Radical dications

Reactions with Parting of Radicals. The one-electron oxidation of cationic dyes yields a corresponding radical dication. The stabihty of the radicals depends on the molecular stmcture and concentration of the radical particles. They are susceptible to radical—radical dimerization at unsubstituted, even-membered methine carbon atoms (77) (Fig. 6). 

According to the electron-transfer mechanism of spectral sensitization (92,93), the transfer of an electron from the excited sensitizer molecule to the silver haHde and the injection of photoelectrons into the conduction band ate the primary processes. Thus, the lowest vacant level of the sensitizer dye is situated higher than the bottom of the conduction band. The regeneration of the sensitizer is possible by reactions of the positive hole to form radical dications (94). If the highest filled level of the dye is situated below the top of the valence band, desensitization occurs because of hole production. 

Closely related behavior had previously been observed with the Cu complex 14, prepared from the corresponding thiobisphenol ligand (HL ) (Scheme 7) [158]. The species formed upon oxidation was proposed at the time to be a dinuclear diphenoxyl radical dication 15. The catalytic behav-... 

The +C(SH)3 cation and the radical dication derived from it have been the subject of high-level calculations.67 The ability of two adjacent sulfur atoms to stabilize cations, anions, and radicals makes these species usefiil for relating bond-breaking and electron-transfer energies.68,69 Electrophilicity parameters for the dithiocarbenium ions (27) have been worked out,70 and the stabilities of the cations (28), (29) and (30) have been estimated using PM3 calculations.71 Cation (31) can be captured by solvent or azide ion, or it may ring close to (32), which subsequently alkylates another (31) cation as shown.72... 

The methane dication (CH42+) with a lifetime of only 3 jxs803 has been detected experimentally in the gas phase.804-807 A square-planar D4h symmetrical structure was predicted by early calculations.808,809 However, the planar but not square C2v symmetrical form (443) was shown to be the preferred structure by Wong and Radom.810 The cation contains an, vp2-hybridized carbon, an empty p-orbital perpendicular to the plane of the molecule, and a 2e-3c interaction. It can be visualized as a complex of a hydrogen molecule weakly bounded by a 2e-3c bond to the CH22+ core as shown by ab initio calculations [MP2 and B3LYP levels with 6-311++G(2df,2pd) basis set].811 The pentacoordinate CH52+ radical dication was studied by Stahl... 

Depending on the amount of thianthrenium perchlorate, /V,/V,/V, /V, /V",/V"-hexakis (anisyl)-l,3,5-triaminobenzene gives its cation radical, dication diradical, and trication triradical as well (Stickley et al. 1997). These species are stable in methylene chloride at low temperatures (at 298 K they can exist for several days). Spin and charge are localized at each oxidized nitrogen atom. The dication diradical and trication triradical structures are ground-state triplet and quartet molecules (Sato et al. 1997). 

As mentioned in Section 6.3.1, the radical cations of many heteroaromatics have been formed anodically, thermally, or photochemically. With electron-rich heterocycles, one-electron oxidation is often followed by dimerization, either via coupling of two radical cations or via radical cation-substrate reaction, which in turn is most often followed by a second electron-transfer step leading to the same non-radical dication formed by the previous mechanism (see Scheme 11). The course of the reac-... 

Since the highest occupied molecular orbital of the triaminocyclopropenylium ion is greatly lowered by the conjugatively electron-donating amino groups, the ion undergoes facile one-electron oxidation either electrochemically ° or by the action of sulfuric acid ° or anti-mony(V) chloride. The produced radical dication, which is red-violet in color, is so stable that it can be directly observed by ESR ° and also isolated as the bis(hexachloroantimonate) salts 2 in more than 90% yield. " ... 

When one electron is removed from the radical dication the... 

Super-dry electrolyte media, e.g., dry CHjCN under vacuum-line conditions, are used to study the second oxidation of (h -C5H5)jNi the wave is highly reversible at RT compared to the — 40°C required for reversibility if the solvent is less stringently prepared (Fig. D". Addition of activated alumina directly to the electrochemical cell and stirring the solution briefly before making the scan can be used to study the cation radical/dication redox behavior of aromatics . 

Neutral carotenoid Cation radical Dication Car Car+ Car2+... 

Reaction intermediates such as radicals, carbonium ions, carbanions, cation and anion radicals, dications and dianions can be produced. For example, six reversible redox reactions of 9,9 -bianthryl-10,10 -dicarbonitrile in 0.1 M Bu4NPFg/propio-nitrile solution yield a spectrum of products ranging from the tetra-anion to the dication... 

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