Electron delocalization between the reagent and reactant molecules is the principal driving force of chemical reactions. It brings about the formation of new bonds and the cleavage of old bonds. By taking the aromatic substitution reaction as an example, we have shown the orbitals participating in electron delocalization. The interacting orbitals obtained are localized around the reaction sites, showing the chemical bonds that should be generated and broken transiently along the reaction path. By projecting a reference orbital function that has been chosen to specify the bond being formed on to the MOs of the reactant molecules, the reactive orbitals that are very similar to the interacting orbital have been obtained. The local potential of the reaction site for electron donation estimated for substituted benzene molecules by using these projected orbitals shows a fair correlation with the experimental scale of the electron-donating and -withdrawing strength of substituent groups. The reactivity is shown to be governed by local electronegativity and local chemical hardness and also by the localizability of interaction on the reaction site. 0 1996 John Wiley & Sons, Inc.
ical concept of bonds. The formation of new bonds between the reagent and the reactant is brought about by mutual electron delocalization [2]. In this interaction, the highest occupied (H0)MO and the lowest unoccupied (LU)MO play usually the dominant roles 13-51. The energy levels of the HOMO and the LUMO have been connected with some important quantities of molecules, e.g., the "electronegativity" by Mulliken [6] and the "absolute hardness" and "chemical potential" by Parr et al.