Radicals in Solids

2 Radicals in Solid H20. - 5.2.1 ESR Spectra and Models of Defects. Primary defects of radiolysis of water is hydrogen atom (H°), OH radicals and their 

Hydrogen atom, II0 irradiation of solid H20 at 4.2K produces signals of H° with A = 50 mT which is only stable for several hours due to formation of H2 

H02 radical H02 radicals are observed when OH radicals are annealed at 130K. 

Trapped electron et A self-trapped electron like an F centre in alkali halides is observed at g = 2.001 with a linewidth of 4.3 mT mostly in glassy ice doped with alkaline ions. 

SOf in solid H20 acid rain and Antarctic ice core. The signal intensity at g — 2.0036 and g = 2.0020 is enhanced considerably in y-irradiated solid H20 doped with S032. One can detect the concentration down to 10-100 ppm of S032- in irradiated solid H20. 03 radicals are stable after OH radicals are annealed out.120 

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FID does not die away before the deadtime has elapsed. In die case of inliomogeneously broadened EPR lines (as typical for free radicals in solids) the dephasing of the magnetizations of the individual spin packets (which all possess slightly different resonance frequencies) will be complete within the detection deadtime and, therefore, the FID signal will usually be undetectable. 

Evidence indicates [28,29] that in most cases, for organic materials, the predominant intermediate in radiation chemistry is the free radical. It is only the highly localized concentrations of radicals formed by radiation, compared to those formed by other means, that can make recombination more favored compared with other possible radical reactions involving other species present in the polymer [30]. Also, the mobility of the radicals in solid polymers is much less than that of radicals in the liquid or gas phase with the result that the radical lifetimes in polymers can be very long (i.e., minutes, days, weeks, or longer at room temperature). The fate of long-lived radicals in irradiated polymers has been extensively studied by electron-spin resonance and UV spectroscopy, especially in the case of allyl or polyene radicals [30-32]. 

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

ORGANIC ION-RADICALS IN SOLID PHASES 2.7.1 Organic Ion-Radicals in Frozen Solutions... 

A. Lund and M. Shiotani (eds.) EPR of Free Radicals in Solids. Trends in Methods and Applications. 2003 ISBN 1 -4020-1249-7... 

Solid CH4 on Triton and Terrestrial Methane Hydrate. - 5.4.1 Radicals in Solid CH4 and on Triton. Methane (CH4) and nitrogen are gaseous molecules on Earth but are frozen and solids under the lowest temperature of 37K at Triton, a satellite of Neptune. Craters of solidified CH4 and N2 and black smoke consisting of gaseous N2 and solid CH4 were discovered by the Voyager II a few km above the ground volcanic eruptions in 1989. 

Nuclear tunneling has been most explicitly demonstrated in low-temperature reactions of atoms and free radicals in solids. The common feature of these reactions is the use of radiation to obtain unstable species which undergo reaction. As a rule, the active particles were recorded with the help of the EPR spectroscopy. A wide class of reactions has been studied... 

Attempts were made to explain trends in the magnitudes of hyperfine coupling constants in terms of hyperconjugation, localization on oxygen, and so forth (Symons, 1959). Also, results for radicals in solids were being related to their more stable, and generally more complicated, counterparts in solution. It was realized that the equivalence of /3-protons, in radicals such as MeaCOH, must be the result of rapid... 

Scheme 8 Unimolecular photochemical methods to generate phenoxyl radicals in solid state.

When the electron spins are coupled with nuclear spins, the cross relaxation accompanying the change of the nuclear spin state can occur. In this case the apparent spectral overlap of the A and the B spins is not necessary. The spectral averaging of Eq. (3) is therefore a difficult task. Instead, we assume that the spectral overlap function in Eq. (2) is given by a constant F. Then, the spatial averaging of Eq. (3) is necessary for correlating the observed relaxation kinetics with the theory. The result of the spatial averaging will be shown for the two extreme cases of the spatial distribution of radicals in solids. 

Fig. 20. Scheme for reactions of cyclohexyl radicals in solid matrix. 

Although employing inorganic radicals in solid electrolytes opens the possibility to detect gaseous oxides by a potentiometric method, it is a chemical choice rather than a material choice for solid electrolytes. Since salts involving inorgaific radicals are not usually good solid electrolytes due to limited chemical stability. 

Solid Phase Experiments. The production of trapped free radicals in solid rare gases is a very simple matter, if the rare gas is present in at least 25 to 1 excess and the number of collisions between the free radicals and other molecules or the wall is held to a minimum (46). Use of the optical spectra of the free radical, if known, offers a usually reliable method by which it can be identified. 

The formation free radicals in solid state,the diffusion of free radicals into polymer matrix,the kinetics of hydrogen atom abstraction. 

Organic Radicals in Solution Organic Radicals in Solids Triplets and Biradicals Applicatins of E.S.R. in Polymer Chemistry Spin Labels Biological Systems Free Radical Studies in Biology and Medicine Loop-Gap Resonators. 

THE RATE OF REACTION AND DECAY OF RADICALS IN SOLIDS 3.2.1 Kinetic theories for radical recombination... 

The reactions of radicals formed in polymers by radiochemical, photochemical and mechanical methods have been reviewed and discussed recently by Butyagin et al. [214]. They identify the three main types of radical in solid polymers as... 

A particularly clear-cut case of this behavior has been observed in the CN radical in solid Ne. The major relaxation pathway for excited vibrational levels of the low-lying A TI electronic state involves crossing into the nearest available level of the ground state followed by... 

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