Theoretical investigations are carried out on the title reactions by means of direct dynamics method. The optimized geometries, frequencies and minimum energy path are obtained at the MP2/6-31+G(d,p) level, and energetic information is further refined at the MC-QCISD (single-point) level. The rate constants for both reactions are calculated by canonical variational transition-state theory with the small-curvature tunneling correction in a wide temperature range 200-3000 K, The theoretical rate constant is in good agreement with the available experimental data. Furthermore, the effects of halogen substitution on the rate constants of this class of hydrogen abstraction reactions are also discussed. (c) 2007 Elsevier B.V. All rights reserved.