Systematic Application of the Principle of Detailed Balancing to Complex Homogeneous Chemical Reaction Mechanisms.

Abstract

It is not uncommon for proposed complex reaction mechanisms to violate the principle of detailed balancing. Here, we draw attention to three ways in which such violations can occur: reversible reaction loops where the rate constants do not attain closure, illegal loops, and reversible steps having rate equations in the forward and reverse directions that are inconsistent with the equilibrium expressions. We present two simple methods to test whether a proposed mechanism is consistent with the first two aspects of the principle of detailed balancing. Both methods are restricted to closed homogeneous isothermal reactions having mechanisms that consist of stoichiometrically balanced reaction steps. The first method is restricted to mechanisms in which all reaction steps are reversible; values of Δf G° are assigned to all reaction species, equilibrium constants are then computed for all steps, and all rate constants for elementary steps are constrained by the relationship Keq = kf/ kr. The second method is applicable to mechanisms that can consist of a series of reversible and/or irreversible reaction steps. One first examines the subset of reversible steps to determine whether any of these steps are stoichiometrically equivalent to a combination of any of the other steps. If so, the forward and reverse rate expressions must yield equilibrium constants that are in agreement with the stoichiometric relationships. Next, the complete set of steps is examined to look for "illegal reaction loops". Both of these procedures are performed by constructing matrices that represent the stoichiometries of the various reaction steps and then performing row reductions to identify basis sets of loops. A method based on linear programming is described that determines whether a mechanism contains any illegal loops. These methods are applied in the analysis of several published reaction mechanisms.