Direct dynamics for free radical kinetics in solution: Solvent effect on the rate constant for the reaction of methanol with atomic hydrogen

We calculate the rate constant for the reaction H-. + CH3OH --> H-2 + (CH2O H)-C-. both in the gas phase and in aqueous solution at 298 K. To accomplis h this, we apply two different methods to estimate the electronic energies along the reaction path. First, we use specific reaction parameters (SRP) t o mix the exchange and correlation energies in Becke's adiabatic connection theory (AC-SRP) to optimize the model for the specific bond-breaking, bond -making combination under consideration. Second, we obtain the potential en ergy using a linear combination of the Hartree-Fock method and AML with spe cific reaction parameters (HF parallel to AM1-SRP); in this linear mixing m ethod, eight NDDO parameters and the linear mixing parameter are simultaneo usly optimized by a genetic algorithm. To calculate the reaction rate const ants in solution, the solute atomic charges are represented by class IV cha rges, the electric polarization of the solvent is determined from the elect ronic charge distribution of the solute self-consistently, and the solute e lectronic, solvent electric polarization terms are augmented by first-solva tion-shell terms calculated by the SM5.42 solvation model. Reaction rate co nstants of the hydrogen transfer reaction and the kinetic isotope effects a re studied both in the gas phase at 200-2400 K and in aqueous solution at 2 98 K. The AC-SRP and HF parallel to AM1-SRP methods, although quite differe nt, give qualitatively similar pictures of the reaction at the separable eq uilibrium solvation level; however, it is found that a full equilibrium sol vation path (ESP) calculation, which involves optimization of structures al ong the reaction path in the presence of solvent, is essential to reproduce the speedup of the reaction due to solvation, The final calculation, based on the HF parallel to AM1-SRP electronic structure calculations and ESP dy namics with variational transition state theory in curvilinear coordinates with the microcanonical optimized multidimensional tunneling approximation, agrees well with experiment not only for the speedup due to the solvation but also for the D-. + CH3OH and H-. + CD3OH kinetic isotope effects.