The Visual VB Rule for Chemical Reactions

A chemical reaction always involves bond-breaking/making processes or valence electron rearrangements, which can be characterized by the variation of VB structures. According to the resonance theory [1, 50], the evolution of a system in the elementary reaction process can be interpreted through the resonance among the correlated VB structures corresponding to reactant, product and some intermediate states. Because only symmetry-adapted VB structures can effectively resonate, all VB structures involved in the description of a reaction will thus retain the symmetry shared by both reactant and product states in the elementary process. Therefore, we postulate that the VB structures of the reactant and the product states for concerted reactions should preserve symmetry-adaptation, called the VB structure symmetry-adaptation (VBSSA) rule. 

In fact, chemical reactions are generally very localized, and most parts of the reactants and products are therefore conserved. This feature lingers in VB structures, e.g., only partial VB structural segments are directly involved in the 

---