Insights into the applicability of the R dot approach for reaction mechanism kinetics studies

An alternate method, which we call as the R dot approach, to obtain the quasi steady state (QSS) rate expression for linear kinetics mechanisms (both sequential reaction mechanism and mechanisms with parallel pathways) had been recently proposed. This method requires the use of intermediate reaction routes (IRR's) to solve for the unknown intermediate species concentrations. In mechanisms where there are many choices of IRR's, it was not clear which one must be chosen so as to obtain the correct rate expression matching with the QSS rate expression. In this work, it is shown that the correct choice is the one which will make the rate limiting step (RLS) reversibility equal to the overall reversibility. This also provides a condition to check the applicability of the R dot method to a particular linear kinetics mechanism. For a mechanism with non linear kinetics, the R dot method may sometimes provide unsatisfactory approximations even if the applicability conditions are satisfied. These are illustrated through four mechanism examples.

► Resolves a specific question about a previously proposed method for deriving rate expressions for reaction mechanisms.
► A condition to check the applicability of the R dot method to a particular linear kinetics mechanism is proposed.
► Paper establishes the limitations of the R dot method and its applicability to a general reaction mechanism.