Reactions of atomic hydrogen with dichloromethane in a flow system

The reaction kinetics of atomic hydrogen with dichloromethane were studied in a tubular flow reactor with 2.6 cm I.D. Pyrex tube at pressure of 2.1 to 2.7 torr absolute and room temperature . Hydrogen atoms were generated by a microwave discharge plasma. Atomic hydrogen concentration were measured by chemiluminescence titration with nitrogen dioxide. The concentrations of hydrogen atom ranged from 2.0 * 1014 to 5.0 * 1015 atoms per cm3. Chlorocarbon reagent and products were monitored by online gas chromatography (FID). Major products observed were hydrogen chloride and methane. The conversion of dichloromethane increases first to a maximum and then decreases with increasing concentration of dichloromethane. A detailed reaction mechanism was developed incorporating QRRK activated complex theory and transition state theory. Sensitivity analysis on mechanism allows identification of the important rate constants. The results of modeling mechanism are quite consistent with the experimental observation when the concentration ratio of dichloromethane to hydrogen atom is less than 0.15. This mechanism describes a path toward the formation of the major products. Through computer modeling of the reaction scheme and comparison with experimental data, the room temperature rate constants of the primary steps were determined as follows. H + CH2Cl2 --k1--\u3e HCl + CH2Cl k1 = 3.63 * 109 cm3 mole-1 s-1 H + CH2Cl2 --k2--\u3e H2 + CHCl2 k2 = 2.08 * 109 cm3 mole-1 s-