Bond Energies and Bond Dissociation Energies

The problem of bond dissociation, R1R2 → R1 • +R2 •, is addressed from the view point that the fragments, R1 and R2, may not be individually electroneutral in the host molecule, whereas the corresponding radicals certainly are. An expression is derived for the charge neutralization energy, CNE, accounting for the neutralization of R1 by R2. This leads to a new formula for the dissociation energy, D* = ε + CNE + ΔEnb + RE(R1) + RE(R2), where ε is the charge-dependent bond energy, ΔEnb is a small nonbonded contribution and the last two terms are reorganizational energies which measure the relaxation of an electroneutral fragment to yield the final product. This new formula is general. For diatomics it reduces to D* = ε. For a bond in the "interior" of a molecule (i.e. a bond linking sufficiently large groups), the appropriate expression is D* ≈ ε + RE(R1) + RE(R2). Peripheral bonds (e.g., C-X with X = H, Cl, Br, I) are described by D* ≈ constant + RE. Finally, bonds involving the "exterior" of a molecule (e-g., hydrogen bonds) are described by D* = CNE + ΔEnb. Even though the latter "bonds" may be relatively weak, any charge imbalance resulting from their formation is capable of inducing significant modifications in the "interior" of the bonded partners and thus can affect their reactivities. This is where detailed charge analyses and the calculation of charge-dependent bond energies can prove valuable.