ELSEVIER Chemical Physics 200 (1995) 87-106 Chemical Physics Electron-proton free-energy surfaces for proton transfer reaction in polar solvents: test calculations for carbon-carbon reaction centres M.V. Basilevsky, A.V. Soudackov, M.V. Vener Karpov Institute of Physical Chemistry, ul. Vorontsovo Pole 10, 103064 Moscow, Russia Received 7 April 1995 Abstract The proton transfer reaction R- + HR ~ RH + R- of benzyl-type compounds in a polar solvent has been studied theoretically in terms of a proton adiabatic dynamical treatment of a mixed quantum-classical reacting system. The gas phase potentials were obtained using the PM3 method and then approximated by appropriate (LEPS type) quasi-analytical functions. Polar medium degrees of freedom were introduced, similar to the Marcus electron transfer theory, in terms of a two-state electronic Hamiltonian. At this level, three-dimensional free energy surfaces were obtained, including a pair of intrasolute coordinates, the C-H stretch and heavy-atom vibrational mode of the reaction centre, together with the collective medium polarization mode. At the next stage, two-dimensional electron-proton free energy surfaces (EP FESs) corresponding to the adiabatic approximation with respect to C-H stretch were generated for the two lowest proton levels. Their main features are described. 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 (,-, 0.4-0.6) because the reaction coordinate at the transition state of the ground 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 sufficiently high classical barriers (> 10 kcal/mol) on ground-state two-dimensional EP FESs for proton transfer reactions in polar solvents. 1. Introduction Among various proton transfer (PT) reactions in- teresting and important are those proceeding in con- densed phase: in solution or the protein environment of an enzyme system. The crucial role of medium ef- fects underlying the energetics, kinetics and mecha- nisms of PT processes has been commented in Refs. [ 1-3]. The medium influence is especially remark- able for electrically charged PT systems, when a so- lute reacting species is an ion, organic or inorganic. Electrostatically induced solvation phenomena mainly determine the energetical profile of such a reaction. The dynamical role of a solvent in elementary PT processes has being extensively discussed [4-19]. The medium motion may constitute an important element of a reaction coordinate within the transi- tion state (TS) region of the reaction configurational space. Thereby, a close resemblance of mechanisms of PT and electron transfer (ET) processes becomes clearly visible. In the present work we focus on a special ap- proximate PT treatment corresponding to the proton- adiabatic mechanism. It represents an idealized limit- 0301-0104/95/$09.50 (~) 1995 Elsevier Science B.V. All rights reserved SSD1 030 1-01 04(95)00227-8