Theoretical study of the solvent effect on the hydrogen abstraction reaction of the methyl radical with hydrogen peroxide

The hydrogen abstraction reaction of the methyl radical with hydrogen perox ide (CH3. + HOOH --> CH4 + HOO.) in both the gas phase and aqueous solution was studied by means of quantum chemical calculations. The gas phase react ion was described at the MP2, QCISD(T) and CCSD(T) levels of theory. To eva luate the effect of the solvent? different continuum solvation models were used. First, as a necessary calibration, the performance of the polarizable continuum model (PCM), the conductor like screening model (CPCM) and the c onductor like screening model for real solvents (COSMO-RS) was tested in th e calculation of the free energies of hydration for a set of 16 neutral org anic molecules. The PCM and CPCM solvation models reproduce the experimenta l hydration free energies best, with an rms value of about 0.2 kcal mol(-1) . The solvent effect on both the activation and reaction energies of the re action of the methyl radical with hydrogen peroxide was subsequently calcul ated. All solvation models confirm the experimentally observed decrease in reaction rate in going from gas phase to aqueous solution. The PCM and CPCM suggest an increase in activation energy by about 4 kcal mol(-1), correspo nding thus to a 1000-fold decrease in reaction rate, in good agreement with experiment. The reason for such a strong solvent effect is a larger stabil ization of hydrogen peroxide by water.