Theoretical studies on the substitution reactions CH3X+H→CH4+X (X=F, Cl, Br)

The new class of substitution reactions CH3X+H→CH4+X(X=F, Cl, Br) have been studied theoretically. The results show that in the CH3F+H reaction, the substitution channel does not compete with abstraction channel. In the CH3Cl+H and CH3Br+H reactions, the abstraction channels still dominate but the substitution channels should notably contribute at high temperature. A new method is proposed by topological analysis on IRC path. The relative variation trend between the breaking bond and the forming bond can be observed clearly along the reaction path. In addition, we propose a simple topological method to judge that a reaction is exothermic or endothermic.