Delocalized cation radicals

Oxidation of naphthalene (xii) and durene (xiii) was investigated in [C2mim] [N(Tf)2], [C4mim][N(Tf)2], [C4mim][PF6], and [Ni,8,8,8][N(Tf)2] ILs [10], An irreversible electrochemical process was detected for xii. Similar results are commonly observed in molecular solvents, where the reactivity of the radical cation of a polyaromatic hydrocarbon is known to decrease with the size of the aromatic hydrocarbon [10]. Less delocalized cation radicals of alkylbenzenes, like the one obtained from xiii, are known to display a different reactivity due to the C-H acidity, and the deprotonation from the cation radical is the predominant type of decay (Eq. 11). The nature of the electrochemical mechanism was found to follow the same ECE-DISPl mechanism (DISP = disproportiOTiation) as that in acetonitrile (Eqs. 15.10-15.13) [10]. 

Calculate the difference in energy between localized and delocalized forms for ally cation, radical and anion. Does it increase, decrease or remain approximately the same with increasing number of n electrons Rationalize your result. 

A study of the electrochemical oxidation and reduction of certain isoindoles (and isobenzofurans) has been made, using cyclic voltammetry. The reduction wave was found to be twice the height of the oxidation wave, and conventional polarography confirmed that reduction involved a two-electron transfer. Peak potential measurements and electrochemiluminescence intensities (see Section IV, E) are consistent vidth cation radicals as intermediates. The relatively long lifetime of these intermediates is attributed to steric shielding by the phenyl groups rather than electron delocalization (Table VIII). 

The cation radical heptafulvalene, on the other hand, undergoes no symmetry reduction. Both the rings show a moderate double-bond fixation (Fig. 7), and the unpaired spin is delocalized throughout the molecule (Table 3). 

Inspection of Fig. 7 reveals that in the cation radical of XXI a marked bond fixation exists in one of the rings, while bond lengths are nearly equalized in the other ring, on which the unpaired spin is delocalized (Table 3). On the other hand, in the anion radical of XXI, there is a moderate bond fixation in both rings, and the unpaired spin is delocalized over the entire molecule. 

Le Mest Y, L Her M, Hendricks NH, Kim K, Collman JP. 1992. Electrochemical and spectroscopic properties of dimeric cofacial porphyrins with nonelectroactive metal centers. Delocalization processes in the porphyrin rr-cation-radical systems. Inorg Chem 31 835... 

The oxidation generates highly delocalized phenoxy radicals (PhO, Scheme 2.21), which may initiate (i) a radical polymerization process, trapping the reactant (CF) to give a benzyl radical intermediate (QMR), or it may (ii) follow a radical coupling to produce the p-QM p-O-QM, which being a reactive electrophile could undergo cationic polymerization. 

In a second possibility, the polaron-like hopping model, a structural distortion of the DNA stabilizes and delocalizes the radical cation over several bases. Migration of the charge occurs by thermal motions of the DNA and its environment when bases are added to or removed from the polaron [23]. 

Bock et al. studied the cation radicals of various organosilanes having a -system in the / position extensively and found that the charge is delocalized considerably into the silyl groups [9,10]. Kira and Sakurai et al. also carried out an ESR study of cation radicals of allylsilanes and revealed the large polarization of the SOMO (singly occupied molecular orbital) as a consequence of the a-% interaction [11],... 

The redox properties of cyclic polysilanes are interesting because they resemble those of aromatic hydrocarbons. For instance, cyclic polysilanes can be reduced to anion radicals or oxidized to cation radicals. ESR spectra for both the cation and anion radicals indicate that the unpaired electron is fully delocalized over the ring [17,19,20]. The aromatic properties of the cyclic polysilanes are ascribed to a high energy delocalized HOMO and a relatively low energy LUMO. Because the HOMO and LUMO levels lie at similar level to those of benzene, cyclic polysilanes can serve either as electron donors or electron acceptors. 

The reactions of electrogenerated cation radicals of diarylsulfldes are mainly orbital-controlled and at this level the electronic structure of their frontier orbitals (HOMO-SOMO) has very interesting synthetic consequences. The 3p orbitals of sulfur are conjugated with only one aromatic ring even if there are two aryls bound to sulfur. Therefore, only one ring can be activated electrochemically. The degree of the charge delocalization in the ArS moiety of a cation radical on the one hand, and the availability of p- and o-positions for the substitution on the other, determine quite different reactivity of such species. 

