ESR signals

Electron Spin Resonance Spectroscopy. Several ESR studies have been reported for adsorption systems [85-90]. ESR signals are strong enough to allow the detection of quite small amounts of unpaired electrons, and the shape of the signal can, in the case of adsorbed transition metal ions, give an indication of the geometry of the adsorption site. Ref. 91 provides a contemporary example of the use of ESR and of electron spin echo modulation (ESEM) to locate the environment of Cu(II) relative to in a microporous aluminophosphate molecular sieve. 

The nature of the intermediates impHcated in the photooxidation of water with Ti02 has been identified in several reports using spin traps by the electron spin resonance (esr) technique under ambient conditions (53). No evidence for OH species, even at 4.2 K, was found (43), but the esr signal... 

Mechanistic studies on the formation of PPS from polymerization of copper(I) 4-bromobenzenethiolate in quinoline under inert atmosphere at 200°C have been pubUshed (91). PPS synthesized by this synthetic procedure is characterized by high molar mass at low conversions and esr signals consistent with a single-electron-transfer mechanism, the Sj l-type mechanism described earlier (22). 

Janssen and co-workers prepared t-butyl 1-pyrazolecarboxylate (111) via 1-pyrazolecar-bonyl chloride (75JOC915). The thermolysis of this compound in benzene solution at 140 °C led to 1-phenylpyrazole. The IV-pyrazolyl radical (112), which is proposed as an intermediate, proved elusive and neither CIDNP nor ESR signals were observed. 

Fig. 1. The resistivity (solid and dotted line)[6] and the inverse scattering time (plain dots) [7] which is proportional to the product T. Ag (T is the linewidth and Ag the -shift of the ESR signal). For a metal, the resistivity and ip are expected to decrease when lowering the temperature [7].

When equimolar quantities of 80a and its dication 110 are combined in acetonitrile, single electron transfer occurs and the coproportionation product was obtained (95TL2741).Tliis deeply red-colored, air-sensitive radical cation 111 showed a strong ESR signal (g = 2.0034). On the other hand, the excellent electron donor 80a could be prepared by electrolytic reduction starting from 110. It was necessary to carry out the reduction with scrupulous exclusion of oxygen. Tlius, the electrolysis of 110 at -1.10 V initially gave rise to an intense red color, which was presumably due to the formation of 111. Upon further reduction, the red color faded and the tetraaza-fulvalene 80a was isolated at a 62% yield (Scheme 45). 

Treatment of a series of 1,2-dithiols (136) including ds-cyclohexane-1,2-dithiol with AI2CI6 in CH2CI2 at 25°C led to persistent ESR signals due to the corresponding 1,2-dithietane radical cations (g = 2.0187 0.0003)... 

Fjg. 19. Dependence of the intensity of the ESR signal on the wavelength of incident light of a complex of a polyfschiff base) (prepared from 4,4 -diacetyldiphenyl sulfide) with bromine mole ratio 1 4.8... 

Fig. 20. Increase of the intensity of the ESR signal during the irradiation and its decay in darkness of a bromine complex of a poly(schiff base) prepared by polycondensation of 4,4 -diacetyldiphenyl sulfide and p-phenyienediamine mole ratio 1 S.8...

Thus, irradiation in the transfer band results in practically the whole molecular complex being found in the complete transfer state. The course of the curve corresponding to the increasing ESR signal (Fig. 20) is typical of a number of PCSs and is well described by the relationship AI = where a is constant. 

Ward (Ref 15) and Brown and Williams (Ref 16) studied nitrobenzene with radiation > 3300A using tetrahydrofuran (THF) as solvent in degassed solns. The effects of low temp were also studied. An ESR signal, attributed to the C6HsN(02)H radical, was obtained with THF as solvent. No ESR signal was obtained when perfluoronaphthalene was added. They concluded that the triplet state of nitrobenzene was an intermediate... 

Scaiano and Kim-Thuan (1983) searched without success for the electronic spectrum of the phenyl cation using laser techniques. Ambroz et al. (1980) photolysed solutions of three arenediazonium salts in a glass matrix of 3 M LiCl in 1 1 (v/v) water/acetone at 77 K. With 2,4,5-trimethoxybenzenediazonium hexafluorophos-phate Ambroz et al. observed two relatively weak absorption bands at 415 and 442 nm (no e-values given) and a reduction in the intensity of the 370 nm band of the diazonium ion. The absence of any ESR signals indicates that these new bands are not due to aryl radicals, but to the aryl cation in its triplet ground state. 

The formation of adduct is followed by fragmentation and subsequent H-atom abstraction reaction from the sulfinic acid produced. Strong acid solutions of aromatic sulfoxides like thianthrene 5-oxide (7) or phenothiazine 5-oxide (8) gives rise to ESR signals, which... 

