Radical cations resonance-stabilized

Contrary to the high selectivity shown in cationic and anionic polymerization, radical initiators bring about the polymerization of almost any carbon-carbon double bond. Radical species are neutral and do not have stringent requirements for attacking the re-bond or for the stabilization of the propagating radical species. Resonance stabilization of the propagating radical occurs with almost all substituents, for example... 

We saw in Section 6.9 that the stability order of alkyl carbocations is 3° > 2° > 1° > —CH3. To this list we must also add the resonance-stabilized allvl and benzyl cations. Just as allylic radicals are unusually stable because the... 

Aliphatic amines undergo a characteristic a cleavage in the mass spectrometer, similar to that observed for alcohols. A C-C bond nearest the nitrogen atom is broken, yielding an alkyl radical and a resonance-stabilized, nitrogen-containing cation. 

In addition to fragmentation by the McLafferty rearrangement, aldehydes and ketones also undergo cleavage of the bond between the carbonyl group and the a carbon, a so-called a cleavage. Alpha cleavage yields a neutral radical and a resonance-stabilized acyl cation. 

The different hole injection and transport materials can be compared relative to TPD. The spiro derivative Spiro-TAD (36a) has a lower first oxidation potential that can be explained by the better resonance stabilization of the radical cation [87]. The material exhibits two successive one-electron oxidations (0.23 and 0.38 V vs. Fc/Fc+) and one subsequent formal two-electron oxidation (0.58 V) to the tetracation (Fig. 3.28). 

Fig. 30 Correlation of resonance stabilization energy for diaryl telluride radical cations with Pa. Data from Ref. 113.

In summary, the highest resonance stabilization is observed between N and N and the highest rotational barrier for N-centered 3e-n bonds was predicted and found for hydrazine cation-radicals (Nelsen 1992). The barriers are, of course, lowered by delocalization onto substituents and by steric strain in the nearly planar, most stable form for species, which bear N-containing substituents. 

The difference in conjugation between neutral molecules and their ion-radicals can also be traced for keto-enol tautomerism. As a rule, enols are usually less stable than ketones. Under the equilibrium conditions, enols exist only at a very low concentration. However, the situation becomes different in the corresponding cation-radicals, where gas-phase experiments have shown that enol cation-radicals are usually more stable than their keto tautomers. This is because enol cation-radicals profit from allylic resonance stabilization that is not available to ketones (Bednarek et al. 2001, references therein). 

The effect of a substituent on the reactivity of a monomer in cationic copolymerization depends on the extent to which it increases the electron density on the double bond and on its ability to resonance stabilize the carbocation that is formed. However, the order of monomer reactivities in cationic copolymerization (as in anionic copolymerization) is not nearly as well defined as in radical copolymerization. Reactivity is often influenced to a larger degree by the reaction conditions (solvent, counterion, temperature) than by the structure of the monomer. There are relatively few reports in the literature in which monomer reactivity has been studied for a wide range of different monomers under conditions of the same solvent, counterion, and reaction temperature. 

The various methods that convert N-vinylcarbazole to the cyclodimer all depend on the generation of a radical cation one resonance contributor (102) to which shows the radical character essential to explain the head-to-head linking and the role of nitrogen in stabilizing the cationic center. This much-studied dimerization process, including a consideration of the role of oxygen, has been discussed elsewhere. 

In the 1930s, Michaelis compared radical cations with trivalent-carbon or divalent-nitrogen intermediates using potentiometric methods. He rationalized their unusual stability as follows The fact that such radicals are capable of existence at all, can be attributed to a particular symmetry of structure resulting in resonance a... 

Conjugated conducting polymers consist of a backbone of resonance-stabilized aromatic molecules. Most frequently, the charged and typically planar oxidized form possesses a delocalized -electron band structure and is doped with counteranions (p-doping). The band gap (defined as the onset of the tt-tt transition) between the valence band and the conduction band is considered responsible for the intrinsic optical properties. Investigations of the mechanism have revealed that the charge transport is based on the formation of radical cations delocalized over several monomer units, called polarons [27]. 

Unfortunately, while it is clear that the allyl cation, radical, and anion all enjoy some degree of resonance stabilization, neither experiment, in the form of measured rotational barriers, nor higher levels of theory support the notion that in all three cases the magnitude is the same (see, for instance, Gobbi and Frenking 1994 Mo el al. 1996). So, what aspects of Hiickel theory render it incapable of accurately distinguishing between these three allyl systems ... 

P-Cleavage next to the ester carbonyl is promoted by the ortho hydroxyl group, through a six-membered ring (Scheme 5.19). The radical cation produced, with m/z 120, is resonance stabilized and is observed as a major fragment in the EI MS. 

The product iodine forms a radical cation with DPD, which is a red dye (see Figure 2-8). The radical cation DPD 0 is stabilized by resonance and forms a fairly stable color with one absorption maximum at 510 nm and one at 551 nm. The concentration of ozone is proportional to the intensity of the dye and can be calculated according to equation (2-3). advantages see iodometric method... 

Although some radicals and cation radicals are postulated for chemical and electrochemical transformations of 2-benzopyrylium cations (Sections III,F,1 and IV,B)> attempts to record their electron spin resonance (ESR) spectra failed, obviously because of a low stability of these radicals. However, the structural combination of hydroxy aryl and 2-benzopyrylium fragments favors the formation of radical cations 301-303, and their ESR spectra were recorded on oxidation of the corresponding 2-benzopyrylium salts with lead tetraacetate (87RRC417). 

Equation 27 is a useful tool in chemistry for assigning spectra. The slope and intercept of such relationships depend on the nature of the basic site, whether charge is localized in orbitals of Tt or cr symmetry and whether the system is closed or open shell. But also caution must be exercised in developing relationships between ionization potentials and molecular properties such as base strength the correct ionization potential must be chosen and molecules in which the presence of equivalent lone-pairs leads to resonance stabilization of the radical cation should be regarded as potential exceptions. 

One-electron oxidation of toluene results in the formation of a cation radical in which the donor effect of the methyl group stabilizes the unit positive charge. Furthermore, the proton abstraction from this stabilized cation radical leads to the conjugate base, namely, the benzyl radical. This radical also belongs to the it type. Hence, there is resonance stabilization in the benzyl radical. This stabilization is greater in the benzyl radical than in the tt cation radical of toluene. As a result, the proton expulsion appears to be a favorable reaction, and the acid-base equilibrium is shifted to the right. This is the main cause of the acidylation effects that the one-electron oxidation brings. 

In the cation radicals, the N-S fragments with their nearest environments are planar (Zverev, Musin, Yanilkin 1997) N-X rotation disrupts the 3c-tt bond. The resonance stabilization between N and X disappears. 

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