Standard homolytic bond dissociation

It is clear that proper description of the energetics of homolytic bond dissociation requires models that account for electron correlation. Are correlated models also needed for accurate descriptions of relative homolytic bond dissociation energies where the relevant reactions are expressed as isodesmic processes A single example suggests that they may not be. Table 6-15 compares calculated and measured CH bond dissociation energies in hydrocarbons, R-H, relative to the CH bond energy in methane as a standard ... 

Overall, the performance of Hartree-Fock models is very poor. In most cases, activation energies are overestimated by large amounts. This is not surprising in view of previous comparisons involving homolytic bond dissociation energies (see Table 6-2), which were too small. In terms of mean absolute deviation from the standard (MP2/6-311+G ) calculations, STO-3G yields the poorest results and 3-2IG the best results. 6-3IG and 6-311+G models provide nearly identical activation energies (just as they did for transition-... 

Carbon-centered radicals play an important role in organic synthesis, biological chemistry, and polymer chemistry. The radical chemistry observed in these areas can, to a good part, be rationalized by the thermodynamic stability of the open shell species involved. Challenges associated with the experimental determination of homolytic bond dissociation energies (BDEs) have lead to the widespread use of theoretically calculated values. These can be presented either directly as the enthalpy for the C-H bond dissociation reaction described in Equation 5.1, the gas-phase thermodynamic values at the standard state of 298.15K and 1 bar pressure being the most commonly reported values. 

In a landmark paper, Breslow and coworkers described the determination of pA), values of weak hydrocarbon acids by use of thermochemical cycles involving electrochemical reduction data for triarylmethyl, cycloheptatrienyl, and triphenyl- and trialkylcyclopropenyl cations and radicals [9aj. Later, they derived pATa data from standard oxidation potentials and bond-dissociation energies [9b, c]. The methodology was further developed by Nicholas and Arnold [10a] for the determination of cation radical acidities, and later modified and extensively used by Bordwell and coworkers [10b, c] so that homolytic bond-dissociation energies and cation radical... 

The one-electron oxidation potentials of the nucleophiles in aqueous solution were obtained from a thermodynamic cycle involving the gas-phase bond-dissociation energies and the aqueous pKa values of the H-nuc species. Also, the one-electron oxidation potentials, the standard free energies for the cation-nucleophile combination reaction, and the standard free energies for the homolytic bond dissociations of the adducts are interrelated by a thermodynamic cycle. 

The standard enthalpy change for the reverse of equation 1 is the heterolytic bond dissociation energy for BH (equal by convention to the proton affinity of B). The homolytic bond dissociation energy for BH", i.e. the standard enthalpy change for reaction 5, has been obtained [17,18] for some bases from the proton affinity and the... 

Stability in chemistry is not an absolute, but a relative concept. Let us consider the standard heats of reaction AH0 of the homolytic dissociation reaction R—H —> R + H. It reflects, on the one hand, the strength of this C—H bond and, on the other hand, the stability of the radical R produced. So the dissociation enthalpy of the R—H bond depends in many ways on the structure of R. But it is not possible to tell clearly whether this is due to an effect on the bond energy of the broken R—H bond and/or an effect on the stability of the radical R that is formed. 

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