Temperature dependence of some electron-solute reactions in n-hexane and iso-octane

The rate constants for the reactions of electrons with biphenyl, pyrene, perylene and carbon tetrachloride in n-hexane can all be expressed by the relation $\text{k = 3·6(±1) × 10}{}^{14}\text{exp}\text{[}\text{−14,600(±1000)}\text{RT}\text{]}\text{mol}{}^{\mathrm{-1}}\text{dm}{}^3\text{s}{}^{\mathrm{-1}}$ over the temperature range 300-185 K. These are considered normal diffusion-controlled reactions, the activation energy (in J mol⁻¹) being that for electron diffusion. For reaction with oxygen, $\text{k = 5×10}{}^{12}\text{exp (}\text{−9600}\text{RT}\text{]}$ and the relatively slower rate for oxygen thus derives from the pre-exponential factor not from a higher activation energy, and is explained in terms of a reversible electron addition.

In iso-octane there is more variation with solute than in n-hexane. Forcarbon tetrachloride, biphenyl and oxygen the activation energies are 2000, 2800 and 3200 J mol⁻¹, respectively, and the pre-exponential factors 1·5×10¹³, 4·0×10¹³ and 1·0×10¹², respectively. The rate constant with pyrene does not follow the Arrhenius expression and is about twice those for carbon tetrachloride and biphenyl at room temperature.