Mv. Basilevsky et al., ELECTRON-PROTON FREE-ENERGY SURFACES FOR PROTON-TRANSFER REACTION IN POLAR-SOLVENTS - TEST CALCULATIONS FOR CARBON-CARBON REACTION CENTERS, Chemical physics, 200(1-2), 1995, pp. 87-106
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.