Anthracene cation

Kira and coworkers25 found that in deaerated DMSO solution of frans-stilbene both the solute cation and anion are produced and the anions are eliminated by aeration. Since they found26 that the absorption spectra of the anthracene cation and anion are quite similar, they suggested25 that the absorption spectrum observed by Hayon for anthracene solution in DMSO is a superposition of the spectra of the solute cation and anion. This observation casts a serious question on the yield of solvated electrons found by Hayon23. 

DP) and leads (Equation 4.5) to the formation of an anthracene cation radical as a result of the single-electron transfer process. The resulting ion-radical pair [AN, DP is the critical intermediate that subsequently evolves to cycloadduct (AD). 

According to the results of Ben Taarit and co-workers (76) and Neikam (77) Ce(III) Y zeolites will not form anthracene cation radicals but upon oxidation to Ce(IV) the radicals are readily formed. This experiment suggests that one role of oxygen during calcination may be to oxidize certain cations. The surface may be oxidized by molecules other than oxygen since the chlorination of 7-alumina by carbon tetrachloride considerably increases the sites responsible for the acceptor character. These sites, which oxidize perylene into the paramagnetic radical ion, have been attributed to biocoordinated positive aluminum atoms (78). 

In a similar vein, the cleavage of the photodimer of anthracene occurs in the dark in the presence of nitrosonium cation to afford the anthracene cation radical as its 7t-dimer via a thermal electron transfer196 (equation 66). 

Fig. 12 (a) The absorption spectra of singlet excited anthracene ( ANT ) and anthracene cation radical (ANT+ ) obtained upon 25-ps laser excitation of anthracene in the presence of excess maleic anhydride (MA). (b) The authentic spectrum of singlet excited anthracene ( ANT ). Reproduced with permission from Ref. 212. 

Furthermore, kinetic analysis of the decay rate of anthracene cation radical, together with quantum yield measurements, establishes that the ion-radical pair in equation (76) is the critical reactive intermediate in osmylation reaction. Subsequent rapid ion-pair collapse then leads to the osmium adduct with a rate constant k 109 s 1 in competition with back electron-transfer, i.e.,... 

Fig. 14 Transient absorption spectrum of anthracene cation radical (ANT+ ) obtained upon 30-ps laser excitation of the [ANT, OsOJ charge-transfer complex in dichloro-methane. The inset shows the authentic spectrum of ANT+ obtained by an independent (electrochemical) method. Reproduced with permission from Ref. 96b.

Since the latter conditions pertain to aromatic nitration solely via the homolytic annihilation of the cation radical in Scheme 16, it follows from the isomeric distributions in (81) that the electrophilic nitrations of the less reactive aromatic donors (toluene, mesitylene, anisole, etc.) also proceed via Scheme 19. If so, why do the electrophilic and charge-transfer pathways diverge when the less reactive aromatic donors are treated with other /V-nitropyridinium reagents, particularly those derived from the electron-rich MeOPy and MePy The conundrum is cleanly resolved in Fig. 17, which shows the rate of homolytic annihilation of aromatic cation radicals by NO, (k2) to be singularly insensitive to cation-radical stability, as evaluated by x. By contrast, the rate of nucleophilic annihilation of ArH+- by pyridine (k2) shows a distinctive downward trend decreasing monotonically from toluene cation radical to anthracene cation radical. Indeed, the... 

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. 

Fig. 15 Structures of representative benzanthracene derivatives and their derived carbocations/carboxonium ions (and comparison with model anthracene cations).

FIGURE 12. Extinction of the anthracene cation-radical by benzyltriphenylphosphonium tetrafluoroborate (in MeCN Xexe = 340 nm, Ames = 730 nm [anthracene] = 5.10 5 mol.1"l)... 

Since overlap of the spectra of the TNB anion radical and the anthracene cation radical is virtually confined to the central feature of the anion spectrum, observation of the intensity of one of the outer features permits separate assessment of the anion-radical concentration (Figure 2c). As in a previous investigation (2) a quantitative study of the enhancement of the ion-radical spectrum in the presence of coadsorbate was therefore possible by using a calibration curve in which the intensity of the outer line of the TNB spectrum was plotted against the doubly integrated area of the whole of the TNB spectrum in a separate series of experiments. Figure 3 shows the effect of added anthracene and perylene on the surface concentration of TNB anion radicals. A tenfold increase in the TNB radical concentration was observed in the presence of either hydrocarbon. Addition of naphthalene, on the other hand, produced no enhancement of the TNB anion-radical concentration. 

