Abstract: Reaction class isodesmic reaction method, which is the extension of the isodesmic reaction method usually used for the calculation of the thermodynamic properties to calculate the reaction barrier and reaction enthalpy for reactions in a class, is proposed. The pyrolysis of β alkyl hydrogen peroxide generating olefin and HO2 radical, an important class of reactions in the combustion of the hydrocarbon in low-temperature phase, is studied. Calculations for five representative reactions in this reaction class using reaction class isodesmic reaction method and direct calculation method respectively, at HF、DFT、MP2、CCSD(T) level reveal that a low-level ab initio method such as B3LYP method using reaction class isodesmic reaction method can reproduce reaction barriers by high-level ab initio methods such as CCSD(T). In this study, reaction barriers and the reaction enthalpies for 16 reactions of this class at B3LYP/6-311G(d,p) level using reaction class isodesmic reaction method are calculated and a linear free energy relationship between the reaction barrier and the reaction energy is obtained: delta V=71.02+0.41?delta E.

Key words: Reaction class isodesmic reaction method, Reaction barrier, Linear free energy relationship