What Controls Selectivity of Hydroxyl Radicals in Aqueous Solution? Indications for a Cage Effect

The oxidation of three isotopologues of methylcyclohexane (MCH: C₇H₁₄, C₇D₁₄, c-C₆D₁₁-CH₃) by OH-radicals (^•OH) in aqueous solution was investigated. Intermolecular and intramolecular H/D kinetic isotope effects (KIE = k_H:k_D) for the abstraction of H and D atoms by ^•OH were measured. These KIEs reflect inter- and intramolecular selectivities of hydrogen abstraction, i.e., the selection of ^•OH attack on carbon-hydrogen bonds in different molecules and in different positions of one molecule, respectively. The intermolecular selectivity of ^•OH attack in aqueous solution is largely discriminated against in comparison with the intramolecular selectivity. The observed extent of discrimination cannot be explained by partial diffusion control of the overall reaction rates. A cage model, where ^•OH and hydrocarbon molecules are entrapped in a solvent cage, is more appropriate. The much higher intramolecular KIEs compared to the intermolecular KIEs of the same chemical reaction, R-H + ^•OH → R^• + H₂O, indicate a high degree of mobility of the two reaction partners inside of the solvent cage. This mobility is sufficient to develop an intramolecular selectivity comparable to that of gas-phase reactions of ^•OH. Furthermore, literature data on KIEs of H-abstraction by ^•OH in aqueous and gas phases are discussed. There is a general tendency toward lower selectivities in the aqueous phase.