An Ab intio Investigation of the Reactions of 1,1- and 1,2-Dichloroethanes with Hydroxyl Radical

The hydrogen abstraction reactions by OH radical from 1,1- and 1,2-dichloroethane have been investigated by ab initio molecular orbital theory. Optimized geometries and harmonic vibrational frequencies have been calculated for all reactants, transition states, and products at the (U)HF/6-31G(d,p) and (U)MP2=full/6-311G(d,p) levels of theory. Single-point QCISD(T)/6-311G(d,p) //(U)MP2=full/6-311G(d,p) calculations have also been carried out for the inclusion of higher order electron correlation. Three distinct transition-state structures have been located for the reaction between 1,1-dichloroethane and OH radical (one for a-abstraction and two for beta-abstraction). Four transition-state structures have been located for the reaction between 1,2-dichloroethane and OH radical. The calculated values for the barrier heights, reaction enthalpies, and the change in entropy were found to be in good agreement with available experimental values. In addition, the rate constants calculated using the transition state theory were in reasonable agreement with experimentally derived values. Non-Arrhenius behavior of the rate constants for these reactions was found to arise from the tunneling effects and availability of multiple reaction channels.