in aqueous solution and in the gas phase has opposite signs. Values for the KIE and k 6 /k 5 decrease with increasing temperature from 5 to 55°C in the gas phase, while these values increase in solution. We propose that this phenomenon is a consequence of a solvent cage effect.The reactions of hydroxyl radicals with hydrocarbons play an important role in biochemical processes and the chemistry of the atmosphere and natural water supplies [1][2][3][4]. The kinetics of the reactions of OH radicals with saturated hydrocarbons (RH)including cycloalkanes, has been studied extensively in the gas phase [1]. The absolute rate constants for reaction (1) have been measured in aqueous solution only for methane [5,6], while relative rate constants in the methane series have been measured for butane [7-10] and cyclopentane [11,12].In previous work [13,14], we carried out a systematic study of the kinetics and selectivity of the reactions of OH radicals with a wide range of normal, iso, and cycloalkanes in the oxidative system containing H 2 O 2 , Fe 2+ , Fe 3+ , and water similar to Fenton's reagent [3]. We found unusually low substrate selectivity and hydrogen kinetic isotope effect (KIE) for reaction (1) in aqueous solution in comparison with the gas-phase reactions. These findings were qualitatively explained by diffusion complications for the reactions in solution related to the cage effect [13].A new approach toward the study of the mechanisms for activation of C-H bonds of saturated hydrocarbons by oxidizing agents, metal complexes, and electrophiles developed in previous work [13,15] involves study of the temperature dependence of the KIE for cyclohexane/cyclohexane-d 6 pairs and the ratios of the reaction rates for the cyclopentane/cyclohexane pair and analysis of the compensation ratios obtained. There have been no data on the temperature dependence of the rate constants of the reactions of alkanes with OH radicals in aqueous solution.

Recently, we have discovered the unusual inverse temperature dependence of the kinetic isotope effect (KIE) for the OH + c -C 6 H 12 /c -C 6 D 12 reaction in water. Temperature increase causes a KIE increase; this is valid for both the Fenton system (I) and the HOONO H 2 O system (II) (a new source of OH radicals in alkane reactions), whereas in the gas phase KIE decreases with increasing temperature. Results of these studies are considered. The KIE temperature dependences for both reactions in solution,The anomalous temperature behavior of KIE in water is described by a cage effect model at the step of the reactant encounter, which was proposed earlier by Rudakov. The model can be represented in a concise form:H 2 O, where RH··OH * is the oriented encounter complex in the water cage. It is suggested that elementary acts of H-abstraction (step 2) are identical for both gas phase and solution. It allows calculation of the competition parameter between steps 2 and -1 (the alkane leaves the cage via diffusion), by using the experimental KIEs.Correspondence to: E. S. Rudakov; e-mail: rudakove@ukr.net; V. L. Lobachev; e-mail: lmarvl@ivc.com.ua. c 2011 Wiley Periodicals, Inc. RUDAKOV, LOBACHEV, AND VOLKOVAA temperature-dependence average for systems I and II was found as (k −1 /k 2H ) = (245 ± 35) exp[(18.5 ± 0.5)/RT]. The hypothetical dependence of (k H /k D ) w on T was calculated within the overall temperature range (by extrapolations outside the experimental area, 278-328 K), and its connection with that of (k H /k D ) g on T is considered.

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