Reaction-path dynamics of hydroxyl radical reactions with ethane and haloethanes

A detailed analysis of reaction-path dynamics of hydrogen abstraction from ethane, fluoroethane, and chloroethane by hydroxyl radical has been performed by the variational transition-state theory augmented with multidimensional semiclassical tunneling approximations. The minimum energy path and its first and second derivatives were calculated at the MP2(full)/6-31G(d,p) level of theory. The calculated barrier heights were further improved by Gaussian-2(MP2) methodology. This dual-level dynamic approach has been used to calculate the reaction rate constants for temperatures from 200 to 1000 K. The contribution from tunneling effect was evaluated using the semiclassical zero-curvature and small-curvature tunneling approximations. The calculated thermal reaction rate constants agree well with the experimental results. The variational effects on the location of central dynamical bottleneck are significant for all three reactions. The competition between potential and vibrational energies in determining the location of the variational transition-state shifts the dynamical bottleneck toward reactants. The influence of halogen substitution on the reaction rate constant is also discussed.