Electron spin resonance spectra observation

The mass spectra of several unsaturated sulfur heterocycles show the presence of 1,2-dithiete cation radicals. 1,2-Dithiete cation radicals also are obtained by treatment of a-hydroxyketones or a-diketones with sodium sulfide, sodium thiosulfate or sodium dithionite, and sulfuric acid. ° Bis(trifluoromethyl)-l,2-dithiete yields a cation radical directly when dissolved in sulfuric acid. ° The electron-spin resonance spectrum of the benzo-1,2-dithiete cation radical (formed in chlorina-tions with sulfur-containing reagents) has been observed.Thermolysis of a 1,4-dithiin may go via a l,2-dithiete. ° ... 

Three types of radical can be detected in irradiated polyethylene [4 — 65]. The methylene radical, —CH2—CH—CH2 — (I), possesses a sextet electron spin resonance spectrum. It is formed exclusively during irradiation at liquid nitrogen temperature. At or near room temperature, the ESR spectrum is the superposition of this sextet, which progressively disappears, and a more stable septet assigned to the allyl radical —CH—CH=CH2 — (II), Fig. 11. At very high doses, a singlet assigned to the polyenyl radical is observed. 

A sextet has been observed in the electron spin resonance spectrum of irradiated polyethylene at 77°K [31, 32] and assigned to the central radical (II). Moreover, five hyperfine components have been detected and attributed to the terminal radical (I) [32]. According to Ranby and Yoshida [32] free radicals (I) and (II) are formed in primary photochemical processes by fission of C—C and C—H bonds. Such a conclusion seems, however, doubtful in the light of the chemical evidence presented above which supports the initiation of the photodegradation of polyethylene by adventitious impurities. Moreover Tsuji and Takeshita [33] have recently observed that alkyl radicals produced at liquid nitrogen temperature by ultraviolet irradiation of polyethylene in vacuo are transformed into transient acyl radicals when the sample is allowed to warm up to about—125°C ... 

The most probable order of d levels in copper phthalocyanine is illustrated in Fig. 19 (131, 145). The calculated energies (131) are appended. The relative order of the ea and b2g orbitals is the inverse of that found for copper acetylacetonate (238). Although the data for cobalt phthalocyanine (139) cannot be assigned unambiguously, the orbital levels are probably in the same relative order. The hole would then be in the d orbital rather than in the dxt-yt (as in copper phthalocyanine) and this is in accord with the observation that the electron-spin resonance spectrum of cobalt phthalocyanine is solvent dependent, while that of copper phthalocyanine is not (131, 139). The alternative assignment of the cobalt data places the hole in the dxy orbital lying some 16,000 cm-1 above the dxz,dyX pair, which seems unlikely. 

Another difference is that the 5/ orbitals have a greater spatial extension relative to the 6,s and 6

electron-spin resonance spectrum of UF3 in a CaF2 lattice shows structure attributable to the interaction of fluorine nuclei and the electron spin of the U3+ ion. This implies a small overlap of 5/ orbitals with fluorine and constitutes an/covalent contribution to the ionic bonding. With the neodymium ion a similar effect is not observed. Because they occupy inner orbitals the 4/ electrons in the lanthanides are not accessible for bonding purposes and virtually no compound in which 4f orbitals are used can be said to exist. 

The G.F.F. theory was put to a more quantitative test by Luckhurst and OrgeF " who examined the electron spin resonance spectrum of fluorenone ketyl, generated by electrochemical reduction, in mixtures of dimethylformamide and methanol. In the range of mol fractions of methanol less than 0.15, the G.F.F. theory was able to account for the variation of all four observed splitting constants on the assumption that the alcohol and the ketyl formed a 1 1 adduct. In this region of alcohol concentrations the dimethylformamide is in excess and it is not necessary to postulate a stoichiometry for the solvation of the ketyl by dimethylformamide. At concentrations of methanol in excess of 0.15 the above description of the system was found to be inadequate, presumably because of the formation of l 2ketyl-alcohol adducts or because of a more general form of solvent effect upon the 1 1 adduct. 

The electron spin resonance spectrum of the radical ion complex of pyracene in tetrahydrofuran (Fig. 15) revealed that the structure of the complex is d)mamic in nature. A marked line-width alternation is observed 112-114), which arises from a d)mamic equilibrium existing between the two possible conformations of the ion pair with the counter ion in position A and in position B. 

The electron spin resonance of the nitroxalkylcorrinoids can be readily observed in aqueous solution at room temperature. Both the cobalamin and cobinamide show nitrogen hyperfine coupling constants of 17.2 gauss. A typical spectrum is shown in Fig. 20. The line widths for the low, intermediate, and high field peaks are 1.87, 1.87, and 2.20... 

