Stabilization energies radical

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

Total radical stabilization energy of 19.8 kcal/mol implies 10 kcal/mol of excess radical stabilization relative to the combined substituents. CH—N(CH3)2 rotational barrier is > 17 kcal/mol, implying strong resonance interaction. 

According to these data, which structural features provide stabilization of radial centers Determine the level of agreement between these data and the radical stabilization energies given in Table 12.7 if the standard C—H bond dissociation energy is taken to be 98.8 kcal/mol. (Compare the calculated and observed bond dissociation energies for the benzyl, allyl, and vinyl systems.)... 

Figure 7.24. Solid-state photochemical decarbonylation model for ketones. The dashed path corresponds to the experimentally determined energies of acetone (in kcal/mol). The effects of substituents with radical stabilizing energies (RSEs) are illustrated by the solid line in the reaction coordinate. See color insert.

Table 1 Radical stabilization energies (in kj/mol) of monosubstituted methyl radicals at 0 K according to Eq. 1... 

When considering the stability of spin-delocalized radicals the use of isodesmic reaction Eq. 1 presents one further problem, which can be illustrated using the 1-methyl allyl radical 24. The description of this radical through resonance structures 24a and 24b indicates that 24 may formally be considered to either be a methyl-substituted allyl radical or a methylvinyl-substituted methyl radical. While this discussion is rather pointless for a delocalized, resonance-stabilized radical such as 24, there are indeed two options for the localized closed shell reference compound. When selecting 1-butene (25) as the closed shell parent, C - H abstraction at the C3 position leads to 24 with a radical stabilization energy of - 91.3 kj/mol, while C - H abstraction from the Cl position of trans-2-butene (26) generates the same radical with a RSE value of - 79.5 kj/mol (Scheme 6). The difference between these two values (12 kj/mol) reflects nothing else but the stability difference of the two parents 25 and 26. 

F. G. Bordwell, X.-M. Zhang. From Equilibrium Acidities to Radical Stabilization Energies. Acc. Chem. Res. 1993, 26, 510-517. 

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. 

The radical stabilization energy (RSE) of a substituted methyl radical CH2X is generally defined as the difference between the C-H bond dissociation energy in methane and the C-H BDE in the substituted methane CH3X ... 

Table 6.11 Calculated radical stabilization energies at 0 K for the cyanomethyl radical (kJ/mol).8...

Table 6.13 Comparison of experimental radical stabilization energies at 0 K (k J/mol) of substituted methyl radicals with those calculated with DFT-based electronic structure methods.

Table 7 Radical-stabilization energies (RSE) (kcal mol ) for radicals derived from a-substituted acetophenones, PhCOCHjR."...

Although it is generally agreed that the thermal isomerization of bicyclo[2.2.0]hexanes to hexa-l,5-dienes takes place via diradical intermediates,113 118 121,123 125 experimental evidence has been obtained which implies otherwise.115,116 While a radical stabilization energy of approximately 4 kcal mol"1 was obtained for the pyrolysis of methyl 4-chlorobicy-clo[2.2.0]hexane-l-carboxylate (28 b) to methyl 5-chlorohexa-l,5-diene-2-carboxylate (29b),115116 as related to the parent 2-chlorohexa-1,5-diene (29a),115-l16-118 kinetic studies have indicated that there is a small but significant increase in activation energy of about 1 kcal mol-1 for the gas-phase and solution pyrolysis of l-chloro-4-methylbicy-clo[2.2.0]hexane (28c), as compared to l-chlorobicyclo[2.2.0]hexane (28a).115-116 In the light of this result, the commonly accepted diradical mechanism must be questioned and it is likely that the isomerization of these compounds occurs via a concerted process. 

A quantity called the radical stabilization energy (RSE) may be defined to relate the stabilities of substituted carbon radicals to the methyl radical. The effects of adjacent X , Z, and C substituents on the RSEs of carbon-centered radicals has been widely investigated [142,143]. The expectations based on simple orbital interaction theory as espoused above are widely supported by the experimental findings, except that when the the n donor or n acceptor ability of the group is weak and the inductive electron-withdrawing power is large, as in F3C and (Me N+CHj, the net effect is to destabilize the radical relative to the methyl radical [143]. The BDE of a C—H bond of a compound R—H is another measure of stability of the product radical, R. It is related to the RSE by... 

TABLE 7.1. Radical Stabilization Energies (kJ/mol) of Mono- and Disubstituted Methyl Radicals,... 

---