Average ionization energies of fragments in molecules from fragment transfer energies

A method is described to determine the changes in the stabilities of fragments X and Y that result from the formation or cleavage of the X-Y bond. Fragment transfer energies are defined as the differences between the energies of the free fragments and of the fragment in the molecule. The sum of these transfer energies equals the bond dissociation energy, and they allow one to discuss the relation between fragment energies and bond stabilities. The method is illustrated by considering a reaction of the type (X-Y)+ --> X+ + Y and, specifically, the dediazoniation reactions of a wide variety of diazonium ions. X-N bonding in the diazonium ions results because the X+ cations are more stabilized than the N2 groups are destabilized by the charge transfer associated with bond formation, that is, cations force N2 to form diazonium ions. The N2 transfer energies are found to be correlated with the N2 charges supporting the generally made assumption that fragment stabilities might be discussed in terms of fragment charges. The correlation is well approximated by a linear function and its slope defines the average ionization energy of a fragment in a molecule. It is found that the average ionization energy of N2 in the diazonium ions roughly equals the ionization energy of free N2, and consequences are discussed.