Dual-level direct dynamics of the hydroxyl radical reaction with ethane and haloethanes: Toward a general reaction parameter method

The dynamics of hydroxyl radical reactions with ethane, fluoroethane, and c hloroethane have been examined in terms of variational transition state the ory augmented with multidimensional semiclassical tunneling corrections. Di fferences in reactivity for hydrogen abstraction from both the primary and the secondary carbon atoms are examined in terms of energetic and entropic effects on the location of the dynamical bottleneck. Interpolated variation al transition state theory is used to calculate reaction rate constants at the [G2(MP2)//MP2/6-31G(d,p)]/SCT level of theory. A vibrational-mode corre lation analysis is performed; i.e., the character of the vibrational modes are identified as a function of the reaction coordinate and a statistical d iabatic model is used to provide qualitative analysis of a possible vibrati onal-state specific chemistry for this reaction. A significant enhancement of the reaction rate is predicted for the excitation of the pertinent C-U s tretching mode of the reactant hydrocarbon molecule. The standard PM3 Hamil tonian is reparametrized (via a genetic algorithm) to obtain reliable semie mpirical potential energy surfaces for the reaction of ethane with the OH r adical. The specific reaction parameters (SRP) so obtained are then used to predict the reaction rate constants for both the fluoroethane and chloroet hane abstraction reactions. The temperature dependence of the rate constant s calculated at the [G2(MP2)//MP2/6-31 G(d,p)///PM3-SRP]/mu OMT level of th eory are compared with those of experiment and are found to be in very good agreement. (The computed rate constants differ from experiment by, at most , a factor of 2.5.) We demonstrate that the specific reaction parameters ca n be used for analogous reactions of the same mechanism, implying a general reaction parameter set (GRP) for related molecules. Perhaps reaction rates for larger hydrocarbons (that are of interest in atmospheric and combustio n chemistry) can be obtained reliably at low computational cost.