Ionization Energies and Entropies of Cycloalkanes. Kinetics of Free Energy Controlled Charge-Transfer Reactions.

Article abstract of DOI:10.1021/j100215a030

Enthalpies and entropies of ionization (ΔH⁰_ion and ΔS⁰_ion) of alkylcyclohexanes, as well as cycloheptane, cyclooctane, and trans-Decalin, have been determined by charge-transfer equilibrium measurements.Values of ΔH_ion, in units of kcal mol^-1 (or eV), range from 229.6 (9.96) for cycloheptane to 210.7 (9.14) for trans-Decalin.A major effect of alkyl substitution is observed following substitution at a site α to a tertiary hydrogen atom (as from methylcyclohexane to 1,2-dimethylcyclohexane), or following replacement of a tertiary hydrogen atom (as from methylcyclohexane to 1,1-dimethylcyclohexane).In both cases, ΔH⁰ _ion decreases by ca. 5 kcal mol^-1.Entropies of ionization are near zero for alkylcyclohexanes but range up to 5 cal deg^-1 mol^-1 for nonsubstituted cycloalkanes (cyclooctane).The charge-transfer reactions involving the cycloalkanes are shown to be fast processes; i.e., the sum of the reaction efficiencies (r=k/k_collision) of the forward and reverse processes is near unity.The efficiencies of these processes appear to be determined uniquely by the overall free energy change (or equilibrium constant K).Specifically, the reaction efficiencies are defined, within a factor of 2 by the relation r=K/(1+K), which can be justified by using transition-state theory applied to the decomposition of a collision complex over surfaces lacking energy barriers.These reactions are defined as intrinsically fast processes in that they are slowed only by the overall reaction thermochemistry and not by any properties or reactions of the intermediate complex.