Contra-thermodynamic behavior in intermolecular hydrogen transfer of alkylperoxy radicals

Quantum chemical investigation of bimolecular hydrogen transfer involving alkylperoxy radicals, a key reaction family in the free-radical oxidation of hydrocarbons, was performed to establish structure-reactivity relationships. Eight different reactions were investigated featuring four different alkane substrates (methane, ethane, propane and isobutane) and two different alkylperoxy radicals (methylperoxy and iso-propylperoxy). Including forward and reverse pairs, sixteen different activation energies and enthalpies of reaction were used to formulate structure-reactivity relationships to describe this chemistry. We observed that the enthalpy of formation of loosely bound intermediate states has a strong inverse correlation with the overall heat of reaction and that this results in unique contra-thermodynamic behavior such that more exothermic reactions have higher activation barriers. A new structure-reactivity relationship was proposed that fits the calculated data extremely well: E_A = E_o + αΔH_rxn where α = -0.70 for ΔH_rxn < 0, and α = 1.10 for ΔH_rxn > 0 and E_o = 3.05 kcal mol^-1.