Radicals, and bond dissociation energy

Table I. Recommended Values for Bond Dissociation Energies and Radical Thermochemistry in Aromatic Oxidation...

Of fundamental importance to free-radical chemistry are bond dissociation energies and radical heats and entropies of formation. Bond dissociation energy is defined as the energy required to break a particular bond to form two radicals. More precisely, bond dissociation energy of the R—X bond, D(R—X), is the enthalpy change of Reaction 9.7.37... 

Table 9.3 Selected Bond Dissociation Energies and Radical Heats of Formation at 25°C (kcal mole-1)0...

Tables 9.3 and 9.4 list selected bond dissociation energies and radical heats of formation. Note particularly that the decrease in energy required to remove hydrogen in the series methane, primary, secondary, tertiary, parallels increasing radical stability, and that aldehydic, allylic, and benzylic hydrogens have bond dissociation energies substantially lower than do alkyl hydrogens.

Table 15 shows the new values of bond dissociation energies and radical enthalpies of formation derived using the relationship... 

Brocks JJ, Beckhaus H-D, Beckwith ALJ, Ruchardt C (1998) Estimation of bond dissociation energies and radical stabilization energies by ESR spectroscopy. J Org Chem 63 1935-1943 Buxton GV, Langan JR, Lindsay Smith JR (1986) Aromatic hydroxylation. 8. A radiation chemical study of the oxidation of hydroxycyclohexadienyl radicals. J Phys Chem 90 6309-6313 Buxton GV, GreenstockCL, Helman WP, Ross AB (1988) Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals ( OH/O -) in aqueous solution. J Phys Chem Ref Data 17 513-886... 

Henry, D. J. Parkinson, C. J. Mayer, P. M. Radom, L. Bond dissociation energies and radical stabilization energies associated with substituted methyl radicals, J. Phys. Chem. A 2001,105, 6750-6756. 

Table 12.4. Substituent Effects on Radical Stability from Measurements of Bond Dissociation Energies and Theoretical Calculations of Radical Stabilization Energies...

The ntility of the experimental methods are illnstrated in this chapter by considering their applications to the stndy of reactive molecules, including radicals, car-benes and diradicals, carbynes and triradicals, and even transition states. These are provided in Section 5.4, which inclndes resnlts for representative bond dissociation energies and an extensive list of thermochemical results for carbenes, diradicals, carbynes, and triradicals. Section 5.5 provides a comparison and assessment of the resnlts obtained for selected carbenes and diradicals, whereas spectroscopic considerations are addressed in Section 5.6. 

B Homolytic Bond Dissociation Energies and the Relative Stabilities of Radicals ... 

Thus, acidity can be determined from independent measures of the bond dissociation energy and electron affinity, or the acidity provides a measure of the electron affinity of the corresponding radical if the bond dissociation energy is known. 

In this chapter, we look closely at the performance of several ab initio techniques in the prediction of radical thermochemistry with the aim of demonstrating which procedures are best suited in representative situations. We restrict our attention to several areas in which we have had a recent active interest, namely, the determination of radical heats of formation (AHf), bond dissociation energies (BDEs), radical stabilization energies (RSEs), and selected radical reaction barriers and reaction enthalpies. We focus particularly on the results of our recent studies. 

During coal conversion, the coal structure influences both thermal and catalytic reactions. Thermal reactions of solid coals initiate the breakage of weak bonds at rates proportional to their bond dissociation energies. The radicals thus produced require stabilization by hydrogenation or addition of small molecules otherwise the radicals couple to produce much more thermally stable bonds, which eventually leads finally to the formation of infusible and insoluble coke. 

The approach taken in our laboratory combines both of these trends. Specifically, we have developed a new experiment that allows us to study, for the first time, the photodissociation spectroscopy and dynamics of an important class of molecules reactive free radicals. This work is motivated in part by the desire to obtain accurate bond dissociation energies for radicals, in order to better determine their possible role in complex chemical mechanisms such as typically occur in combustion or atmospheric chemistry. Moreover, since radicals are open-shell species, one expects many more low-lying electronic states than in closed-shell molecules of similar size and composition. Thus, the spectroscopy and dissociation dynamics of these excited states should, in many cases, be qualitatively different from that of closed-shell species. 

Armstrong DA, Yu D, Rauk A (1996) Oxidative damage to the glycyl a-carbon site in proteins an ab initio study of the C-H bond dissociation energy and the reduction potential of the C-H centered radical. Can J Chem 74 1192-1199... 

Due to sharp and selective reduction in bond dissociation energy of radical cations [106,107], the cleavage of strong -C-C- or C heteroatom bonds has been observed during PET reactions. In this context, examples of -C-C- bond cleavage from jS-phenylethers [108], bibenzyls and pinacols [25,109,110] may be mentioned. In an extensive study, Whitten et al. [Ill, 112] have reported clean -C-C- bond cleavage from the reaction of /J-aminoalcohols and vinylog-ous amino alcohols. 

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... 

With the Ea of the molecules and radicals, bond dissociation energies, and electron impact data, the Morse potential energy curves for the C6F5X compounds can be calculated. They are shown in Figures 11.7 and 11.8. There are two curves dissociating to each of the complementary limits C6F5 + X(—) and C6F5(-) + X. The... 

We will show later on that the results of Table XXIII may be used for rationalizing thermochemical data such as C—C and C—H bond dissociation energies and also the reactivity of carbon-centered radicals. 

Egger K.W. and Cocks A.T., Homopolar and heterpolor bond dissociation energy and heat of formation of radicals and ions in the gas phase. I. Data on organic molecules . Helvetica Chimica Acta, 56,5, Nr. 148-149, 1516-1536. (1973)... 

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