Region for which the rate constants increase with increasing exergonicity of the reaction in plots relating rate constants for @[email protected], or quantities related to it, with the standard Gibbs energy for the reaction (\(\Delta _{\text{ET}}G^{\text{o}}\)). This region is predicted by the Marcus theory for @[email protected] for the case of \(-\Delta _{\text{ET}}G^{\text{o}}\leq \lambda \) in the Marcus equation.