Empirical correlation found between the observed second-order rate constant, $k_{\text{q}}$, for an intermolecular electron-transfer reaction and the Gibbs energy of the photoinduced electron transfer process within the encounter complex (${\mathrm{\Delta }}_{\text{ET}}{G}^{\text{⚬}}$): $$k_{\text{q}}=\frac{k_{\text{d}}}{1+\frac{k_{\text{d}}}{K_{\text{d}}\text{Z}}[\text{exp}\left(\frac{\mathrm{\Delta }{G}^{‡}}{RT}\right)+\text{exp}\left(\frac{{\mathrm{\Delta }}_{\text{ET}}{G}^o}{RT}\right)]}$$ with $k_{\text{d}}$ and $k_{\text{-d}}$ the rate constant for the formation and separation, respectively, of the encounter (precursor) complex, $K_{\text{d}}={}^{k_{\text{d}}}{/}_{k_{\text{-d}}}$, $Z$ the universal collision frequency factor, $R$ the gas constant, $T$ the absolute temperature and $\mathrm{\Delta }{G}^{‡}$ the activation Gibbs energy of the forward electron transfer reaction.