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 + CHCl3−xFx is interpreted by a sequence of rapid atom-switching reactions. The fast formation of FCl is explained by reactions between F and radicals of CCl3−xFx (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.