Semiempirical modeling free energy surfaces for proton transfer in polar aprotic solvents

A method of calculation of a free-energy surface (FES) of the proton transf er (PT) reaction in a polar aprotic solvent is developed. This is based on the two-state (valence bond) VB description of the solute combined with rec ent continuum medium models. Its essential new feature is an explicit quant um-chemical treatment of VB wave functions, including internal electronic s tructure of a chemical subsystem. The FES includes a pair of intrasolute co ordinates, R, the distance between hydrogen-bonded atoms and s, the proton coordinate, together with the collective medium polarization mode. Two hydr ogen-bonded systems immersed in a polar solvent (Freon) were considered. Th e first one is the H5O2+ ion, a model system which was used as a benchmark testifying the validity of our semiempirical calculations. The second syste m is the neutral (CN)(CH3)N-H ... N(CH3)(3) complex in Freon. PT for this s ystem has been studied experimentally. The dependencies of basic parameters controlling FES properties (the overlap integral, the coupling matrix elem ent and the reorganization energy E-r) on intrasolute coordinates R and s a re evaluated and discussed. In particular, for the neutral complex, E-r dep ends on s linearly, and its dependence on R is weak. The FES, for the neutr al system, has two potential wells separated by the energy barrier of simil ar to 7 kcal/mol. Quantum-mechanical averaging over the proton coordinate, s, reduces the barrier from 7.0 to 1.2 kcal/mol. The value of the nonadiaba tic parameter on the averaged FES is equal to 0.13. This implies that the P T in the second system corresponds to an intermediate dynamic regime and th at proton tunneling effects are hardly significant for this reaction. (C) 2 000 Elsevier Science B.V. All rights reserved.