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

Abstract

The hydrogen abstraction reaction of the methyl radical with hydrogen peroxide (CH₃˙ + HOOH → CH₄ + HOO˙) in both the gas phase and aqueous solution was studied by means of quantum chemical calculations. The gas phase reaction was described at the MP2, QCISD(T) and CCSD(T) levels of theory. To evaluate 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 conductor like screening model for real solvents (COSMO-RS) was tested in the calculation of the free energies of hydration for a set of 16 neutral organic molecules. The PCM and CPCM solvation models reproduce the experimental 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 reaction of the methyl radical with hydrogen peroxide was subsequently calculated. 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, corresponding 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 stabilization of hydrogen peroxide by water.