Proton Coupling Constants for Anthracene Cations and Anions and their Methyl Derivatives (Bolton et al., 1962b Brivati et al., 1961a)... 

Given that triplet-triplet energy transfer proceeds via a Dexter (electron exchange) mechanism, it is not surprising that electron transfer can also occur via upper triplet states. Two-color experiments with anthracene in acetonitrile in the presence of ethylbromoacetate, a dissociative electron acceptor, showed that excitation to an upper triplet state led to depletion of the T-T absorption and concurrent production of the anthracene cation radical as a result of electron transfer (Scheme 1) [52]. 

In another example, Yildirim et al. photochemically generated anthracene radical cations in the presence of TEMPO [29]. TEMPO immediately trapped the radical to form the TEMPO-anthracene cation, which was subsequently used to initiate cationic polymerization of cyclohexene oxide (CHOX). The resulting alkoxyamine-functional polycyclohexene oxide was used to macroinitiate styrene polymerization, resulting in the formation of S-6/-CHOX (Scheme 8.9). 

Figure 10. Transient absorption spectrum of anthracene cation radical obtained at 35 ps following the 532-nm CT excitation of the anthracene-0s04 complex in dichloromethane. The inset shows the formation and decay of the cation radical within about 30 ps [161].

Fio. 18. EPB spectrum of the anthracene cation radical, formed by adsorption of anthracene vapor in vacuo on silica-alumina (95 5) at low coverage. According to Karakchiev, Barachevsky and Holmogorov. 

A detailed spin assignment is not available for the polymer at the present. However, compared with splittings in anthracene cation and anion (26), the 9-G coupling constant obtained here does clearly point out the delocalized ir-electron nature of the paramagnetic center. 

Scheme 4 describes the electron transfer photosensitization of iodonium salts by anthracene [61,70,91-94]. Singlet anthracene reacts with diphenyliodonium cation by diffusion controlled electron transfer in acetonitrile solution. In-cage decomposition of diphenyliodo radical competes with rapid back electron transfer to yield the singlet radical pair of anthracene cation... 

This result contrasts with the observations on anthracene sensitized photolysis, wherein no arylated diarylsulfides are observed. Naphthalene cation radical is sufficiently oxidizing (naphthalene Eox = 1.54 V versus SCE) to oxidize diphenylsulfide ( ox = 1.31 V versus SCE), whereas anthracene cation radical cannot (anthracene Eox = 1.09 V versus SCE) [94], Thus, diphenylsulfide cation radical/phenyl radical pair is formed by two sequential single electron transfer reactions in-cage, the subsequent chemistry being the same... 

In 1967, Forbes et al. further examined free radicals present in tobacco smoke condensates by ESR (1211). They prepared and examined sulfuric acid solutions of smoke condensates. They reported that the free radical species present in sulfuric acid solutions of smoke condensates corresponded to a modified benzopyrene-type cation radical and to the spectrum of a modified anthracene cation radical, with the... 

Both silica and alumina have served as a host for oxidation of benzene by ultraviolet irradiation, leading to the benzene dimer cation radical (benzene) (Tanei, 1968). Photoionization here is thought to be biphotonic (p. 180). On the other hand, the formation of perylene and anthracene cation radicals on silica alumina is enhanced by ultraviolet irradiation, and the process is found to be monophotonic (Takimoto and Miura, 1972). The fate of the photo-ejected electron is, of course, not known, a state of ignorance which pertains to all of the cation-radical forming reactions on catalyst surfaces. 

Hammerich O, Parker VD (1974) Reaction of the anthracene cation radical with acetonitrile. A novel anodic acetamidation. J Chem Soc Chem Commun 7 275-276... 

Dihydro-9-methyl-9,10-o-benKno-9-arsonia-anthracene cation As, (sp )3 1.903—1.908(6), 403... 

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