An AM1 MO study of bond dissociation energies in substituted benzene and toluene derivatives relative to the principle of maximum hardness

The heats of formation of a series of m- and p-substituted benzene and toluene derivatives, Ar-Y and ArCH2-Y, and their phenyl or benzyl cations, anions, and radicals were calculated by the semiempirical AM1 MO method. Using this data and either the experimental values for the Y species or those obtained by the ab initio CBS-4 method, the heterolytic and homolytic bond dissociation energies (BDEs) were calculated along with the electron transfer energies for the ions. While the values of the homolytic BDEs were essentially independent of the ring substituents, a plot of the heterolytic BDEs versus the electron transfer energies gave a straight line of unit slope with an intercept at Delta H-homo thus confirming that Delta H-het = Delta H-ET + Delta H-homo. Likewise, a plot of the appropriate HOMO or LUMO energies of the phenyl, benzyl, or Y ions versus Delta H-het, gave a linear plot in agreement with the principle of maximum hardness. A positive charge adjacent to the bond being broken increases the Delta H-homo value while a negative charge decreases Delta H-homo. (C) 1998 Elsevier Science Ltd. All rights reserved.