ELECTRON-PROTON FREE-ENERGY SURFACES FOR PROTON-TRANSFER REACTION IN POLAR-SOLVENTS - TEST CALCULATIONS FOR CARBON-CARBON REACTION CENTERS

The proton transfer reaction R(-) + HR --> RH + R(-) of benzyl-type co mpounds in a polar solvent has been studied theoretically in terms of a proton adiabatic dynamical treatment of a mixed quantum-classical re acting system. The gas phase potentials were obtained using the PM3 me thod and then approximated by appropriate (LEPS type) quasi-analytical functions. Polar medium degrees of freedom were introduced, similar t o the Marcus electron transfer theory, in terms of a two-state electro nic Hamiltonian. At this level, three-dimensional free energy surfaces were obtained, including a pair of intrasolute coordinates, the C-H s tretch and heavy-atom vibrational mode of the reaction centre, togethe r with the collective medium polarization mode. At the next stage, two -dimensional electron-proton free energy surfaces (EP FESs) correspond ing to the adiabatic approximation with respect to C-H stretch were ge nerated for the two lowest proton levels. Their main features are desc ribed. The reaction with R = benzyl proved to be proton-adiabatic. Its rate constant transmission factor calculated in terms of the Kramers- Grote-Hynes theory is significantly less than unity (similar to 0.4-0. 6) because the reaction coordinate at the transition state of the grou nd state EP FES coincides with the medium mode. The reaction with R = fluorenyl does not obey the proton-adiabaticity condition and needs a special kinetic treatment. A remarkable observation is that the double adiabatic electron-proton approximation is incapable of providing suf ficiently high classical barriers (> 10 kcal/mol) on ground-state two- dimensional EP FESs for proton transfer reactions in polar solvents.