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

Charge Distribution and Chemical Effects. XLII. Bond Dissociation Energy and Radical Formation

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., . 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, , shows in what manner charge neutralization energies disguise genuine contributing CH bond energies to create a perception of seemingly constant CH bond contributions

X-alpha local spin density calculations of the 1:1 hydrogen-bonded complexes formed by water, ammonia, and hydrogen fluoride

The dissociation energies of the 1:1 hydrogen-bonded complexes formed by NH3, H2O, and HF were computed in the LCGTO-Xα local spin density approximation using extended basis sets. Attention was given to the appropriate selection of α. The order of stability of the various complexes reflects well their acid–base properties, in general agreement with experimental data and refined post Hartree–Fock computations. Keywords: hydrogen bonds, Xα method, local spin density method

On the calculation of atomization energies of organic molecules in the Xα local spin density approximation

The atomization energies of selected alkanes, alkyl radicals, and amines are deduced from Xα local spin density calculations of E, the total energy of a molecule in its potential minimum. Both the exchange-correlation energy, , and itself obey, in a first approximation, simple atom-by-atom additivity rules. The appropriate α's for use in Xα(LSD) calculations of organic molecules thus appear as weighted averages and depend on the particular composition of each molecule. The at least approximate validity of this averaging procedure is illustrated by the accuracy of calculated atomization energies