Radical anions electronic absorption spectra

The delocalization of the conduction electron onto the side chains would be expected if the pendant groups were replaced with more electrophilic substituents than the phenyl group. However, this is not the case. Figure 22 shows the absorption spectrum of poly-(methylnaphthylsilane) radical anion. The absorption spectrum is very similar to that of the naphthalene radical anion, which implies that the unpaired electron is localized on the pendant group. Increase of the electron affinity of pendant groups does not necessarily cause the delocalization. 

Isomerization. Finally, some investigations of unstable radical anions will be discussed. Radical anions of ds-stilbene rapidly isomerize to radical anions of trans-stilbene therefore, their spectra or relative electron affinities cannot be studied by conventional techniques. Flashing a solution of perylene radical anions mixed with cis-stilbene yields cis-stilbene radical anions, which persist for 200-300 jls before the ejected electrons are recaptured by the perylene formed by the photolysis. The difference spectrum of the photolyzed solution is shown in Figure 12. The concentration of the ds-stilbene radical anions formed can be determined by the degree of bleaching the absorption peak of the perylene radical anions. The absorption spectrum of cis-stilbene radical anions can also be determined in this way (19). Similar experiments, results of which are shown in Figure 13, permit the determination of the absorption spectrum of trans-stilbene radical anions. Because this spectrum has been established by other methods, the reliability of the flash-photolytic method can be determined by comparing the differ-... 

Matrix-isolated alkali atoms (or small clusters) also undergo easy photoionization, and the electrons released in this process may attach themselves to nearby substrates to form the corresponding radical anions. However, one drawback of alkah metal atoms or clusters is that they tend to swamp the electronic absorption spectrum of the target reactive intermediate that can only thus be detected by IR. 

The anion-radical 128 of beiizo[c]thiophene-l,3-dione was observed by ESR spectroscopy several years ago its electronic absorption spectrum has been recorded more recently. ... 

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

Ito and Matsuda studied the y-radiolysis of 2-methyltetrahydrofuran (MTHF) solutions of diphenyl sulfone and dibenzothiophene-S,S-dioxide (DBTSD) at 77 K. They found that the radical anions of these sulfone compounds are formed and have intense absorption bands at 1030 nm and 850 nm, respectively. The blue glassy solution of y-irradiated diphenyl sulfone has absorption bands at both 1030 nm and 360 nm while the absorption spectrum of the benzenesulfonyl radical formed by UV irradiation of diphenyl sulfone solution at 77 K showed only a peak at 382 nm. Gamma-irradiated phenyl methyl sulfone solution showed an absorption band only at 385 nm. Consequently the appearance of the absorption bands in 800-1030 nm of diphenyl sulfone and DBTSD may suggest that the unpaired electron is delocalized on two phenyl rings. The same authors studied the radiolysis of MTHF solutions of disulfones (diphenyl and dihexyl disulfones). They found a blue coloring of the solution by the y-radiolysis of diphenyl disulfone and dihexyl disulfone due to absorption peaks at 695 nm and 690 nm respectively, besides smaller absorptions at 300-400 nm. Comparing these results to the previous observation, that phenyl methyl sulfone solution absorbs only at 398 nm, results in the conclusion that the absorption band at 690 nm is due to the linked two sulfone moieties. The authors found that substituents on the phenyl ring lead to shifts in the absorption maxima of the... 

The intensity of absorption gives the product G , where G is the observed yield and is the molar extinction coefficient. The absolute value of was determined by Fielden and Hart (1967) using an H2-saturated alkaline solution and an alkaline permanganate-formate solution, where all radicals are converted into Mn042. They thus obtained = 1.09 x 104 M- cm1 at 578 nm, which is almost identical with that obtained by Rabani et al. (1965), who converted the hydrated electron into the nitroform anion in a neutral solution of tetrani-tromethane. From the shape of the absorption spectrum and the absolute value of at 578 nm, one can then find the absolute extinction coefficient at all wavelengths. In particular, at the peak of absorption, (720)/ (578) = 1.7 gives at 720 nm as 1.85 X 104 M 1cm 1. 

