Reaction Control of Transient Systems: Identifying the Dominant Transition and Intermediate States

Elucidating the energetics for all of the states in a complex reaction pathway, and identifying the kinetically feasible mechanisms for a chemical process is costly and time-consuming.  Many of the intermediate and transition states along the reaction pathway only weakly influence the overall net rate; and therefore, a desirable alternative is to determine the states that have the most influence and precisely calculate their properties.  An approach that has been proposed to determine the most influential states is the degree of rate control (C. Stegelmann, et al., JACS 131, 13563 (2009)).  While most reports have only studied stationary (steady-state) processes, there are many applications where the reactions operate in a transient mode.  In this work a variant of the degree of rate control method is introduced for transient systems; the differences between the transient and stationary approaches are highlighted.  The role that an intermediate species' concentration has on the rate controlling states is discussed and a method to account for the effects is described.  The transient variation of the degree of rate control is demonstrated for processes with increasing complexity to show how using the transient degree of rate control provides important insight into each system.  The influence of reversibility between intermediate states, energetic interactions between species in the intermediate states, and how the approach is used to identify the net activation barrier are also discussed.