Free radical content

Chemisorption of oxygen on char has often been discussed previously in terms of free radical concentration in the char (1.5,6). For cellulose chars Bradbury and Shafizadeh (1) found that free spin concentration reached a sharp maximum at HTT 550°C, coinciding with maximum CSA and drew the obvious conclusion that the extent of CSA was at least partly related to free radical content of the char. However, in subsequent work on cellulose char, DeGroot and Shafizadeh (3) have found that unpaired spin concentration continues to increase up to HTT 700"C. Ihe CSA of the char must therefore depend on factors other than free radical concentration. 

Evidence for such stable free radicals has been obtained from electron spin resonance measurements. A sample of the 77-23 PVC-styrene, which had been exposed to 0.8 megarad of gamma-radiation at room temperature, displayed resonance peaks comparable to 3 X 10 8 mole per gram of free radicals (compared with a diphenyl picrylhydrazyl standard). When an identical sample was heated for 10 minutes at 75°C. following irradiation, the free radical population had fallen below detection limits. Heating evidently destroyed or decreased the free radical content by reaction or termination. 

One of the most striking properties of humic acid is its change in free radical content upon conversion to the solid sodium salt. This change is reversible upon reacidification of the salt, the spin content returns to its original level (28). In general, the line widths increased about 50% on conversion to the salt. Table III illustrates this effect for many different humic acids. These results seem entirely consistent with the known properties of the salts of quinhydrone as shown in Figure 6 (28). 

Table III. Free Radical Content of Humic Acids and Salts...

Figure 1. Free radical content of fungally decayed sweet gum...

The free radical content in kraft lignin has been determined by Steelink (39) as about 0.0001 per OCH3. Even with a model e value of 200,000 as obtained for a tetra-/ T/-butylquinonemethide radical (P), only about 7% of the absorption of kraft lignin at 420 m/z would be explained. 

Early work on a series of carbonized coals gave 3 x 1019 free radicals per gram (1 free-radical per 1600 carbon atoms). It was also established that the free-radical content of coal at first increases slowly (in the range 70 to 90% carbon) (Ladner and Wheatley, 1965), rises markedly (90 to 94% w/w C), and then decreases to limits below detectability. Thus, in a coal having 70% carbon, there is one radical per 50,000 carbon atoms, but this is increased to one radical per 1000 carbon atoms in coal with 94% w/w carbon. 

Because of the free-radical character of the radiative decomposition of starch, the biological studies have given major concern to the mutagenicity of the products. Several studies have revealed that free-radical content of -irradiated starch does not cause detectable perturbations in the physiological functions of experimental mice and rats through several subsequent populations. Starch samples tested were irradiated at doses between 20 and 600 krads.122 225 At these dose levels... 

For a number of carbohydrate-Gly melanoidins (170 °C, 20 min, dialysed), free-radical content (EPR) and oxygen scavenging ran parallel,81 except for maltotriose, where oxygen scavenging was greater than expected on the basis of such a relationship. Yet, the overall implication is that the oxygen-consuming properties are based on a radical mechanism. 

As antioxidants are free radical quenchers, the level of the added antioxidants in the refined oil decreases while the oil is in storage or in transit. However, one must recognize the fact that, because of the presence of antioxidants, the free radical content of the oil is going to be low at the time it is added to the fryer (provided the oil is not stored for too long or oxidized as a result of mishandling before use). 

The strength and sensitivity of the -H NMR signals of oil shales are sufficient such that pulsed NMR techniques afford a means of dynamically observing thermal transformation processes. Parameters can be extracted from the NMR data that relate to the hydrogen content, phase structure, molecular mobility and free radical content of the specimen. 

Volk and Schnitzer (1973) concluded that variations in the functional group components and spectral properties of humic acids from a group of Florida mucks indicated that higher rates of humification were related to (1) greater amounts of carboxyl, phenolic hydroxyl, quinone, and ketonic carbonyl groups (2) fewer alcoholic hydroxyl groups and aliphatic structures, as per IR evidence and (3) increments in EJEf, ratios and free-radical contents as revealed by ESR spectroscopy (Table 12). 