Methods of electron spectroscopy are widely used to follow the electron-transfer process. Thus, the progress of electron transfer from naphthalene anion-radical to cup-stacked carbon nanotubes is easily detected by monitoring the UV absorption spectrum. Namely, the absorption band around 500-900 nm due to naphthalene anion-radical completely disappears after reduction of the nanotubes. At the same time, the reduced nanotubes exhibit ESR spectrum characterized with g-factor of 2.0025 (Saito et al. 2006). This g-value is close to the free spin g-factor of 2.0023 that is diagnostic of the delocalized electron on carbon nanomaterials (Stinchcombe et al. 1993). It should be parallelly... 

In order to measure the absorption spectra, the radical anions were generated electrochemically in the optical path of a spectrophotometer. The absorption spectrum of 3,5-dinitroanisole radical anion (Figure 11, curve c) is very similar to that of the 550-570 nm species produced photochemically. So we believe this species to be the radical anion formed by electron transfer from the nucleophile to the excited 3,5-dinitroanisole and decaying by interaction with its surroundings including the nucleophile radical cation. The behaviour described seems to be rather general for aromatic nitro-compounds since it is observed with a series of these compounds with various nucleophilic reagents. 

Electron photodetachment upon laser excitation of the solvent anion above 1.76 eV was observed (Fig. 2a,c) [18]. The cross section of photodetachment linearly increases between 1.76 and 3 eV (Fig. 2b). Under the same physical conditions, the photodetachment and absorption spectra of the solvent anion are identical (Fig. 2b) [20], suggesting a bound-to-CB transition the quantum yield of the photodetachment is close to unity. The photodetachment spectrum is similar to the photoelectron spectra of (C02) 9 clusters observed by Tsukuda et al. [24] in the gas phase it is distinctly different from the electron photodetachment spectra of CO2 in hydrocarbon liquids [27]. This suggests that a C-C bound, 7)2, symmetric dimer anion constitutes the core of the solvent radical anion [18,19]. 

Replacing all the pendant groups with aryl groups attains the delocalization of the unpaired electron. Figure 23 compares the absorption spectra of the radical anions of poly-(4-ethylphenyl-phenylsilane) and poly(dicyclohexylsilane) in MTHF at 77 K. Although the molecular structures of these polysilanes are similar, the absorption spectra are different. The absorption spectrum of the poly(dicyclohexylsilane) radical anions is similar to that of... 

Other transient radicals such as (SCN)2 [78], carbonate radical (COj ) [79], Ag and Ag " [80], and benzophenone ketyl and anion radicals [81] have been observed from room temperature to 400°C in supercritical water. The (SCN)2 radical formation in aqueous solution has been widely taken as a standard and useful dosimeter in pulse radiolysis study [82,83], The lifetime of the (SCN)2 radical is longer than 10 psec at room temperature and becomes shorter with increasing temperature. This dosimeter is not useful anymore at elevated temperatures. The absorption spectrum of the (SCN)2 radical again shows a red shift with increasing temperature, but the degree of the shift is not significant as compared with the case of the hydrated electron. It is known that the (SCN) radical is equilibrated with SCN , and precise dynamic equilibration as a function of temperature has been analyzed to reproduce the observation [78],... 

A compound that is transparent within a spectral domain when in its isolated state can sometimes absorb when in the presence of a species with which it can interact through a donor-acceptor relationship (D-A). This phenomenon is related to the passage of an electron from a bonding orbital of the donor (which becomes a radical cation) to an unoccupied orbital of the acceptor (which becomes a radical anion), which has a close energy level (Fig. 11.6). The position of the absorption band in the spectrum is a function of the ionisation potential of the donor and the electron affinity of the acceptor. The value of e for these transitions is usually large. 

Pulse radiolysis has been used to afford the radical cations of thioanisole, p-methylthioanisole, and benzyl phenyl sulfides. The absorption spectra of these radicals and their reaction with a number of nucleophiles and electron donor s are reported.302 Exposure of trimethylphosphine sulfide to 60Co 7-radiation at 77 K gave the radical anion Me3PS , identified by its ESR spectrum, from which other radicals, including Me2PS, were formed at higher temperatures.303... 

