Unpaired spin concentrations

F ore 220. The variation of free spin concentration (unpaired electrons) ( sg ) 1 signal bandwidth at half-peak height (G) with (a) HTT (°C) and (b) carbon content (wt%) for carbon from PVC (fusible and graphitizable). 

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. 

Figure 2. Unpaired spin concentration in pure macerals...

Table III. Unpaired Spin Concentrations and Line Widths of Macerals before and after Laboratory Pyrolysis...

Thus we are led to the conclusions that fusinites achieved both their high carbon contents and their unpaired spin concentration before incorporation into the sediment and most probably by exposure to elevated temperatures. A commonly held view of the origin of some fusinites is that pyrolysis occurred as a result of forest fires, perhaps ignited by lightning or meteorites. An alternative... 

Oxidation of the polymers from Scheme 1-66 leads to the polymeric cation radicals with ferromagnetic coupling of spins. Surprisingly, however, the spin concentration in these polymer networks was extremely low. Only a few percent of the cation radical units actually carried an unpaired electron. 

Free Radicals in Macerals. Electron spin resonance (ESR) has been used to study carbon free radicals in coals, and to some extent, separated macerals. The technique provides information on radical density and the environment of the radicals. The resonance position, termed the g-value, is dependent on the structure of the molecule which contains the free electron. The line width is also sensitive to the environment of the unpaired electron. In an early study, Kroger (71) reported that the spin concentration varied between maceral groups with liptinite < vitrinite inertinite. For this limited set of samples the spin concentration increases with rank for liptinites and vitrinites and decreases for the micrinite samples. On the other hand, van Krevelen (72) found the same general results except... 

Such a kink is represented in Fig. 5, where the bond alternation has been supposed to take place on a single carbon atom. By counting the valence electrons on this carbon atom one can see that there is an odd electron left. Thus one unpaired spin is present. Indeed, the presence of spins was early detected by ESR in undoped /rans-(CH)x [62], with a spin concentration of about 1 spin per ca. 3000 CH units. However, with a theoretical soliton formation energy of 0.45 eV, one would expect a much smaller concentration of thermal solitons at room temperature. Very likely the spins in trans-po yacetylene, which essentially appear during the cis —> tram isomerization [63], are related to an odd number of tt electrons in conjugated segments accidentally formed during isomerization. 

The case U = 0 is of special interest. In such a case, making one bipolaron from two polarons does not cost energy. The maximum spin concentration is i, which means that 50% of the boxes possess unpaired spins. This result can be explained as follows. Since (1) at the maximum spin concentration one has q = 1, and (2) placing one or two balls in the box is equivalent, there are four equiprobable cases (1) neutral site (no ball), (2) polaron up, (3) polaron down, and (4) bipolaron (two balls). Two among four of these states are magnetic, which gives for the spin concentration. The room-temperature data for polypyrrole and poly aniline can almost be fitted with U 0, which means that in these compounds polarons and bipolarons would be degenerate. 

Figure 9 illustrates the temperature dependence of unpaired spin concentration for PMQl, PMQ2, and PMQ3. 

The integrated intensities of ESR signals can be used to estimate the number of unpaired electrons in materials. For coals, the concentrations of unpaired electrons rise nearly exponentially with increasing coal rank up to approximately 94% C, after which the spin concentrations decrease rapidly. The variation of spin concentration with coal rank has been observed by a number of researchers (11-15). 

When samples of unetched film were heated under vacuum above 250 C, additional unpaired spins were produced. Figure 3 shows kinetic plots for the thermally induced increase in spin concentration at four different temperatures. The ordinate is the relative ESR signal intensity, N /N, where the subscripts refer to the film before (o) and after heating for time t. After an initial rapid increase in signal intensity, the rate levels off to a constant value at each temperature. The slopes of the straight-line portions of the kinetic plots were used to obtain the Arrhenius plot shown in Figure 4, from which an activation energy of 68 16 kcal/ mole was calculated for the thermal production of unpaired spins. 

Spin Concentrations. It Is of Interest to answer the question, what Is the mean molecular weight per unpaired electron(l.e., per radical sites) Based on the assumption of two unpalr electrons per "molecule" and a total spin concentration of 6 x lO spins per grajg, the mean molecular weight per pair of unpaired electrons Is 2 x 10. This Is a number that falls Into the range of expected molecular weights for this particular polymer system. 

