Physical Mechanism of the Elementary Act and a Basic Model

To formulate the basic model, we consider the transfer of a proton from a donor AHZ,+1 to an acceptor B 2 in the bulk of the solution. For reactions in the condensed phase, at any fixed distance R between the reactants, the transition probability per unit time W(R) may be introduced. Therefore, we will consider first the transition of the proton at a fixed distance R and then we will discuss the dependence of the transition probability on the distance between the reactants. 

Unlike the simplest outer-sphere electron transfer reactions where the electrons are the only quantum subsystem and only two types of transitions are possible (adiabatic and nonadiabatic ones), the situation for proton transfer reactions is more complicated. Three types of transitions may be considered here5  

Entirely nonadiabatic transitions, in which the electrons cannot adiabatically follow the change in the positions of the proton and the medium molecules. 

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