Mesitylfhjorenyl anion (9MsF ) is unreactive towards Mel at temperatures below —78 °C.100 Above —60 °C the absorption spectrum of 9MsF in the presence of Mel is replaced by that of the corresponding 9-mesitylfluorenyl radical (9MsF), and 9-methyl-9-mesitylfluorene is formed in low yield, hi a study of the electron-transfer photochemistry of chrysanthemol, an intramolecular S 2 reaction of a vinylcycloprop-ane radical cation has been observed.101 hi a long series of studies of the reactivity of the acids of trivalent phosphorus and their derivatives, the behaviour of P—O nucleophiles towards arylmethyl bromide systems has been examined.102 Further evidence for an X-philic substitution/SET tandem mechanism has been obtained. 

The radical anion Cw, can also be easily obtained by photoinduced electron transfer from various strong electron donors such as tertiary amines, fer-rocenes, tetrathiafulvalenes, thiophenes, etc. In homogeneous systems back-electron transfer to the reactant pair plays a dominant role resulting in a extremely short lifetime of Qo. In these cases no net formation of Qo is observed. These problems were circumvented by Fukuzumi et al. by using NADH analogues as electron donors [154,155], In these cases selective one-electron reduction of C6o to Qo takes place by the irradiation of C6o with a Xe lamp (X > 540 nm) in a deaerated benzonitrile solution upon the addition of 1-benzyl-1,4-dihydronicoti-namide (BNAH) or the corresponding dimer [(BNA)2] (Scheme 15) [154], The formation of C60 is confirmed by the observation of the absorption band at 1080 nm in the near infrared (NIR) spectrum assigned to the fullerene radical cation. 

Photosensitized electron transfer reactions conducted in the presence of molecular oxygen occasionally yield oxygenated products. The mechanism proposed to account for many of these reactions [145-147] is initiated by electron transfer to the photo-excited acceptor. Subsequently, a secondary electron transfer from the acceptor anion to oxygen forms a superoxide anion, which couples with the donor radical cation. The key step, Eq. (18), is supported by spectroscopic evidence. The absorption [148] and ESR spectra [146] of trans-stilbene radical cation and 9-cyanophenanthrene radical anion have been observed upon optical irradiation and the anion spectrum was found to decay rapidly in the presence of oxygen. 

The products of the (e q + RCH=CH2) reaction are RCH—CH2 earbanions. Some of these have been identified by their chemical reactivity. Others have been observed through their absorption spectra by means of pulse-radiolysis techniques. The carbanion of acrylamide, for instance, has been shown to dimerize, to react with other free radicals, inducing anionic polymerization, and to react with oxygen, Ag+ and Fe(CN) - ions, presumably by electron-transfer reactions (Chambers et al., 1967). The absorption spectrum of the product of the (dimethyl fumarate + ey5) reaction has been observed in alkaline solution. The rate... 

In a previous study we have found that, at low temperature, PS-I electron transfer is largely blocked away from A, and that the state (P-700+, A, ) decays with a half-time of 130us. Analysis of the absorption spectrum of that state showed that A, is presumably a quinone radical anion (Brettel et al, 1986). Chemical analysis, following separation by HPLC, has shown that phylloquinone (a naphthoquinone also named vitamin Kj) is the only quinone present in PS-I. We have found 2 moles of phylloquinone per PS-I. Extraction with dry hexane does not change the electron transfer reactions this treatment only extracts only one phylloquinone per PS-I (Biggins and Mathis, 1987). 

In doped PMMA, solute ions were efficiently formed on exposure to ionizing radiation cations were formed for most cases, but anions were formed only when the solute had a high electron affinity [86]. Kira et al. irradiated solid PMMA containing excess biphenyl and a small amount of a second solute by electron-pulses and observed the absorption spectrum of the biphenyl radical-cation produced by the following reactions [109] ... 

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