Bond dissociation energy intrinsic

As with the Marcus-Hush model of outer-sphere electron transfers, the activation free energy, AG, is a quadratic function of the free energy of the reaction, AG°, as depicted by equation (7), where the intrinsic barrier free energy (equation 8) is the sum of two contributions. One involves the solvent reorganization free energy, 2q, as in the Marcus-Hush model of outer-sphere electron transfer. The other, which represents the contribution of bond breaking, is one-fourth of the bond dissociation energy (BDE). This approach is... 

Fig. 9 Deprotonation of cations radicals of synthetic analogs of NADH by oxygen or nitrogen bases in acetonitrile. Correlation between the intrinsic barrier and the homolytic bond dissociation energy of the cation radical (AH + —> A+ + H ).

Examination of the peak potential locations and transfer coefficient values in a series of 16 cyclic and acyclic dibromides according to the procedures detailed in Chapter 3 points to a first dissociative electron transfer rate-determining step (Scheme 4.1). It is followed by another dissociative electron transfer step, leading directly to the olefin. Intrinsic barriers for the first, rate-determining step range from 0.6 to 0.8 eV, consisting mostly of the bond dissociation contribution (one-fourth of the bond dissociation energy). 

There is a clear antiperiplanar preference for the reaction (Scheme 4.2) due to the stabilization of the radical by coupling of the unpaired electron with bromine (ESR) in the first case. The weaker bond dissociation energy leads to a more favorable standard potential and a weaker intrinsic barrier. When the two conformers are present and can convert one into the other, the reduction follows a CE mechanism (Section 2.2.2), which goes through the more reducible of the two.1 2... 

The (at least approximate) validity of Eqs. (6.12)-(6.14) is clearly demonstrated by the remarkable quality of intrinsic CN bond energies and of bond dissociation energies calculated by means of nitrogen net atomic charges deduced in this manner. 

Straightforward applications of the theory are presented in the atom-by-atom approach, as exemplified in Table 15.4, using charges deduced from NMR shifts, Eq. (6.8) for the carbon atoms, and Eqs. (6.12)-(6.14) for the nitrogen atoms. (CN bond dissociation energies and comparisons with the corresponding intrinsic bond energies are described in Chapter 12 for both alkylamines and selected nitroalkanes.)... 

The separate and quantitative evaluation of accelerating steric effects on the homolytic cleavage of C—C-bonds allowed the definition of intrinsic barriers for the bond dissociation reaction, i.e. AG or AH at Hs = 0 in Fig. 2 or 3 or in Eq. (1-11) in general. AH is equal to or slightly than the corresponding bond dissociation energy (BDE) 6a-811 as pointed out earlier. 

Seminal work by Saveant and his group [39] has contributed to our understanding of the dynamics of dissociative ET, compared with the corresponding stepwise process [40], Obviously, bond-dissociation energy, rather than bond dissociation free energy, represents the important contribution to the intrinsic barrier [41]. One must, moreover, take into account that dissociative ET can occur in the adiabatic (e.g. tert-butyl bromide) and in the non-adiabatic regime (di-tert-butyl peroxide)... 

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