In several cycloalkanes, ionization of the solvent or photoexcitation of a solute radical cation results in the formation of rapidly hopping solvent holes. Here we report the temperature dependence of scavenging rate constants and conductivity signals (220 to 350 K) for the solvent holes in liquid cyclohexane, methylcyclohexane, and Decalins. Our results indicate that the hopping in Decalins and methylcyclohexane is thermally activated; the activation energies are of the same order as the energy of solvent reorganization. In cyclohexane, the hopping does not seem to be thermally activated. We discuss the hypothesis that the mobile solvent holes in cycloalkanes are small polarons migrating by phonon-assisted hopping.

Mobile solvent holes in cyclohexane can be reversibly trapped by methylcyclohexane (ΔG° = −0.11 eV), 1,1-dimethylcyclopentane (ΔG° = −0.20 eV), trans-1,2-dimethylcyclopentane (ΔG° = −0.25 eV), and 2,3-dimethylpentane (ΔG° = −0.21 eV). The two dimethylcyclopentanes are identified as “special” impurities responsible for bimodal scavenging kinetics of the solvent holes. The mechanism of the reversible trapping is shown to be charge transfer. Neither the difference in the ionization potentials ΔIPliq of the solvent and the solute, nor the driving force ΔG° of the scavenging reaction are found to correlate with the rate constants of this charge transfer. The proposed explanation is that these rate constants are controlled by the height of the activation barrier that can be estimated from the difference in the vertical IP for the solute and adiabatic IP of the solvent. The correlation of the rate constants with this difference suggests that electron transfer involving mobile holes occurs much faster than the relaxation time of the solute radical cations. This work gives the first data on adiabatic IP for saturated hydrocarbons in solution.

Using time-resolved dc photoconductivity, the migration patterns and reactions of solvent hole in liquid methylcyclohexane between 230 and 350 K have been studied. It is shown that solvent holes in liquid methylcyclohexane are reversibly scavenged by solutes whose liquid-state ionization potentials are 0.2-0.3 eV below that of the solvent. The reversible electron transfer is driven mainly by the reaction heat (60-90%); further decrease in the free energy is due to increase in entropy following the destruction of solvent structure around the hole. Between 133 and 360 K, the solvent hole diffuses with activation energy of 7.8 kJ/mol; the fastest electron-transfer reactions (ca. 8.7 × 10 10 M -1 s -1 at 25 °C) have activation energies between 3.9 and 5.3 kJ/mol (250 to 350 K). Unusually large scavenging radii, 1.5-3 nm, were obtained for these charge-transfer reactions.