Reaction activity and mechanism of R3-CH structure oxidation in coal self-heating

•R3-CH structure especially the H atom is the most active site in coal self-heating with oxygen.•The free energy barrier of oxygen attacking R3-CH site is 8.28 kJ/mol.•The aromatic hydrocarbon radicals and hydroxyl radicals are significant for coal self-heating.•The carbon radical combining with an O plays a vital role in initial coal spontaneous combustion. R3-CH structure is a typical active group during coal self-heating. In this paper, Ph-CH(CH3)–CH2-Ph was selected to analyze reaction activity as well as mechanism between R3-CH structure and oxygen in coal self-heating by Multiwfn and Gaussian calculation. The results demonstrate that R3-CH structure especially the H atom is the most active site in coal self-heating with oxygen. The oxygen attacks R3-CH site in the Ph-CH(CH3)–CH2-Ph, which results in the breaking of C–H bond and formation of O–H bond. The free energy barrier of this oxidation reaction is 8.28 kJ/mol and the heat release of the process is 86.89 kJ/mol, which indicates that it can easily occur at room temperature. From the perspective of products, the aromatic hydrocarbon radicals and hydroxyl radicals related to the initial spontaneous combustion of coal are generated, which is of great significance for the further development of coal self-heating. Subsequently, the carbon radical is combined with a single oxygen atom to generate oxygen-containing free radical which plays vital roles in initial coal spontaneous combustion.