Charge distributions and chemical effects. XLIII. Bond dissociation energies and radical formation

Abstract

The problem of bond dissociation, R1R2 → R1• + R2•, is addressed from the viewpoint that the fragments, R1 and R2, may not be individually electroneutral in the host molecule, whereas the corresponding radicals certainly are. The mutual charge neutralization of R1 by R2 during the cleavage of the bond linking R1 to R2 is described by an expression featuring only molecular ground-state properties. This expression translates directly into a new energy formula for the dissociation energy, D*(R1R2) = ε(R1R2) + CNE − E*nb + RE(R1) + RE(R2), where both the molecule and the radicals are taken at their potential minimum. The charge neutralization energy, CNE, profoundly affects the relationship between the dissociation (D*) and contributing bond energy (ε), i.e., the energy in the unperturbed molecule. Nonbonded interactions between R1 and R2, E*nb, are almost negligible. The reorganizational energy, RE, measures the energy difference between R• and the corresponding electroneutral group found in the symmetric molecule RR. Numerical applications to alkanes reveal an important cancellation of individual CNE terms accompanying the mutual charge neutralization of alkyl groups during the cleavage of CC bonds, i.e., [Formula: see text]. Theoretical εCC's lead to valid CC bond dissociation energies. In CH bond dissociations, on the other hand, the sum εCH + CNE remains nearly constant although individual εCH's may differ from one another by as much as 6 kcal mol−1. The appropriate approximation, [Formula: see text], shows in what manner charge neutralization energies disguise genuine contributing CH bond energies to create a perception of seemingly constant CH bond contributions.