An Electronegativity Model for Polar Ground-State Effects on Bond Dissociation Energies

Abstract

Homolytic bond dissociation energies are a composite of the radical stabilization energies (RSE) of the product radicals and the polar ground-state stabilization energies (PSE) of the reactant molecules. Substituent effects on the PSE are rationalized in terms of changes in the difference of group electronegativities. Thus, the PSE is composed of a bond polarity term, which measures the contribution due to the change in the electronegativity difference between the atoms in the bond, which is broken, and a polar relaxation term, which accounts for the substituent-dependent group electronegativity changes in the remaining bonds. A semi-quantitative model based on Pauling's bonding theory is suitable to assess the direction and relative magnitude of such effects. For the cleavage of benzylic and related bonds, the polar relaxation energy can be neglected (one-bond approximation) to allow the use of correlation analyses and substituent parameters for the interpretation of aryl substituent effects on the PSE. Accordingly, the plots of the PSEs versus substituent parameters should be linear, curved or parabolic depending on the electronegativity difference of the atoms in the bond being broken (⌬EN); moreover, the slopes ( values) should increase linearly with ⌬EN. The predicted dependences of the PSEs on aryl substituents are compared with known experimental results and with the data obtained from semiempirical calculations.

Aryl substituent effects and solvent effects on the decarbonylation of phenacetyl radicals

“…26 This polar effect was attributed to a ground-state stabilization, 26 which arises from the action of the substituents on the strength of the polar bond. 27,28 A similar polar effect is expected for the decarbonylation, but the data set is too small and displays insufficient variation to separate a potential polar effect from the larger radical effect. In fact, a two-parameter Hammett treatment 26 revealed that the inclusion of a polar contribution (s pol ) does not improve the correlation between logk X CO ak H CO and s rad significantly.…”

Investigation of Polar and Stereoelectronic Effects on Pure Excited-state Hydrogen Atom Abstractions from Phenols and Alkylbenzenes†

“…A strong correlation of the BDE with the substituent constant σ + has been previously established (44, 46), which was therefore also employed in the present analysis in order to provide a thermodynamic foundation for the investigated kinetic substituent effects. The detailed underlying reasons for the better correlation with σ + and the observed negative ρ value are in debate (44, 47–50), but it appears appropriate to argue that the removal of a hydrogen atom from the oxygen results in a net loss of electron density on the oxygen, which is better accommodated by electron‐donating substituents (negative ρ value).…”

One-Electron Reduction Potential and the β-Fragmentation of Acetylthiyl Radical, Comparisons with Benzoylthiyl Radical and the Oxygen Counterparts

“…The difference of the C−C bond energy in Ph−CH 3 as compared to that in CH 3 −CH 3 is 11.8 kcal/mol. In terms of radical stabilization energy, , the difference between half of the BDEs of a symmetrical and nonpolar C−C bond in Ph−Ph and the one in CH 3 −CH 3 is 12.3 kcal/mol, i.e., the difference in stability of the methyl versus phenyl radicals. Thus, the 11 kcal/mol higher C−C bond energy of PhC(O)O• as compared to that of the CH 3 C(O)O• radical is mainly attributable to the difference in the stability of the methyl vs phenyl radicals.…”