Kinetics of the Reactions of Partially Halogenated Methyl Radicals (CH2Cl, CH2Br, CH2I, and CHCl2) with Molecular Chlorine

Article abstract of DOI:10.1021/j100179a033

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 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?, including both random and an estimated 20percent 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.