Valence bond theoretical study for chemical reactivity

Based on the symmetry of the valence bond wavefunction and the correspondence between bonding patterns and bonded tableaus for chemical bonds in the ground and excited states, the conservation of orbital symmetry and the bond symmetry rule can be expressed as "symmetry-adaptation of the valence bond (VB) structure". The symmetry-adaptation of the valence bond structure can directly predict the chemical reactivity by a symmetry analysis of the VB structures of reactants and products without carrying out explicitly theoretical calculations. Curve-crossing diagrams for atom exchange reactions H + HLi --> H-2 + Li and H + LiH --> HLiH (cyclic) --> HLi + H have been constructed with the multistructure VB approach and the valence bond structure symmetry rule. The formation mechanism of the barrier and the transition state was discussed. The calculated results show that the H exchange reaction H + LiH <--> HLi+ H is a two-step reaction.