In 1968 DairOlio et al. published the first report of analogous electrosyntheses in other systems. They had observed the formation of brittle, filmlike pyrrole black on a Pt-electrode during the anodic oxidation of pyrrole in dilute sulphuric acid. Conductivity measurements carried out on the isolated solid state materials gave a value of 8 Scm . In addition, a strong ESR signal was evidence of a high number of unpaired spins. Earlier, in 1961, H. Lund had reported — in a virtually unobtainable publication — that PPy can be produced by electrochemical polymerization. 

Whereas the intermediate existence of polarons has been unequivocally proved by ESR measurements and optical absorption data, up to now, the existent of bipolarons has been only indirectly deduced from the absence of the ESR signal and the disappearance of the visible polaron bands from the optical absorption spectrum On the other hand, spinfree — diionic-charge — states in aromatics, whose optical properties bear a remarkably resemblence to the predictions of the bipolaron model, have long been known Further evidence of bipolarons is the fact that doped... 

This color transformation has been observed in dibenzo-p-dioxin (Structure I) and in its bromo, chloro, nitro, methyl, and ethyl derivatives in addition, the observed electron spin resonance (ESR) signals indicated the presence of paramagnetic species (2, 3). This phenomenon has been attributed to the formation of cation radicals in acid solution. 

Figure 10. ESR signal amplitude growth with time under constant irradiation...

Three isomeric tetrachlorodibenzo-p-dioxins were studied. All were insoluble in TFMS acid. To dissolve these compounds and form cation radicals, UV irradiation was necessary. The 1,2,3,4-tetrachloro compound was particularly sensitive to UV irradiation, and as a solid, even turned pink when exposed to ordinary fluorescent light. When subjected to constant UV irradiation, radical ions were induced rapidly. The change in the cation radical concentration was monitored by the ESR signal as illustrated in Figure 10. To determine whether the tetrachloro isomer had been converted to lower chlorinated derivatives after UV irradiation, the dissolved dioxin was then poured into ice water and recovered. The GLC retention time of the recovered dioxin was unchanged in addition, no new GLC peaks were observed. Moreover, the ESR spectrum see Figure 11) for the recovered material was not altered between widely... 

Only in a tartrate buffer is the rate appreciable and it is significant that only in this buffer does the normally diamagnetic Mo(V) display esr signals g = 1.937 and g = 1.945) indicating the presence of monomeric Mo(V) which is regarded as the active species in the proposed mechanism, viz. 

Oxidation of isopropyl alcohol by chromic acid in concentrated acetic acid solution has recently been studied by Wiberg and Schafer S spectrophotometri-cally. At 385 nm a rapid increase in absorbance (with a half life of about 6 sec) due to mono- and diester formation was noted. When the reaction was examined at 510 nm, first a rapid increase, then a decrease of the absorbance was found. Since at this wavelength only chromium species can absorb, the intermediate could be chromium(V) or (IV). The esr spectra of reaction mixtures showed a relatively sharp signal with a. g = 1.9805 value corresponding to chromium(V). The fact that the relative concentrations of the intermediate determined from the spectral data agree well with the intensity of esr signals, indicates that the same species is responsible for the both phenomena. It is then clear that the oxidation of isopropyl alcohol proceeds via chromium(V). 

Figure 9. ESR signal for various reduced Figure 10. Number of paramagnetic species as solids a function of the reduction temperature.

Figure 11. Observed ESR signals after reduction, hydrogen desorption and air exposure.

Figure 2 compares ESR spectra of BaTi40g, Na2Ti60i3 and Ba4Tii303o. No ESR signals were observed without uv irradiation. In accordance with previous observation[3], uv irradiation of BaTi40g at 77 K in the presence of 30 Torr oxygen... 

No new ESR signals were observed upon adsorption of NjO onto RhO /CeO, outgassed... 

In a. p. and s.o. ZV(a) and ZV(i) samples, no ESR signals were detected. In a.p. ZV(acac), a weak ESR signal of vanadyl species was detected (5% of total V), absent after the s.o. treatment. The spectra of samples reduced with CO at 400 to 623 K consisted of a signal showing a resolved hyperfine structure (Vh), overlapping a broad (AHnp = 300 Gauss) and nearly-isotropic band (Vb, giso = 1 -97) (Fig. 3). When recorded at 77 K, both Vh and Vb maintained the same shape as at RT, and their Intensity as a function of temperature followed the Curie law. 

However, there is no indication that the presence of the observed signals correlates with the polymerization efficiency of the catalyst. In fact, systems which exhibit these signals are less effective catalysts and in some cases do not even polymerize ethylene under the chosen conditions. In contrast, systems without EPR signals correlated to Ti species are foimd to be catalytically active. It has to be emphasized at this point that the lack of an ESR signal associated to Ti + ions, in cases where no additional argon or electron bombardment has been applied, cannot be interpreted as an indication of the absence of Ti + centers at the surface. It has been discussed in the literature that small spin-lattice-relaxation times, dipole coupling, and super exchange may leave a very small fraction of Ti " that is detectable in an EPR experiment [115,116]. From a combination of XPS and EPR results it unhkely that Ti " centers play an important role in the catalytic activity of the catalysts. 

---