How does a weak interaction change from a reactive complex to a saddle point in a reaction?

Abstract

An activation strain model was used to reveal the behavior of interaction, which was divided into four types as it proceeded from the reaction complex to the saddle point. For the 1,3-dipolar cycloaddition reaction, the interaction remained nearly zero at the first stage, but then decreased dramatically to become negative in the final stage. For the hydrogen atom transfer reaction, it started with a large positive value, which then fell to zero in the end. For Sn2 reaction, it decreased asymptotically from the beginning to the end. For the free radical addition reaction, meanwhile, it began on a zero point and climbed steadily up to a high positive value, but then decreased slightly at the saddle point. The use of an independent gradient model reveals a weak interaction containing both attractive and repulsive forces. These four types of behavior are the result of a compromise/balance between attraction and repulsion forces.