Kinetics and products studies of reactions between fluorine atoms and CHF3, CHClF2, CHCl2F and CHCl3

Abstract

The absolute rate coefficients for the hydrogen abstraction reactions from CHF₃ (k₁), CHClF₂ (k₂), CHCl₂F (k₃) and CHCl₃ (k₄) by fluorine atoms in the gas phase have been measured over the temperature range 297–421 K using the discharge flow/mass spectrometric technique (DF/MS). Experiments were performed under pseudo-first-order conditions with F atoms in large excess with respect to the halomethane. The temperature dependence of the bimolecular rate coefficients is expressed in the Arrhenius form: k₁ = (2.1_−0.7^+1.1) × 10⁻¹¹ exp[−(1470 ± 150)/T], k₂ = (2.4_−0.6^+0.9) × 10⁻¹¹ exp[−(1040 ± 100)/T], k₃ = (2.6_−0.7^+1.0) × 10⁻¹¹ exp[−(850 ± 120)/T] and k₄ = (1.7_−0.4^+0.5) × 10⁻¹¹ exp[−(550 ± 90)/T]. The units of the rate constants are cm³ molecule⁻¹ s⁻¹ and the quoted uncertainties are 2σ obtained from values incorporating systematic errors due to secondary reactions. Thanks to the use of mass spectrometry, CF₄ and FCl as end products have been identified and quantified. The fast formation of CF₄ observed for each reaction of the series F + CHCl_3−xF_x is interpreted by a sequence of rapid atom-switching reactions. The fast formation of FCl is explained by reactions between F and radicals of CCl_3−xF_x (x = 0, 1 and 2) type. A mechanism of secondary reactions is proposed to account for the experimental concentration profiles determined as a function of time for CF₄ and FCl.