KINETICS OF THE REACTIONS OF PARTIALLY HALOGENATED METHYL RADICALS (CH2CL, CH2BR, CH2I, AND CHCL2) WITH MOLECULAR CHLORINE

The gas-phase kinetics of the reactions of four partially halogenated methyl radicals (CH2Cl, CH2Br, CH2I, and CHCl2) with Cl2 have been studied as a function of temperature using a tubular reactor coupled to a photoionization mass spectrometer. Radicals were homogeneously generated by pulsed 193- and/or 248-nm laser photolysis. Decays of the radical concentrations were monitored in time-resolved experiments as a function of [Cl2] to obtain bimolecular rate constants for the R + Cl2 --> RCl + Cl reactions studied. The following Arrhenius expressions (k = A exp(-E/RT)) were obtained (the numbers in brackets are log (A/(cm3 molecule-1 s-1)), E/(kJ mol-1); the temperature ranges are also indicated): R = CH2Cl [-11.82 +/- 0.12, 4.1 +/- 1.3, 295-719 K]; R = CH2Br [-11.91 +/- 0.14, 2.4 +/- 1.4, 295-524 K]; R = CH2I [-11.94 +/- 0.19, 0.8 +/- 2.2, 295-524 K]; R = CHCl2 [-12.07 +/- 0.15, 10.3 +/- 2.0, 357-719 K]. Errors are 1-sigma, including both random and an estimated 20% systematic error in the individual bimolecular rate constants. The Arrhenius parameters of these and two other R + Cl2 reactions are compared with theoretical determinations based on semiempirical AM1 calculations of transition-state energies, structures, and vibration frequencies. The calculations qualitatively reproduce the observed trends in both the Arrhenius A factors and in the activation energies. The use of molecular properties to account for reactivity differences among all the R + Cl2 reactions which have been studied to date are also explored using free-energy correlations with these properties.