Direct measurements of the rate coefficients for the reactions of some hydrocarbons with chlorine atoms at high temperatures

The reactions of methane, ethylene, ethane, and propane with chlorine atoms have been studied at hightemperatures by using a laser photolysis-shock tube apparatus coupled with atomic resonance absorption spectroscopy. Chlorine atoms were produced by the laser photolysis of silicon tetrachloride. The overall rate coefficients were experimentally determined from the decay of chlorine atoms as: k (CH 4 +Cl)=10 −9.26±0.26 exp[−(35.7±6.6) kJ mol −1 / RT ] cm 3 molecule −1 s −1 (1100–1550 K) k (C 2 H 4 +Cl)=10 −9.67±0.16 exp[−(35.8±3.9) kJ mol −1 / RT ] cm 3 molecule −1 s −1 (1100–1550 K) k (C 2 H 6 +Cl)=10 −9.96±0.05 cm 3 molecule −1 s −1 (1100–1400 K) k (C 3 H 8 +Cl)=10 −9.77±0.06 cm 3 molecule −1 s −1 (1100–1400 K) where the error limits are given at the two standard deviation level. With systematic errors also considered, the reliabilities, Δ[log k (RH+Cl)], were evaluated to be ±0.2 for reactions CH 4 +Cl and C 2 H 4 +Cl and to be ±0.15 for reactions C 2 H 6 +Cl and C 3 H 8 +Cl. These results are in good agreement with the extrapolations of the kinetic data measured up to 800 K by Pilgrim et al. To examine the product channels of these reactions, an ab initio molecular orbital calculation was performed at the QCISD(T)/6-311G ( d,p )//MP2(full)/6–31G( d ) level. The calculation shows that the energetically favorable channels of CH 4 _Cl, C 2 H 6 +Cl, and C 3 H 8 +Cl are H abstraction by Cl atoms. In the C 2 H 4 +Cl system, the measured rate coefficient has no total density dependence so that the Cl addition to ethylene is negligible in this experimental temperature range, although the addition channel has no energy barrier. Based on transition state theory, the rate coefficients were also calculated and discussed. The reactions of hydrocarbons with Cl atoms are suggested to follow the Evans-Polanyi law.