COLLECTIVE MEDIUM COORDINATES AND THEIR APPLICATION IN THE THEORY OF CHEMICAL-REACTIONS

A collective medium coordinate (CMC) represents a highly correlated mo tion of a large ensemble of medium particles, which, as a single varia ble, undergoes a significant change during a chemical process proceedi ng in this medium. CMCs naturally arise when charge transfer reactions in polar solvents are considered. In this context, the most important problems addressed by the CMC theory are: (1) quantum chemical comput ations of free energy surfaces explicitly treating a CMC as an essenti al coordinate, including location of minimum (reactants and products) and saddle (transition states) points on these surfaces; (2) derivatio n of stochastic dynamical equations for CMCs and evaluation of their p arameters, such as masses, friction coefficients, etc.; (3) deduction of the rate expression for stochastic dynamical systems including CMCs . These problems are discussed using as examples electron and proton t ransfer and S(N)2 reactions. Qualitative criterion formulated for when an explicit consideration of a CMC is absolutely necessary and when i t can be discarded. (C) 1997 Elsevier Science B.V.