The free radicals in humic substances appear to be remarkably stable with respect to time and chemical attack (Steelink and Tollin, 1962 Steelink et al., 1963 Steelink, 1964). This stability has led some workers to suggest that humic substances comprise a free radical, or mixture of free radicals, of the semiquinone type (Steelink and Tollin, 1962 Steelink et al., 1963 Atherton et al., 1967). The increase in spin concentration upon converting humic acids to the sodium salts led Steelink (1964) to propose that the radical species in humic acid is a semiquinone co-existent with a quinhydrone species. Steelink (1964) also suggests that a free radical is an integral part of the humic macromolecule. The stability of this radical has been attributed to the delocalization of the unpaired electron over an aromatic system (Theng and Posner, 1967), or a shielding effect on the free radical due to the macromo-lecular network (Steelink, 1964). 

Figure 6.26 shows the chemical structure of PPV. Block letters from A to F show the eight carbon sites in one PPV monomer unit. The principal axes of proton hyperfine coupling of an unpaired tt-electron, defined in Figure 6.18, arc also shown for two proton sites of inequivalent bond orientations. ESR spectra of stretch-oriented undoped PPV films have shown anisotropic lineshapes with an average g value of 2.003 and a spin concentration of about 1 spin/10 PPV monomer units. The observed anisotropic behaviour of the. g value and the linewidth, both larger for the stretch direction, are qualitatively similar to those found in stretch-oriented polyacetylene films mentioned in Section 3.2 and... 

The chief application of ESR spectroscopy to chemical problems is the identification of the presence and nature of species containing unpaired electrons. Determination of concentrations of electron spins in a given sample is not very precise in general, only order-of-magnitude estimates of spin concentrations can be obtained. Accurate quantitative analyses can seldom be performed using ESR spectroscopy. 

Several sodium/magnesium doped polyacetylene films were prepared for magnetic measurements. The polyacetylene films were all thermally isomerized to the mns-isomer before doping. The EPR data for three representative samples are given in Table 4. In all cases, a decrease in the unpaired spin concentration was observed. This again indicates spinless charge carriers. A broadening of the EPR peak width was also observed in each... 

For quantitative comparisons between samples, they should be as closely matched in composition as possible. This applies to the use of standard samples against which may be measured unpaired electron concentrations these are more usually referred to as spin concentrations , and are quoted as the number of spins (unpaired electrons) per gram of sample. So, at the very least, the sample tubes should all be filled to the same depth, and at best, the standard should consist of the same material as the sample. Since this is rarely possible, a more practical alternative strategy is to match the materials as closely as possible in their electromagnetic properties. 

Obtaining a reliable measure of the number of unpaired electrons ( spin concentration ) in a sample is often extremely useful. Even reliable relative values measured across a series of samples can often provide useful information. There are various important applications, as may be illustrated by the following incomplete list ESR dating, the determination of oxidized polycyclic aromatic hydrocarbons and of environmental carbon in samples of ambient air, the influence of air pollution (e.g., SO2 and NO2) on plants and soils, quantification of NO2, RO2, and HOI radicals in air samples, radiation dosimetry, redox activities of zeolite catalysts, and the metabolism of spin probes in cells and tissues. 

Figure 25.17 Variation of unpaired spin concentration N with milling time (f) for NR/SRF carbon black compounds extract.

Figure 25.18 Dependence of unpaired spin concentration (TVs) on the milling time (t) for NR/carbon black compounds with various carbon black loading. (Reproduced from ref. 40 with permission from the American Chemical Society.)...

Figure 25.19 shows the maximum unpaired spin concentration versus specific surface area of various carbon blacks. The increasing order of the free radical concentration of various types of carbon blacks is TM-15 < HAF < SRF < FEF < ISAF. The results are related to the concentration of free radicals on the surface of carbon blacks, i.e. concentration of free radicals for the carbon blacks become higher for the carbon blacks having higher specific surface area. This indicates that the interfacial interactions between NR matrix and carbon black are governed by the active surface area of the carbon blacks. 

Figure 25.20(a) and (b) show the variation of unpaired spin concentration as a function of TM-15 and HAF carbon blacks content in NR compounds. 

Figure 25.19 The maximum unpaired spin concentration versus specific surface area of various carbon blacks.

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