Density Functional Theory Investigation of Possible Structures of Radicals in Coal Undergoing O-2 Chemisorption at Ambient Temperature

We have performed a theoretical study, using density functional theory at the B3LYP/6-311G(d, p) level, of the possible radical types and their structures in coal and their stabilities once they come into contact with atmospheric O-2 at ambient temperature. Eight possible radical types were proposed and explored through fully optimized electronic structure calculations. It was found that the radicals arising from dangling bonds on both the aromatic ring and the alkyl chain via covalent bond rupture are highly reactive toward oxidation, which resulted in the formation of stable oxygen-containing radicals. However, singlet occupied molecular orbital analysis suggested that the obtained radical via O-2 addition on the alkyl group should be attributed to the carbon-centered radical. The aryloxy C-O bond scission actually forms a carbon-centered radical. This type of radical and the radical that originated from odd-numbered carbon fused aromatic rings show high stabilities toward oxidation on account of the positive adsorption free energy change. In addition, the oxidation susceptibility of the radicals derived, from five and seven-membered rings, and nitrogen heteroatom defects in polyaromatic units of coal are also low at 298 K. Unlike the above radicals that could yield a superoxide group through O-2 addition, hydrogen abstraction from a semiquinone radical should be the preferred pathway, which forms a stable quinone group: Meanwhile, small mobile OOH is formed, which may initiate, the chain radical oxidation reaction in low rank coal