Identification of ESR Spectra

The total G value for radical formation at 77 K was found to be 4.2. The G value for the photobleached anion radical is 1.2 in accordance with the previous report [41]. The photobleaching transforms the anion radical into the methyl radical with an efficiency of about 10%. A small quantity of the methyl radical exhibits a very prominent spectrum in the cw-ESR measurements because of the narrow width of the spectral lines. 

The ESE method was used to identify the overlapping ESR spectra of the irradiated PMMA more definitely. The measurements of the ESE-detected ESR were made at 77 K with the 90° — t — 180 two-pulse sequence, at various fixed times of longitudinal relaxation, tl5 while the external magnetic field was swept slowly. The t( -resolved ESR spectrum was obtained from the difference between the echo intensity at a fixed x of 0.5 ps with and without a 90° saturation pulse 

Based on these ESR and ESE results, the G values for the initial formation of radicals at 77 K were determined to be 2.0,1.2, and 1,0 for the side-chain radical, the anion radical, and the main-chain radical, respectively. 

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Identification of ESR Spectra of Mechanically Formed Free Radicals... 

In the conventional ESR method using continuous microwave.radiation (cw ESR), the identification and quantification of radical species are made from the spectral shape and the spectral intensity, respectively, under the condition of a low enough level of microwave power incident to the sample cavity. If the power level is too high, the structure of ESR spectra becomes broadened and obscure and the intensity of the spectra is no longer proportional to the radical concentration (power saturation effect). Care is usually taken to avoid these effects in cw ESR measurements. 

The basic methods of the identification and study of matrix-isolated intermediates are infrared (IR), ultraviolet-visible (UV-vis), Raman and electron spin resonance (esr) spectroscopy. The most widely used is IR spectroscopy, which has some significant advantages. One of them is its high information content, and the other lies in the absence of overlapping bands in matrix IR spectra because the peaks are very narrow (about 1 cm ), due to the low temperature and the absence of rotation and interaction between molecules in the matrix. This fact allows the identification of practically all the compounds present, even in multicomponent reaetion mixtures, and the determination of vibrational frequencies of molecules with high accuracy (up to 0.01 cm when Fourier transform infrared spectrometers are used). 

UV-vis spectra of matrix-isolated intermediates are not so informative as matrix IR spectra. As a rule, an assignment of the UV spectrum to any intermediate follows after the identification of the latter by IR or esr spectroscopy. However, UV-vis spectra may sometimes be especially useful. It is well known, for example, that the energy of electronic transitions in singlet ground-state carbenes differs from that of the triplet species. In this way UV spectroscopy allows one to identify the ground state of the intermediate stabilized in the matrix in particular cases. This will be exemplified below. 

The results described in this review show that matrix stabilization of reactive organic intermediates at extremely low temperatures and their subsequent spectroscopic detection are convenient ways of structural investigation of these species. IR spectroscopy is the most useful technique for the identification of matrix-isolated molecules. Nevertheless, the complete study of the spectral properties and the structure of intermediates frozen in inert matrices is achieved when the IR spectroscopy is combined with UV and esr spectroscopic methods. At present theoretical calculations render considerable assistance for the explanation of the experimental spectra. Thus, along with the development of the experimental technique, matrix studies are becoming more and more complex. This fact allows one to expect further progress in the matrix spectroscopy of many more organic intermediates. 

Occasionally, typical pattern can be observed which can be formed according to special rules like multiplets in ESR-, NMR-, and OES spectroscopy or isotopic ratios in MS (molecular peak pattern). There can also be randomly formed pattern within such spectra, being rich in signals like OES (e.g. the known sodium doublet (Na-D) 589.6 and 589.0 nm, and the magnesium quintet 277.67, 277.83, 277.98, 278.14, and 278.30 nm). The identification of species is always made easier when pattern - whatever type - can be compared instead of a number of signals that are irregularly arranged. 

One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiation in the solid state and/or at very low temperatures. Then, the intermediates can be detected at the end of the irradiation by ESR or optical absorption spectroscopy. The ESR study of radicals in the solid state is done on single crystals, polycrystalline samples or frozen aqueous solution. In case of polycrystalline samples or frozen aqueous solution the identification of the radicals from the ESR spectra is difficult in many cases and, for better identification, the ESR experiment should be conducted on irradiated single crystals. Later, the method of spin trapping, developed for the liquid phase5, was extended to polycrystalline solids. In this technique the polycrystalline solids are /-irradiated and subsequently dissolved in a solution containing the spin trap. 