The reported spin concentrations correspond to one free radical per 1100 molecules (number average molecular weight of 951) for an untreated fulvic acid in the work of Schnitzer and Skinner (1969) and one free radical per 250 molecules of humic acid (1.4 x 10 spins/g, with a molecular weight of 20,000) in the work of Steelink (1964). Consequently, the ESR spectrum is providing data on only a small fraction of the total molecules in the humic mixture, as pointed out by Riffaldi and Schnitzer (1972). Hayes et al. (1975) found that the free radical contents of humic and fulvic acids vary with the solvent used in the extraction, suggesting that the free radical in a humic substance may be an artifact of the extractive procedure. With the severity of these limitations in mind, any generalization concerning the functionality or structural nature of humic substances based on ESR data must be conservative. 

The populations and role of semiquinone states assumed to be responsible for the characteristic ESR signal have been extensively studied by ESR spectrometry for all types of melanins (75). The increase in the free radical content after reduction of melanins in nonaqueous media may indicate an increased population of semiquinones (153) and/or quin-hydrone-type complexes. In such a case a maximum intensity signal should be observed with half-oxidized-half-reduced melanin. 

For horseradish peroxidase (HRP), it was long thought that only a small fraction of the free-radical content was ESR detectable conventional ESR spectra show a narrow component equivalent to about 0.01 spins/heme. However, recent work has demonstrated that much of the free-radical spectrum is extremely broad due to interaction of the radical with ferryl ion, probably through an anisotropic exchange interaction [109]. When these broad components are taken into account, the number of free... 

The thermal conversion of polynuclear aromatic compounds to carbon can be considered a free-radical process because it involves a series of bond cleavage reactions. Many of the radical intermediates are expected to be unstable and cannot be detected by conventional spectroscopic techniques. However, the carbonaceous residues from pyrolysis do contain significant amounts of stable free radicals as apparent from EPR experiments. For example, Figure 8 shows the results of EPR measurements of the free-radical content for anthracene and naphthalene pitches after heating from 400 to 700 °C. The concentration of stable free radicals increases with heat treatment temperature as the molecular size grows. The more highly condensed... 

A variety of compounds have been mixed with oil shale and coals as internal standards to determine the percentage of carbons observed in CP and single-pulse NMR measurements.""In some cases, the spinner material, Delrin, has been used as an internal standard." An excellent reference material is tetrakis (trimethyl silyl) silane (TKS). Furthermore, treatment with samarium (II) iodide selectively reduces the free radical content which can lead to an increase in the percentage of observable carbon. For example, treatment of Wyodak sub-bituminous (x>al with samarium (II) iodide has been shown to increase the percentage of observable carbons by 27%, leading to an increase in the carbon aromaticity of around 10% from 0.66 to 0.73. 

Pulsed NMR measurements have been made as a function of temperature to study the pyrolysis behaviour of oil shales. The method is sometimes referred to as NMR Thermal Scanning. To overcome receiver dead time, a 9O-T-9O90 pulse sequence is used to form the solid echo . The echo signal is then decomposed into a rigid (short relaxation time) and a mobile (longer relaxation time) component. These data are then related to various properties of the system. Parameters that can be extracted from the NMR data relate to the hydrogen content, phase structure, molecular mobility, and free radical content. By measuring the temperature dependence of these parameters... 

The highest conductivities thus far observed in organic molecules are found among the donor-acceptor complexes. The activation energies for conduction are small, on the order of 0.1 eV ( 2 kcal) or less. Examples are the complexes of aromatic hydrocarbons with iodine and complexes of amines with chloranil or related quinones. The best conductors have a high free-radical content and are usually not of simple stoichiometry. The complex salts of TCNQ ° (I) are the best of the known organic conductors. The... 

The free radical content is appreciably lower for fulvic acids than for humic acids. 

FIGURE 5 21 Free radical contents as signal heights (cm) per gram fresh relght of the sample ... 

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