Naturally, the cation-radical of diphenylamine is characterized with an analogous positive-charge delocalization (Liu and Lund 2005). The A,A -diphenyl-p-phenylenediamine cation-radical is almost planar and the spin density intrudes outer phenyls. When the outer phenyls contain two methyl groups in ortho positions, the molecule loses planarity. As a result, the spin density concentrates within the inner ring and its adjacent two nitrogen atoms (Nishiumi et al. 2004). 

By and large, we have a strong delocalization of the radical nonsatnration within a molecular carcass. This facilitates deprotonation of the initial cation-radicals. The same phenomenon was observed in the anion-radicals of cyclic depnties—glycine anhydride, alanine anhydride, and sar-cosine anhydride (Tarabek et al. 2007). 

Some cation-radicals can appear as hydrogen acceptors. Thus, fullerene Cgg is oxidized to the cation-radical at a preparative scale by means of photoinduced electron transfer. As in the case of anion-radical, the fullerene Cgo cation-radical bears the highly delocalized positive charge and shows low electrophilicity. This cation-radical reacts with various donors of atomic hydrogen (alcohols, aldehydes, and ethers) yielding the fullerene 1,2-dihydroderivatives (Siedschlag et al. 2000). 

Easily ionizable anthracene forms the cation-radical as a result of sorption within Li-ZSM-5. In case of other alkali cations, anthracene was sorbed within M-ZSM-5 as an intact molecule without ionization (Marquis et al. 2005). Among the counterbalancing alkali cations, only Li+ can induce sufficient polarization energy to initiate spontaneous ionization during the anthracene sorption. The lithium cation has the smallest ion radius and its distance to the oxygen net is the shortest. The ejected electron appears to be delocalized in a restricted space around Li+ ion and Al and Si atoms in the zeolite framework. The anthracene cation-radical appears to be in proximity to the space where the electron is delocalized. This opens a possibility for the anthracene cation-radical to be stabilized by the electron s negative field. In other words, a special driving force for one-electron transfer is formed, in case of Li-ZSM-5. 

A typical example of steric control over spin delocalization has been described for the cation-radical of 3,4-bis(thioisopropyl)-2,5-dimethyl-l-phenylpyrrole (Domingo et al. 2001). Scheme 3.15 depicts this sitnation. In this cation-radical, one thioisopropyl group is almost coplanar with the pyrrole ring, whereas the other one occupies an orthogonal position. Accordingly, the ESR spectra established an eqnilibrinm between the symmetrical and asymmetrical conformations of the cation-radical. This equi-librinm is shifted toward the asymmetrical form at low temperatmes. The main feature of the equilibrium is the widening of spin delocalization, which includes not only the pyrrole ring but also one donor sulfur atom at the expense of the other sulfur atom. The steric control predetermines the discrimination of the other sulfur atom in the spin-delocalization process. 

The cation-radical of permethyldithia[6]radialene provides one, even more pertinent, example of the steric control over spin delocalization (Gleiter et al. 1996). As described, the unpaired electron is delocalized only in one half of this cation-radical, namely, within the limits of the 2,3-dithiatetramethyl-butadiene unit. Owing to the steric demand of the isopropilidene groups, two of the four methylene groups are twisted, whereas the other two are coplanar (see Scheme 3.16). It seems... 

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. 

Cation-radicals containing three electrons in the field of four atoms or anion-radicals with five electrons retained by four atoms represent a special group of multicentered ion-radicals. Thus, nonclassical, cyclically delocalized 3e/4C cation-radicals and 2e/4C (dication)-radicals of substituted cyclobutadienes are known (Allen and Tidwell 2001). The 3e/4N cation-radical and the 5e/4N anion-radical have also been discovered (Exner et al. 1998, 1999, 2000). The reactions in Scheme 3.31 illustrate structures of the 5e/4N anion-radical as well as the corresponding dianion and acety-lated products of the latter. 

In the case of tetra(A-oxide) of the initial bis(diazene), one-electron oxidation in Scheme 3.32 leads to the formation of a novel, O-stabilized, 3e/4N cation-radical with 3-D delocalization of the unpaired electron (Exner et al. 1999). 

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