Dissociation free energy

Theoretical treatments lead to the approximate expression in equations (19) and (20), " where Teff is the effective temperature of the complexes, and the A(AG)s are the differences in their dissociation free energies. If the entropy effects of the two competing fragmentation processes cancel, then the A(AG)s can be substituted by the A(A//)s. 

There are no large differences between the reactivities of PhS , (EtO)2PO and CHjCOCHj" with the same aryl radical, but CN appears to be significantly less reactive. It is not easy to evaluate the respective role of the bond dissociation free energy and of the Nu-/Nu" standard potential in equation (13) in this connection because of the paucity of available data concerning these two quantities. An explanation of the low reactivity of CN" should thus await the availability of such data as well as that of a precise expression of the intrinsic barrier in a model of these intramolecular concerted electron-transfer-bond-breaking (or forming) reactions. 

Figure 10.7 A schematic showing the energy and free energy landscapes for the association of simple spherical molecules A and B with the potential defined by Equation (10.29). (A) The solid shows the energy U(r) and the dashed line shows the free energy, which combines the energy with the entropic contribution of the spherical shell volume 4nr2dr. The transition state for the dissociation reaction occurs at r, the location of the free energy maximum. (B) The association-dissociation free energy landscape is shown for the finite concentration case, where 7rrc [B] = 1.

TABLE 13 Calculated (SM6) Dissociation Free Energies (kcal mol ) and Experimental... 

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)... 

Table 1 Calculated average bond dissociation free energies (kcal/mol) ...

The two-state model implies that quaternary interactions are necessary for the stabilization of the folded state and AG is equal to the dissociation free energy... 

The cycle in Eq. (15) shows that the solution phase dissociation free energy can be decomposed into solvation free energies and a gas phase component. Further, in this case, it is not hard to see that the difference between the free energies characterizing the two acid dissociation reactions above in Eq. (13) corresponds to the free energy difference of Eq. (14). Computationally (or experimentally), one needs to compute... 

The driving force for a HAT reaction, AG°xh/y =- 7 ln. Kxh/y. is best determined by direct equilibrium measurements in the solvent of interest. However, this is typically limited to reactions where IAG°xh/yI is small, less than about 5 kcal mol . Also, this is only possible for reactions in which all of the species are fairly stable, which is unusual for organic radical reactions. The AG° for a HAT reaction is typically more easily derived as the difference in bond dissociation free energies (BDFEs) of X-H and Y-H in the solvent of interest. We have recently reviewed BDFEs of common organic and biochemical species and how they are obtained, so only an overview is given here. 

Table 1.1 Properties of reagents in selected solvents solution bond dissociation free energies (BDFEs) and self-exchange rate constants ( xh/x) in selected solvents. ...

Table 1.3 Self exchange rate constants and bond dissociation free energies (BDFEs) for Bu3PhO( /H) in different solvents."...

Equilibria are reported for the transfer of a hydride, a proton, or a hydrogen atom. The measurements and the determinations of various electrochemical potentials were used to determine 11 possible homolytic and heterolytic Co-H bond-dissociation free energies of 5 and its monohydride derivatives 6. These data are reported in Scheme 1. 

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