Naphthalene anion radical spectrum

The naphthalene anion radical spectrum (Figure 2.2) provided several surprises when Samuel Weissman and his associates1 first obtained it in the early 1950s at Washington University in St. Louis. It was a surprise that such an odd-electron species would be stable, but in the absence of air or other oxidants, [CioHg]- is stable virtually indefinitely. A second surprise was the appearance of hyperfine coupling to the two sets of four equivalent protons. The odd electron was presumed (correctly) to occupy a it molecular orbital... 

When several magnetically equivalent nuclei are present in a radical, some of the multiplet lines appear at exactly the same field position, i.e., are degenerate , resulting in variations in component intensity. Equivalent spin-1/2 nuclei such as 1H, 19F, or 31P result in multiplets with intensities given by binomial coefficients (1 1 for one nucleus, 1 2 1 for two, 1 3 3 1 for three, 1 4 6 4 1 for four, etc.). One of the first aromatic organic radical anions studied by ESR spectroscopy was the naphthalene anion radical,1 the spectrum of which is shown in Figure 2.2. The spectrum consists of 25 lines, a quintet of quintets as expected for hyperfine coupling to two sets of four equivalent protons. 

Figure 2.2 ESR spectrum of the naphthalene anion radical 1 simulated using hyperfine couplings given in Table 2.1.

Things get a little more complicated when a spin 1 nucleus like 14N is added to the picture, but the same technique works again for the determination of the relative intensities of the ESR lines. Consider, for example, the relative intensities of the hyperfine lines arising from the pyrazine anion radical, whose spectrum is shown in Figure 2.3. Like that of the naphthalene anion radical, the spectrum observed for the pyrazine anion radical2 consists of 25 well-resolved... 

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... 

Give the number of lines in the ESR spectrum of the anion radical of each of the following molecules. (Assume all lines are resolved.) (a) Naphthalene (b) anthracene (c) pentacene (d) azulene (e) o-xylene (f) w-xylene (g) p-xylene (h) nitrobenzene (i) />-fluoronitrobenzene. 

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. 

Branching diagrams are often crucial to obtaining an assignment of a spectrum when a radical contains more than one set of equivalent nuclei. The spectrum of the naphthalene anion is shown in Figure 29.6 together with a diagram... 

Hoefelmeyer and Gabai (2000) have synthesized l,8-bis(diphenylboryl)naphthalene and compared the structures of this molecule and the anion radical prepared from it. In the parent neutral molecule, the boron centers are separated by 3 nm. In the anion radical, the boron-boron distance is approximately 1 nm longer than that observed in compounds with single bonds between four-coordinated boron atoms. According to the ESR spectrum of the anion radical, the unpaired electron is preferentially suspended in the field of the two boron atoms. The one-electron a-bond emerges. This one-electron a-bond can obviously be viewed as resulting from the overlap of the formerly vacant and parallel p-orbitals of two... 

The reductive step of the Sternberg procedure actually produces a resonance with a g value higher than that of the starting coal, suggesting possible increased influence of oxygen in the resultant radical ensemble. The top absorption spectrum is of an aliquot removed during reduction and shows the presence of the naphthalene anion. The middle spectrum is of a filtered, reduced Monterey coal (spin density of 7 X 20 spins g). The bottom spectrum was obtained following butyl iodide addition. Spectra are 40 G in width but not concentric in g value. (Potassium was used in these examples.)... 

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. 

Figure 29.6 Spectrum of the naphthalene radical anion and a reconstruction of the spectrum.

A radical anion of an aromatic hydrocarbon was implicated as early as 1866, when Berthelot obtained a black dipotassium salt from naphthalene and potassium [41]. This reaction must have proceeded via the naphthalene radical anion as a more or less fleeting intermediate. Again, Schlenk and co-workers captured the essence of such an intermediate. In the case of anthracene they noticed the existence of two different species, a purple dianion and a blue transient species with a banded spectrum [42]. They identified this intermediate as a monosodium addition product which contains trivalent carbon . Further details were revealed only with the advent of electron paramagnetic resonance spectroscopy. 

