Empirical correlation found between the observed second-order @[email protected], \(k_{\text{q}}\), for an @[email protected] electron-transfer reaction and the Gibbs energy of the @[email protected] process within the @[email protected] (\(\Delta _{\text{ET}}G^{\,\unicode{x26ac}}\)): \[k_{\text{q}} = \frac{k_{\text{d}}}{1+ \frac {k_{\text{d}}}{K_{\text{d}}\, \text{Z}}\left [ \text{exp}\left ( \frac{\Delta G^{\ddagger }}{RT} \right ) + \text{exp}\left ( \frac{\Delta_{\text{ET}} G^{o }}{RT} \right ) \right ]}\] with \(k_{\text{d}}\) and \(k_{\text{-d}}\) the @[email protected] for the formation and separation, respectively, of the @[email protected] (precursor) complex, \(K_{\text{d}} = \: ^{k_{\text{d}}}\! /_{k_{\text{-d}}}\), \(Z\) the universal @[email protected] factor, \(R\) the @[email protected], \(T\) the absolute temperature and \(\Delta G^{\ddagger}\) the @[email protected] Gibbs energy of the forward @[email protected] reaction.