Kinetic Study for the Reactions of Chlorine Atoms with a Series of Hydrofluoroethers

Abstract

The absolute rate constants for the reactions of chlorine atoms with several hydrofluoroethers, CHF2OCHF2 (1), CF3CH2OCH3 (2), CF3CH2OCHF2 (3), and CF3CH2OCH2CF3 (4), were measured in the gas phase over the temperature range 273−363 K. The experiments were performed with a very low pressure reactor (VLPR) in a molecular flow system, where both reactants and products were simultaneously monitored with a quadrupole mass spectrometer. The absolute rate constants are given by the expressions (in cm3 molecule-1 s-1, 2σ uncertainties): k1 = (1.03 ± 0.19) × 10-12 exp(−867 ± 106/T), k2 = (2.26 ± 0.20) × 10-11 exp(0 ± 50/T), k3 = (3.84 ± 1.00) × 10-12 exp(−1451 ± 147/T), and k4 = (4.03 ± 0.87) × 10-12 exp(−680 ± 121/T). All reactions proceed via the abstraction of a hydrogen atom and lead to the formation of HCl. The C−H bond strengths of the fluoroethers under study were determined theoretically by ab initio calculations, using the 6-31G basis set, augmented by polarization and diffusion functions. Electron correlation was taken into account by second-order Møller−Plesset perturbation theory (MP2, frozen core). The theoretical results suggest that the C−H bond strengths are increasing in the order −OCH2−H < −OCH(CF3)−H < −OCF2−H. Furthermore, the calculated C−H bond strengths are correlated with the corresponding rate parameters for the reactions of Cl atoms with CF3CH2OCH3, CF3CH2OCH2CF3, and CHF2OCHF2, respectively.