Dual-Level Direct Dynamics Studies on the Hydrogen Abstraction Reactions of CH2CH3 - n Xn+HBr (X=Cl, Br and n=1, 2)

Abstract

The reactions of the HBr molecule with CH2CH2Cl (reaction R1), CH2CHCl2 (R2), CH2CH2Br (R3) and CH2CHBr2 (R4) are investigated by a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points were calculated at the MPW1K/6–311+G(d,p) and BMK/6–311+G(d,p) levels. To refine the reaction enthalpy and energy barrier height of each reaction, single-point energy calculations were carried out by the G2M(RCC5) method based on the geometries optimized at the above-mentioned two levels. Using the canonical variational transition state theory or the canonical variational transition state theory with the small-curvature tunneling correction, the rate constants of HBr with CH2CH2Cl (R1), CH2CHCl2 (R2), CH2CH2Br (R3), and CH2CHBr2 (R4) were calculated over a wide temperature range of 200–2000 K at the G2M(RCC5)//MPW1K/6–311+G(d,p) level. The effect of chlorine or bromine substitution on the ethyl radical reactivity is discussed. Finally, the total rate constants are fitted by two models, i.e. three-parameter and four-parameter expressions.