CH bond activation of methane in aqueous solution: A hybrid quantum mechanical/effective fragment potential study

In this study, we investigated the CH bond activation of methane catalyzed by the complex [PtCl 4 ] 2− , using the hybrid quantum mechanical/effective fragment potential (EFP) approach. We analyzed the structures, energetic properties, and reaction mechanism involved in the elementary steps that compose the catalytic cycle of the Shilov reaction. Our B3LYP/SBKJC/cc‐pVDZ/EFP results show that the methane activation may proceed through two pathways: (i) electrophilic addition or (ii) direct oxidative addition of the CH bond of the alkane. The electrophilic addition pathway proceeds in two steps with formation of a σ‐methane complex, with a Gibbs free energy barrier of 24.6 kcal mol −1 , followed by the cleavage of the CH bond, with an energy barrier of 4.3 kcal mol −1 . The activation Gibbs free energy, calculated for the methane uptake step was 24.6 kcal mol −1 , which is in good agreement with experimental value of 23.1 kcal mol −1 obtained for a related system. The results shows that the activation of the CH bond promoted by the [PtCl 4 ] 2− catalyst in aqueous solution occurs through a direct oxidative addition of the CH bond, in a single step, with an activation free energy of 25.2 kcal mol −1 , as the electrophilic addition pathway leads to the formation of a σ‐methane intermediate that rapidly undergoes decomposition. The inclusion of long‐range solvent effects with polarizable continuum model does not change the activation energies computed at the B3LYP/SBKJC/cc‐pVDZ/EFP level of theory significantly, indicating that the large EFP water cluster used, obtained from Monte Carlo simulations and analysis of the center‐of‐mass radial pair distribution function, captures the most important solvent effects. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011