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: k1a=(2.8 ± 0.3)× 10–11(T/298)–(0.85 ± 0.14) cm3 molecule–1 s–1. CHCl2: k1b=(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.