As with the nitroxalkylcobalamins (119) and cobinamides, the co-binamides in which nitroxide is coordinated show electron spin resonance spectra very similar to the spectrum of free nitroxide. The high field line is not broadened as much as in the spectrum of a nitroxalkyl-cobinamide. No hyperfine splitting from methyl protons in the 2 or 6 positions can be observed for the bound nitroxide. However, treatment of the coordinate spin labeled compounds with cyanide releases the nitroxide. When this happens, the proton hyperfine can be observed (Fig. 25). Thus treatment with cyanide simply displaces the nitroxide and a spectrum for free nitroxide is observed. 

Although it is very hard to observe the absorption spectrum of eh when metal is dissolved in water because of its high reactivity, some attempts were made in water and ice (Jortner and Stein, 1955 Benett et al., 1964, 1967). Furthermore ESR (electron spin resonance) studies revealed that the trapped or solvated electron in ice interacts with six equivalent protons, thus ruling out H20-. 

To simplify terminology of axial systems, gzz is defined to be g(l (the g-value observed with the symmetry axis of Cu + parallel to the applied field), and gxx (= gyy) is defined to be gA (the g-value observed with the symmetry axis perpendicular to the applied field). An elongated z-axis (depicted in Figure 11 for Cu(H20)5 +) results in gjj > gj. For axially symmetric Cu + rigidly bound in a crystal, the g-value can then vary between the minimum (gj.) and maximum (g(,), depending on orientation of the crystal within the magnetic field. However, for axial Cu + bound in a powdered clay sample, all possible orientations, and therefore all g-values between gA and gj are represented in the "powder" spectrum. Therefore, electron spin resonance occurs only for field values, H, between Hjj and H, where ... 

The effect of temperature on the association of vanadium compounds in asphaltenes was investigated by Tynan and Yen (1969). Using electron spin resonance (ESR), they observed both anisotropic and isotropic hyperfine structures of vanadium, interpreted as bound or associated and free vanadium, from asphaltenes precipitated for a Venezuelan petroleum and reintroduced to various solvents. Higher temperatures and more polar solvents resulted in a transition from bound to free vanadium, as shown in Fig. 12. At 282°C, only 1% of the anisotropic spectrum was observed. An activation energy of 14.3 kcal/mole was observed for the transition. 

Historically, the triphenylmethyl radical (1), studied by Gomberg in 1987, is the first organic free radical. The triphenylmethyl radical can be obtained by the reaction of triphenylmethyl halide with metal Ag as shown in eq. 1.1. This radical (1) and the dimerized compound (2) are in a state of equilibrium. Free radical (1) is observed by electron spin resonance (ESR) and its spectrum shows beautiful hyperfine spin couplings. The spin density in each carbon atom can be obtained by the analysis of these hyperfine spin coupling constants as well as information on the structure of the free radical. 

Direct observation of transition-state selectivity has been observed from the low-temperature cyclization of dienes inside H-mordenite and H-ZSM-5 (9). By using electron spin resonance (ESR) spectroscopy, it has been possible to explore radical formation upon the sorption of dienes on H-mordenite and H-ZSM-5. From the analysis obtained, it was found that the dienes are not very reactive for oligomerization inside H-mordenite channels. Heating H-mordenite with presorbed 1,4-pentadiene or 1,5-hexadiene yields selective cyclization of molecules via cycloalkenie radicals inside the H-mordenite channel. However, in the smaller pores of H-ZSM-5 (although die nature of both acid and redox sites in both zeolites are the same) no eyclo-olefinie radicals are formed as shown by the ESR spectrum. These experiments illustrate the reality of transition-state selectivity inside the pores of zeolites. 

The nature and the possible structures of Xj and X2 are indicated by their electronic spectra. The absorption maximum at 425 nm in benzene and the shape of the spectrum of Xl9 the first short-lived intermediate (Figure 15), closely resemble those of the spectrum (A.max at 422 nm in methylene chloride) of TPP Mn(IV) prepared independently (31, 32, 33, 36), possibly indicating a charge-transfer complex (Figure 17). This assumption of a loose side-on complex resulting from a two-electron transfer is consistent with the previous results of the electron spin resonance (ESR) spectrum observed for an oxygencarrying species, TPP Mn(II) or substituted TPP Mn(II) (37). The... 

We said earlier that we can never prove a mechanism—only disprove it. Unfortunately, just as the correct mechanism seems to be found, there are some observations that make us doubt this mechanism. In Chapter 39 you saw how a technique called electron spin resonance (ESR) detects radicals and gives some information about their structure. When the Cannizzaro reaction was carried out with benzaldehyde and a number of substituted benzaldehydes in an ESR spectrometer, a radical was detected. For each aldehyde used, the ESR spectrum proved to be identical to that formed when the aldehyde was reduced using sodi-... 

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