Ultraviolet absorption spectra of the CH2Cl and CHCl2 radicals and the kinetics of their self-recombination reactions from 273 to 686 K

Abstract

The UV absorption spectra of the chloromethyl (CH₂Cl) and dichloromethyl (CHCl₂) radicals have been determined between 197.5 and 230 nm, together with the absolute rate constants for their association reactions: CH₂Cl + CH₂Cl → Products (1a) CHCl₂+ CHCl₂→ Products (1b) as a function of temperature from 273 to 686 K and between 29 and 760 Torr N₂ total pressure. The transient decays of the radicals were monitored by time-resolved UV absorption following the flash photolysis of Cl₂ mixed with the parent molecules (CH₃Cl or CH₂Cl₂). At a resolution of 2 nm, no vibrational structure was detected in either spectrum which both appear as strong broad electronic bands with maxima around 200 nm (CH₂Cl) and 215 nm (CHCl₂). At these wavelengths the absolute absorption cross-sections were measured as (1.45 ± 0.16)× 10^–17 and (1.37 ± 0.24)× 10^–17 cm² molecule^–1, respectively, relative to that of CH₃O₂ at 240 nm (4.55 × 10^–18 cm² molecule^–1). The experimental results, supported by RRKM calculations, demonstrate that the measured rate constants for removal of the radicals correspond to the high-pressure limiting rate constants for recombination, both of which exhibit a negative temperature dependence, represented by the following expressions: CH₂Cl: k_1a=(2.8 ± 0.3)× 10^–11(T/298)^{–(0.85 ± 0.14)} cm³ molecule^–1 s^–1. CHCl₂: k_1b=(9.3 ± 1.7)× 10^–12(T/298)^{–(0.74 ± 0.10)} cm³ molecule^–1 s^–1. Errors are 1 σ. The present results are compared to existing data on the self-recombination reactions of the CH₃ and CCl₃ radicals; the negative temperature dependences of the self-association rate constants for the series CH₃, CH₂Cl, CHCl₂ and CCl₃ are shown to be consistent within the framework of a recently developed variational transition-state theory method.