ESR of Lignin

ESR studies of lignin model compounds, on the other hand, have yielded information on the detailed structures of phenoxy radicals that are involved in some homolytic splitting reactions of lignin Such information has proven useful in elucidating the mechanisms of these reactions As with other spectrometnc methods, ESR also suffers from its inability to shed light on the true nature of free radicals, phenoxy or otherwise, that exist in lignin macromolecules... 

In other analytical work, Shevchenko et al. [80] found excellent agreement between theoretical and experimental ionization potentials. Semenov and Khodyreva (81) compare photoelectron spectra with CNDO/S3 calculations and found good resnlts. Semiempirical calculations, on a number of lignin model compounds have also been nsed as an aid in the interpretation of ESR spectroscopy [34]. In more general research on substituted aromatic systems [82] correlated experimental p/f valnes to semiempirical heats of formation and HOMO energies, with correlations in the 0.7-0.9 range. 

Y-P Sun, B Yates, J Abbot, and C-L Chen. ESR study of lignin model compounds irradiated by uv (254 nm photons) in the presence of hydrogen peroxide. Holzforschung 50 233-236, 1996. 

During UV irradiation of lignin several other free radicals are formed, which were detected by ESR spectroscopy [981]. Because of the very complicated structure of lignin (4.124) it is difficult to identify the free-radical sites formed. The reactive groups available in lignin consists of various types of ether, primary and secondary hydroxyl groups, carbonyl groups, carboxyl and ester functions. There also exist a number of aromatic and phenolic sites where free radicals can be formed. 

Electron spin resonance (esr), 22 132 for lignin characterization, 15 10 Electron-stimulated desorption-ion angular distribution (ESDIAD), 24 74 Electron transfer (ET), 9 376-381, 388 mechanisms of, 13 444 rate constant for, 13 447 Electron-transfer dynamics, in... 

Figure 3. a, the effect of doping degree on IR spectra (bromine) b, ESR-spectra of sulfur lignin doped with bromine. 

Burdon542 has surveyed the current hypotheses for the structure of humic substances and has concluded that the various products from chemical degradations and NMR data are all consistent with their being mixtures of plant and microbial materials and their microbial degradation products. The examination of soil carbohydrates, proteins, lipids, and aromatics supported this view the presence of colour, fluorescence, ESR signals, mellitic acid, and other features do not contradict it. Regarding the Maillard reaction, some free monosaccharides and the necessary amino species are present in soil, so it may proceed, but only to a small extent it is not a major process. However, in marine environments, the relative abundance of carbohydrates and proteins makes them more probable precursors of humic substances than lignin or polyphenols. 

Electron spin resonance (ESR) or electron paramagnetic resonance (EPR) spectroscopy has developed at an outstanding pace since its discovery in 1945 (Zavoiskii 1945), so that at present the technique is very well understood in its many aspects. In wood chemistry, ESR has become an essential tool for the study of the structure and dynamics of molecular systems containing one or more unpaired electrons, i.e., free radicals. ESR has found applications as a highly sensitive tool for the detection and identification of free radical species in lignin and lignin model compounds (Steelink 1966, Kringstad and Lin 1970). A recent literature review of free radicals in wood chemistry is available (Simkovic 1986). 

ESR is a sensitive tool for detecting free radicals. In addition, ESR represents further improvement over indirect methods normally employed in lignin investigations of free radicals by providing the possibility of specific free radical identification and characterization through spectroscopic detail. The characteristics of an ESR spectrum useful for deductions about physical interactions can be conveniently grouped into the following four parameters (1) g-value, (2) intensity, (3) line shape, and (4) hyperfine structure. 

Theoretically, ESR lines should be infinitely narrow, experimentally, they are broadened by various mechanisms, both intrinsic (determined by sample properties and the physics of the resonance experiment) and extrinsic (dependent on spectrometer operating conditions) Line shapes of ESR signals of peroxy radicals in lignin are often asymmetric because of the solid-state or power spectrum effects of the vanous anisotropic interactions... 

Fig. 5.5.2. Power saturation of ESR spectra of photo irradiated milled wood lignin at 77 K Microwave power (mW) a5,b 10, c 15, d 20, e 30 / 35, g 40...

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