The new class of substitution reactions CH3XCH/CH4CX(XZF, Cl, Br) have been studied theoretically. The results show that in the
CH3FCH reaction, the substitution channel does not compete with abstraction channel. In the CH3ClCH and CH3BrCH 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.
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