Resonance stabilization radicals

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. 

The rationale behind this choice of bond integrals is that the radical stabilizing alpha effect of such radicals are explained not by the usual "resonance form" arguments, but by invoking frontier orbital interactions between the singly occupied molecular orbital of the localized carbon radical and the highest occupied molecular orbital (the non-bonding electrons atomic orbital) of the heteroatom (6). For free radicals the result of the SOMO-HOMO interaction Ts a net "one-half" pi bond (a pi bond plus a one-half... 

The most common and also most effective mechanism of radical stabilization involves the resonant delocalization of the unpaired spin into an adjacent 7r system, the allyl radical being the prototype case. A minimal orbital interaction diagram describing this type of stabilization mechanism involves the unpaired electron located in a 7r-type orbital at the formal radical center and the 7r- and tt -orbitals of the n system (Scheme 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. 

If the radical were restricted to resonance between the KekulA structures A and B, with the free valence on the methyl carbon, resonance would stabilize the radicals to just the same extent as the undissociated molecules, which would then have only the same tendency to dissociate as a hexaalkylethane. But actually the five structures A, B, C, D and E (each with three double bonds) contribute about equally to the structure of the radical, which thus resonates among five structures instead of two and is correspondingly stabilized by the additional resonance energy. 

About the same value, 25 kcal/mole, is found for the resonance energy of the allyl radical 6 the resonance energy stabilizing the product is that... 

One measure of the resonance component of radical stability in polymerizations is the so-called Q value of the monomer, which quantifies the resonance stabilization of the radical (Stevens 1990). However, the experimentally determined value of Q can be influenced by other factors unrelated to resonance. To evaluate the extent to which their measured Q values were consistent with resonance stabilization of the monomer radical, the authors compared isodesmic energies from Eq. (6.14) to measured Q values for R = Me, rBu, PhO, CN, Ph, vinyl, and phenylethynyl. The largest stabilization energy was computed for the R = phenylethynyl case, about 101 kJ mol-1, although at the HF/3-21G level the expected linear correlation between log(2 and stabilization energy was only fair (R2 = 0.86 a better correlation for the non-phenylethynyl substituents had been obtained previously at a higher level of theory). 

Resonance effects, on the other hand, can significantly affect the regiochem-istry of the cyclizadon. Resonance delocalization of the unpaired electron of a free radical stabilizes that radical. This is why the allyl radical is much more stable than the //-propyl radical. Thus, if a double bond is substituted with a group capable of providing resonance stabilization to a free radical, it undergoes free-radical addition much more readily than a double bond which cannot provide such resonance stabilization. 

Another family of inhibitors, hydrogen-donating agents such as iso-propyl and 1,4-di-ivo-propyl benzenes, was investigated by Lamouroux in order to reduce the formation of TBP-TBP dimers, which exhibit a very high plutonium retention of TBP (47). The presence of at least one mobile hydrogen on the iso-propyl group could produce a benzylic tertiary radical stabilized by resonance. The addition of such compounds reduced the concentration of the TBP-TBP dimers by about 50%. 

The SHM predicts the propenyl cation, radical and anion to have the same resonance energy (stabilization energy). Actually, we expect the resonance energy to decrease as we add ji electrons why should this be the case ... 

Correlations in view of the third parameter cr" (the radical stabilizing factor), describing stabilization of the radical state, do not allow reliably to discuss about the contributions to common substituent effect as the dependence between resonance and ct parameters is found out (r23=0.7) [991],... 

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