G value, ESR

As expected, they are deeply coloured (R = Pr, deep red R = cy, red-violet) and have typical magnetic moments of 1.5— 1.6 /xB and ESR g values 2.064, 2.038, 2.007 (for R = Pr1) others [157d] have suggested that the structures were obtained on disordered molecules. In solution, at room temperature, they tend to decompose... 

Iron(III) complexes of 2-acetylpyridine Af-oxide iV-methyl- and 3-azabicyclo[3.2.2.]nonylthiosemicarbazone, 24 and 25, respectively, have been isolated from both iron(III) perchlorate and chloride [117], The perchlorate salt yields low spin, octahedral, monovalent, cationic complexes involving two deprotonated, tridentate thiosemicarbazone ligands coordinated via the N-oxide oxygen, azomethine nitrogen and thiol sulfur based on infrared spectral studies. Their powder ESR g-values are included in Table 1 and indicate that bonding is less covalent than for the analogous thiosemicarbazones prepared from 2-acetylpyridine, 3a and 4. Starting with iron (III) chloride, compounds with the same cations, but with tetrachloroferrate(III) anions, were isolated. 

Exhaustive compilations of magnetic properties of transition metal complexes are available.166-192-194 The following sections describe very briefly the magnetic behaviour of a selection of complexes or groups of complexes, from the various electronic configurations which arise for the transition metals. They are intended to be illustrative of the points made in the earlier sections no attempt at completeness of coverage is made. More detailed and intensive accounts of magnetic properties of transition metal complexes are available.99 166 169-174-175 186-195-197 No mention is made of results for lanthanoid and actinoid elements, nor of ESR g-values. 

Table 1 ESR g-values, hyperfine coupling constants (aN) of 1,2,3-dithiazolyl radicals and cation radicals, and half-wave potentials of 1,2,3-dithiazolyls...

Figure 3. ESR g values of vitrains from selected coals...

Heteroatomic Coals, especially low-rank coals that are rich in oxygen, might be expected to contain heteroatomic free radicals. Yen and Sprang (29) showed that the g values of coals as well as those of other bituminous materials showed a systematic trend with their heteroatom content. In particular, they plotted the ESR g values as a function of where X refers to the atom fractions of oxygen, nitrogen, and sulfur in the coals and values are spin-orbit coupling constants of the respective atoms, i.e., 152 cm , 70 cm , and 382 cm . This... 

ESR g values for West Virginia (Ireland Mine) hvAb coal heat-treated at four different temperatures are shown graphically in Figure 9. The points connected by the dashed lines represent the results of measurements made on neat samples after heat treatment. The general shape of this curve has been established for other coals (8, 15). The point for the sample heat-treated at 450 C falls considerably below the free electron value similar results for other coals have been attributed to the formation of sigma radicals during heat treatment (8). Three experiments were conducted to show that this observed g value was an apparent and not a true value. 

To establish the true relationship between ESR g values and coal heat treatment, samples of Ireland Mine coal diluted with potassium bromide were heat-treated at different temperatures. The results, represented by the open circles in Figure 9, show that the g value is... 

Table II. ESR g Values and Spectral Line Widths for Selected Samples of Solvent-Refined Coal...

ESR spectra of radicals show some similarities with the NMF spectra of spin-paired molecules. The ESR g value corresponds to the NMR chemical shift. Hyperfine splitting in ESR caused by electron-nuclear spin interaction , have an NMR counterpart in the splitting of resonances by miteraclions between two nuclear spins. 

The ESR g values, which arise from spin-orbit coupling, are higher than 2.0023, which is the free electron value in the absence of spin-orbit coupling. The g values for vitrains and fusains... 

FIGURE 10.9 Variation of ESR g value with vitrain rank. (From Retcofsky, H.L. et al.. Organic Chemistry of Coal, J.W. Larsen, Ed., Symposium Series No. 71, American Chemical Society, Washington, DC, 1978, p. 142.)... 

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