In this type of spin traps, 5,5-dimethyl-l-pyrroline-Af-oxide (DMPO) deserves particular mention. DMPO is widely employed as a spin trap in the detection of transient radicals or ion-radicals in chemical and biological systems (see, e.g., Siraki et al. 2007). Characteristic ESR spectra arising from the formation of spin adducts are used for identification of specific spin species. In common opinion, such identification is unambiguous. However, in reactions with superoxide ion (Villamena et al. 2004, 2007b), carbon dioxide anion-radical (Villamena et al. 2006), or carbonate anion-radical (Villamena et al. 2007a), this spin trap gives rise to two adducts. Let us consider the case of carbonate anion-radical. The first trapped product arises from direct addition of carbonate anion-radical, second adduct arises from partial decarboxylation of the first one. Scheme 4.25 illustrates such reactions based on the example of carbonate anion-radical. 

Infrared spectroscopical data encode a lot of structural information and can be analyzed with the help of computational methods (vide supra) aiding in the identification of the observed species. Sometimes, two different electronic states may lie very close in energy and have similar geometries. In such cases (e.g., the quinonoid radicals to be described in Section II.B.), the predicted differences in the IR spectra are too small to allow an unambiguous assignment of the ground-state multiplicity. In this respect, ESR spectroscopy provides valuable comple-... 

ESR Spectroscopy. Electron Spin Resonance spectroscopy is an important technique for investigating the role of radical intermediates in radiation chemistry. The technique has been used widely for many years in the study of radicals occurring in irradiated solid polymers (.6,7). However, by their very nature, such species are reactive and may only exist in low concentration. The identification of these species can also be a problem since in the majority of polymers the environment of the radicals leads to broad, unresolved ESR spectra, which makes detailed spectral analysis difficult. In recent years, many of these problems of sensitivity and resolution have been reduced by more sensitive and stable ESR spectrometers and by development of new methods of data handling and manipulation. 

Analysis of Spectra. An understanding of the mechanism of polymer degradation must involve identification of the radical intermediates. However, anisotropy due to spin lattice interactions in the solid state invariably results in broad, poorly resolved ESR spectra and together with the low concentration of radicals which is usually present, can result in major problems with analysis. We have developed two approaches to this problem 1) increasing resolution and 2) sophisticated analysis routines. 

The sensitivity of esr spectroscopy for detection of radicals is very high. Under favorable conditions, a concentration of radicals as low as 10 12M can be detected readily. Identification of simple hydrocarbon radicals often is possible by analysis of the fine structure in their spectra, which arises from spin-spin splittings involving those protons that are reasonably close to the... 

More recently the use of esr techniques has allowed the unequivocal identification of free radicals in radiolyses. It will therefore be convenient, in this section, first to discuss this evidence and then to summarise some of the other radical characteristics, e.g. structure, spectra, reactivities, which have been deduced from systems where the supposition of free radical processes provides the simplest explanation of results. 

These radical adducts have characteristic resolved ESR spectra which allow the accurate identification of all magnetic properties (g-factors and hyperfine splitting of all magnetic nuclei of the system including 13C, 73Ge and 117Sn and 119Sn of the observed species)33. 

It has generally been accepted that aryloxy radicals are intermediates in the polymerization, largely because the effective reagents are those capable of one-electron transfer. This assumption has been confirmed recently by the identification of both monomeric and polymeric aryloxy radicals in the ESR spectra of polymerizing solutions of 2,6-xylenol (21). The first step in the reaction is the oxidation of the phenol to the aryloxy radical by Cu(II). Carbon-oxygen coupling of two aryloxy radicals yields the cyclohexadienone, which tautomerizers to the dimer (II) (Reaction 3). 

Reactive free radicals also react with the nitrogen of nitroso groups, forming a nitroxide one atom closer to the trapped radical than is the case with nitrone spin traps. This results in ESR spectra containing more chemical structural information. While nitroso spin traps provide radical identification, the resultant adducts are often less stable than those derived from nitrone traps. In particular, nitroso traps are unreliable for oxygen-centered radicals even in vitro. 

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