A more remarkable elongation of the CS lifetime was attained by complex formation of yttrium triflate [Y(OTf)3] with the CS state in photoinduced ET of a ferrocene-anthraquinone (Ec AQ) dyad (53). Photoexcitation of the AQ moiety in Ec AQ in deaerated PhCN with femtosecond (150 fs width) laser light results in appearance of the absorption bands 420 and 600 run at 500 fs, as shown in Eig. 14(a) (53). The absorption bands 420 and 600 nm, which are assigned to AQ by comparison with the absorption spectrum of AQ produced by the chemical reduction of AQ with naphthalene radical anion (53). The decay process obeys first-order kinetics with the lifetime of 12 ps [Eig. um. 

When the life-time of a radical is reduced by rapid intermolecular electron-exchange, all the lines in the spectrum are broadened. The theoretical correlation between line-widths and exchange rates has been discussed (Weissman, 1960 Ward and Weissman, 1957). These authors have determined the rates of electron transfer between naphthalene radical-anion and naphthalene in different solvents and when different metal ions are present. In addition, it is possible to distinguish between the simple exchange reaction, Np -t- Np = Np -j- Np (Np=naphthalene), and that involving the ion-pair, Np Na+ -f Np Np -1- Np Na+, since the e.s.r. spectrum of the ion-pair exhibits a splitting from the Na nucleus (Zandstra and Weissman, 1962). 

The oxidation of compound 32 occurs in a similar manner but even more easily136. Its radical cation can be stored unchanged in acetonitrile for months. The UV spectrum of 32+ (7.max = 480 nm, lg e = 3.1) is very close to that of neutral 32 itself, from which one can conclude that the naphthalene moiety does not take part in the oxidation. As found by cyclovoltammetry, the double proton sponge 35, unlike its isomer 79, is oxidized with reversible two-electron transition, composed of two superimposed one-electron steps at Eli2 —0.50 V (Table 13)45. Apparently, the driving force for this process is the formation of the resonance-stabilized dication 133 (equation 8), which was isolated in the form of black crystals with I3- anions (kmax = 643 nm, lg e = 4.02) and investigated in... 

There has been an incomplete report of the ESR spectrum of PhSnMe with a(2H-ortho) 0.40 mT and aQi-para) 0.82 mT, suggesting that the /s MO is stabilised and that the Me3Sn substituent is electron-attracting.89 The spectrum of the radical anion of l,4-bis(trimethylstannyl)naphthalene (20-18) has also been obtained by treating the substrate with potassium in DME,90 but this is not so informative as naphthalene is not orbitally degenerate, and the spin distribution is not so sensitive to substituent effects. 

Fig. 59. Rapid-scan time-resolved in-situ FT-IR spectrum of the electroreduction of naphthalene (0.2 M) TBAB/tetrahydrofuran. The downward peaks indicate a decrease in naphthalene concentration near the ATR cathode. The positive peaks indicate the formation of naphthalide radical anion and dihydronaphthalene. The time displacement between successive spectra is 115ms. Resolution is 10 cm-1.

Unpaired electrons can be present in ions as well as in the neutral systems that have been considered up to this point. There are many such radical cations and radical anions, and we consider some representative examples in this section. Various aromatic and conjugated polyunsaturated hydrocarbons undergo one-electron reduction by alkali metals. Benzene and naphthalene are examples. The ESR spectrum of the benzene radical anion was shown earlier in Figure 11.2a. These reductions must be carried out in aprotic solvents, and ethers are usually used for that purpose. The ease of formation of the radical anion increases as the number of fused rings increases. The electrochemical reduction potentials of some representative compounds are given in... 

Addition of naphthalene or biphenyl to the glass causes the broad band to be replaced by the characteristic absorption spectrum of the naphthalene or biphenyl radical anion due to... 

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