A model of two interacting states for the SN2-substitution reactions and dissociative electron transfer is considered. It takes into account both direct SN2 effects which are due to the interaction between the bonded and nonbonded molecular modes and indirect SN2 effect emerged from the Franck-Condon principle. Various SN2 effects and the applicability of the Condon approximation are discussed. Equations for the transitional configuration and activation barrier in classical limit are derived for the adiabatic reaction with arbitrary form of the molecular potentials. A general scheme is illustrated by a simple example of the Morse/exponential molecular potentials, and simple algorithms are presented allowing one to plot the dependence of the activation barrier on the driving force using standard PC programs.