Reformulation of activated complex theory

The key assumption of activated complex theory (ACT), that the AC is in thermodynamic equilibrium with the reactants, needs to be reconsidered. This is because the formation of the AC is slower than its collapse to product. However, this can be remedied by assuming that the AC is formed in a rapid pre-equilibrium as a thermally activated species, which collapses to products in a slow step involving the diffusion of another AC molecule (or solvent in the case of a unimolecular reaction). This implies a violation of the principle of microscopic reversibility (PMR), as also the relation between standard free energy change and equilibrium constant (ΔG = -RTlnK). However, it may be argued that not only do these not apply to processes performed irreversibly, but also that irreversibility requires the breakdown of the PMR. Accordingly, catalysts may alter equilibrium constants, and enzymes may regulate biochemical processes in hitherto unsuspected ways.