examined. Also discussed are the possible experimental and theoretical ramifications of these new results.